Antiviral drugs to treat enzootic feline coronavirus infection in kennels and why it might not be a good idea

Niels C. Pedersen, DVM PhD,

Original article: Antiviral Drugs for Control of Enzootic Feline Coronavirus Infection in Pedigreed Catteries and why it may not be a good idea

When discussing feline coronavirus (FCoV) infection in a multicat setting, it is important to understand the correct terminology. The term FCoV is a collective term for two historically named viruses. A coronavirus was eventually identified as the causative agent of feline infectious peritonitis (FIP) in cats, which was named FIP virus or FIPV (Ward, 1970; Zooket al., 1968). Subsequently, FIPV was found to be a mutant form of FCoV that was present in cats infected with a widespread and minimally pathogenic enteric coronavirus and was named feline enteric coronavirus (FECV) (Pedersen et al., 1981). To avoid misunderstandings, this author prefers to refer to the form of FCoV that applies to the immediate discussion. Therefore, it is appropriate to use the term FIPV when discussing the form of FCoV found in a specific type of white blood cell (monocyte/macrophage) in the affected tissues and body fluids of cats with FIP. The term FECV is used to refer to the form of FCoV that causes chronic and intermittent infections of the epithelium in the lower intestine of healthy cats and is excreted in large quantities in the feces. Enzootic is the correct term for infections that occur in animal populations, while endemic is the corresponding term used for humans. Clinical "signs" are what veterinarians and pediatricians observe during physical examination or what owners/parents communicate to them, while symptoms are what patients describe to their doctors. Therefore, "epizootic" and "symptoms" are not strictly veterinary terms.

FECV, like many other microbial infections in cats, is maintained in the population as a chronic or recurrent asymptomatic infection. FECV is first shed in faeces from around 9–10 weeks of life, coinciding with the loss of maternal immunity (Pedersen et al., 2008). Infection occurs via the faecal-oral route and targets the intestinal epithelium, and primary signs of enteritis are mild or usually inconspicuous (Pedersen et al., 2008; Vogel et al., 2010). Subsequent faecal excretion occurs from the colon and usually ceases after several weeks or months (Herrewegh et al., 1997; Pedersen et al., 2008; Vogel et al., 2010) with the development of immunity. The resulting immunity is notoriously short-lived, and repeated infections are common throughout life (Pearson et al., 2016; Pedersen et al., 2008). A stronger immunity appears to develop over time and cats over 3 years of age have been shown to be less likely to become reinfected and become faecal shedders (Addie et al., 2003). Although the level of exposure to FECV is the primary risk factor for FIP in cat breeds (Foley et al., 1997), the health of the immune system at the time of emergence of mutant FIPV is a major determinant of the occurrence of FIP in any population or group of cats.1

FIP is caused by specific mutants that arise during FECV infection (Poland et al., 1996; Vennema et al., 1995).1 These FlP-causing mutants develop with some frequency in the organism, but fortunately most of them are eliminated by the healthy immune system (Poland et al., 1996).1 Given the relationship between FECV enzootic infection and FIP, it is logical to prevent FIP by minimizing FECV exposure. As “no vaccine can produce better immunity than natural infection” and given what is known about the weakness and short-term nature of natural immunity against FECV (Pearson et al., 2016; Pedersen et al., 2008), it is unlikely that it will succeed to develop effective vaccines against FECV.

Although enzootic FECV infection is not amenable to vaccination, thorough carrier testing and strict quarantine can eliminate FECV in a group of breeding research cats (Hickman et al., 1995). However, FECV is so ubiquitous in nature and easily spread by direct and indirect cat-to-cat contact and on human-borne fomites that the strictest quarantine facilities and procedures are required to prevent its spread. How strict must the quarantine be? Experience with testing and removal in conjunction with quarantine to eliminate and prevent FECV infection is limited to one report (Hickman et al., 1995). FECV was eliminated from a specific pathogen-free breed of cats at UC Davis by removing the virus shedders and rigorously tightening quarantine procedures for the remaining colony (Hickman et al., 1995). Nevertheless, FECV re-entered this colony for several years, despite all attempts to prevent its spread (Pedersen NC, UC Davis, unpublished, 2022). The only example of effective quarantine for FECV was described for cats in the Falkland Islands (Addie et al., 2012). These islands in the remote South Atlantic have fortunately remained free of FECV, probably due to their extreme isolation. Measures have been taken to prevent future inadvertent introduction of FECV to the islands (Addie et al., 2012). Based on this experience with feline and murine enteric coronaviruses, it is unlikely that FECV could be kept out of any group of domesticated cats with anything less than the strictest isolation and infection prevention practices.

An interesting approach to prevent or delay FECV infection in kittens in breeding centers has been referred to as "early weaning and isolation" (Addie et al. 19952). It was based on the finding that kittens born to FECV-exposed or infected mothers have maternal immunity to infection up to 9 weeks of age (Pedersen et al., 2008). Therefore, kittens weaned a few weeks before the loss of this immunity (4-6 weeks of age) are usually free of infection and, if removed from the mother and isolated from other cats, could theoretically be kept virus-free. This practice was initially popular, but the necessary facilities and quarantine procedures required to prevent later infection were difficult to maintain in kennels with larger numbers of breeding cats (> 5 cats, Hartmann et al., 2005; > 10 cats Addie et al., 19952). Therefore, elimination of FECV in kittens by early weaning and isolation has been doomed to failure in most common homes/kennels due to the largely unavoidable exposure to FECV that occurs in the breeding, rearing and exhibition of breeding cats.

Another problem with early weaning and isolation is the need to separate virus-free kittens from other cats in a large group. This problem could be avoided if all the cats could get rid of the infection at the same time. This can be achieved by serially testing faeces for FECV excretion over a period of time and culling all shedding cats. However, since a significant proportion of cats in farms involved in FECV enzootic disease shed FECV in their faeces (Foley et al., 1997; Herrewegh et al., 1997), culling cats can have a serious impact on the gene pool (Hickman et al., 1995). . This begs the question – can FECV be eliminated in all cats in a group at the same time? Interestingly, the relatively recent discovery of effective antivirals against FIP has also provided a possible method of eliminating all the spreaders of the virus at the same time (Pedersen et al., 2018, 2019). Early studies of such use of antivirals such as GS-441524, although of a rather preliminary nature, suggest that FECV can be eliminated from a closed population of cats with relatively short treatment (Addie et al., 2023).

Assuming that FECV can be eliminated as an enzootic virus from the feline population by using specific antivirals, what are the pitfalls of doing so? The first pitfalls are the cost of antivirals, the frequent testing of feces required to identify shedding animals, and the establishment and maintenance of adequate quarantine facilities and practices. Therefore, domestic facilities with poor barrier isolation practices are doomed to failure to maintain this group of cats FECV-free for extended periods of time. The second pitfall is related to the normal activities of breeding and exhibiting breeding cats. Breeding cats involves frequent interaction between the cats as well as humans in contact with the cats and with each other. It is also difficult to imagine that a breeder and avid show participant would give up all the joys of breeding and showing their cats by avoiding all such interactions. The final question is: "Now that the cats are free of FECV, what are you going to do with them?". What is the chance that they will remain without FECV for any length of time after leaving the controlled environment? They will have no immunity to FECV and will be very sensitive to the slightest exposure. The same will apply to the group of cats they come from. Finally, the continuous antiviral treatment required to maintain a group of cats free of FECV infection is likely to result in the development of drug resistance. We now know that resistance to GS-441524 can occur in cats treated for FIP, and UC Davis researchers1 and Cornell University3 agree that acquisition of drug resistance in enzootic FECV infections would outweigh any potential benefit of such treatment on FIP incidence. FIP is currently curable in more than 90 % cases4 and even if resistance to antivirals does develop, it is largely confined to the affected cat. It can be argued that HIV-1 infection in humans is currently prevented by antivirals without any reported concerns about drug resistance. Preventive treatment of HIV-1 however, it is not a monotherapy, but includes several drugs of different classes.3 This is not done to increase the effectiveness of treatment, but rather to prevent drug resistance. If the virus develops resistance to one drug in the drug mix, the other drugs will prevent it from replicating.

In conclusion, I would like to paraphrase: "Just because something can be done, should it be done?" The author believes that much larger and better designed studies, conducted over a long period of time, are needed before this practice can be seriously considered. The overall incidence of FIP in smaller and well-maintained farms with enzootic FECV infection is usually less than 1 %, and currently more than 90 % cases of FIP that might arise can be cured.4 A practical way to reduce the incidence of FIP is to keep the number of breeding cats and kittens low, to keep more older cats, to not breed individuals and bloodlines that have given rise to cases of FIP, and to minimize the stress of frequent introductions of new cats and changes in placement or relocated.1 In smaller farms, isolation and early weaning can also be useful.


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  1. Pedersen NC. History of Feline infectious Peritonitis 1963-2022 – First description to Successful Treatment. 2022-final-version.pdf-4.29.22.pdf.
  2. Addie D. Prevention of feline coronavirus (FCoV) infection.
  3. Cornell University blog. Fight FIP. Unraveling feline infectious peritonitis from the ground up.
  4. FIP Treatment – Czechia / Slovakia. Basic data, 2023. FV fteWIW1HXsROLuJ7YY1- i Sf81BCmM9JT9LbCT2mcnwD1rL9IBsLCTB1U59CcnalOGiFqq/pubhtml?gid=1340189982&singl e=true

Antivirals and FIP treatment protocols

Although currently the most widely used active substance in the treatment of FIP is the nucleoside analog GS-441524, in fact there are already several agents with antiviral activity that can be successfully used in the treatment of FIP. In this article, I would like to introduce you to currently used antivirals, or recommendations for their use.

Due to the absence of clinical studies other than GS-441524 and GC-376, unless otherwise stated, the recommended duration of treatment for FIP is still 12 weeks. This does not mean that the treatment cannot be shorter for a specific individual, but at the same time there are also cases where the treatment must be extended. It should also be noted that the treatment should always be terminated only after the assessment of the cat's clinical condition and the results of the blood test.


Currently the most widely used antiviral drug for the treatment of FIP. Nucleoside analog GS-441524 has been the subject of several clinical studies. The first to prove its effectiveness in the treatment of FIP was Dr. Niels Pedersen and his team. You can find his pioneering clinical study here.
The subject of this clinical study was injection form active substance, but it didn't take long, and tablet forms of the drug also appeared on the market. The originally determined dosage was gradually increased over time along with the decreasing price of the treatment, and nowadays it is good to stick to the values listed below. In addition, GS-441524 is a very safe antiviral, and because of minimizing the risk of relapse, it is better overdose, such as underdosing.

Unfortunately, the patent holder of GS-445424, Gilead, never licensed it to another company (with the exception of Bova), and is not even trying to commercialize this substance. For this reason, practically all medicines containing GS-441524 come from the black market.

In the case of a severe condition, it is possible and even advisable to use at least the first 3 days of the so-called booster dosage at the dosage level for neurological FIP, even if the cat does not have neurological FIP. There are even opinions that it is good to use neurological dosage for the first 14 days even in non-neurological forms of FIP (Dr. Addie).

FIP typeGS-441524 - injection solutions
Wet FIP (abdominal effusion, without ocular and neurological symptoms)6 mg/kg once a day sc
Dry FIP (without effusion, or with effusion in the chest cavity without eye and neurological symptoms)8 mg/kg once daily SC
Ocular FIP (ocular symptoms - cloudy eye, blood in the eye chamber, etc.)10 mg/kg once daily SC
Neurological FIP (neurological symptoms, eg anisocoria or mydriasis)12 mg/kg once daily SC
Relapse of FIP (usually associated with neurological manifestations)15 mg/kg once daily SC

Arrival tablets got a little confused with the dosage during treatment, because some manufacturers started to list the so-called equivalent GS content so that the dosage used is "compatible" with injections, while other manufacturers state the real GS content. It is believed that the oral bioavailability of the drug is only about 50% compared to injections, so in practice it is necessary to count on twice the dosage of such tablets compared to injections, or simply use tablets with a known real GS content as tablets with a half-equivalent GS content. There is only one company licensed to use GS-441542 in tablet form in veterinary practice, and that is the British company Bova. Its GS 50 mg tablets are used for the legal treatment of FIP in Australia and the UK. Unfortunately, they are very expensive. It is necessary to take into account the price of 1 tablet approx. 1000 CZK. For this reason, similar to injectable solutions, tablets from the Chinese black market are mainly used for treatment.

Note that for neurological FIP, the recommendation is to split the dose twice a day. This is due to the presumed reduced absorption capacity of the drug in the digestive tract at an equivalent dosage higher than 10mg/kg.

FIP typeGS441524 - tablets with the specified real GS contentGS441524 - tablets with the stated equivalent GS content
Wet FIP (abdominal effusion, without ocular and neurological symptoms)10-12 mg/kg once a day6 mg/kg once a day
Dry FIP (without effusion, or with effusion in the chest cavity without eye and neurological symptoms)12-16 mg/kg once a day8 mg/kg once a day
Ocular FIP (ocular symptoms - cloudy eye, blood in the eye chamber, etc.)20 mg/kg once a day or 10 mg/kg twice a day10 mg/kg once a day or 5 mg/kg twice a day
Neurological FIP (neurological symptoms, eg anisocoria or mydriasis)12 mg/kg twice a day6 mg/kg twice a day
Relapse of FIP (usually associated with neurological manifestations)15 mg/kg twice a day7.5 mg/kg twice a day

As already mentioned, GS-441524 is a safe antiviral, but on the other hand, neutropenia is very often observed after treatment, which can last for a very long time (up to several months). In the case of long-term and very significant neutropenia, the application of filgrastim - a factor that stimulates the formation of hematopoietic cells - can be considered.


Protease inhibitor GC376 is actually a first generation anti-FIP drug. Its effectiveness has been proven in the treatment of wet and dry FIP, but due to the significantly reduced ability to penetrate through the blood-ocular and blood-brain barrier, it is not suitable for the treatment of ocular or neurological forms of FIP. Given that very shortly after pilot study GC376 was lost to Dr. Niels Pedersen with the nucleoside analog GS-441524, the importance of the protease inhibitor GC376 has declined significantly. However, it turns out that it can be, and probably will be in the future, an important component of the combined treatment of FIP, for example together with GS-441524, where the effect of both active substances is mutually potentiated, and as a result is much more pronounced than with each active substance alone .
Currently, the company is trying to launch GC376 on the market Anivive.

FIP typeGC376 - solution for injection
Wet FIP (abdominal effusion, without ocular and neurological symptoms)15 mg/kg 2x daily sc
Dry FIP (without effusion, or with effusion in the chest cavity without eye and neurological symptoms)15 mg/kg 2x daily sc
Ocular FIP (ocular symptoms - cloudy eye, blood in the eye chamber, etc.)it is not used
Neurological FIP (neurological symptoms, eg anisocoria or mydriasis)it is not used
Relapse of FIP (usually associated with neurological manifestations)it is not used

GC376 is a safe antiviral, but its most significant side effect is a delay in the development of permanent teeth in young cats.


This is another drug from Gilead. In fact, it is the so-called prodrug of the above GS-441524. After the application of remdesivir, intracellular metabolism to GS-441524 occurs in the organism. Remdesivir was marketed by Gilead under the trade name Veklury and has played a significant role in the treatment of Covid-19 in humans. However, its use in veterinary practice is very questionable and impractical. Firstly, it lacks approval for veterinary use and secondly, it is very expensive. Application is also a weak point of the drug, as it is intended for intravenous administration. The concentration of Veklura after reconstitution is only 5mg/ml.

The company Bova managed to obtain a license for the use of remdesivir for veterinary use and produces a product with a concentration of 10 mg/ml, which can be used in the form of subcutaneous injections. Unfortunately, the price is very high, so it is not used much in common practice.
Remdesivir has approximately 2x the molecular weight of GS-441524, so the dosage of remdesivir must be approximately 2x higher than that of GS-441524.

FIP typeRemdesivir - solution for injection
Wet FIP (abdominal effusion, without ocular and neurological symptoms)10-12 mg/kg once a day iv/sc
Dry FIP (without effusion, or with effusion in the chest cavity without eye and neurological symptoms)10-12 mg/kg once a day iv/sc
Ocular FIP (ocular symptoms - cloudy eye, blood in the eye chamber, etc.)15 mg/kg once daily iv/sc
Neurological FIP (neurological symptoms, eg anisocoria or mydriasis)20 mg/kg once a day iv/sc
Relapse of FIP (usually associated with neurological manifestations)25 mg/kg once a day iv/sc


Antiviral with a long history primarily intended for the treatment of Covid-19 in humans. Molnupiravir (EIDD-2801) is incorporated into the genome of RNA viruses and causes random mutations resulting in the so-called virus bug disaster. The drug exists in the form tablets or capsules. Most legally manufactured drugs (e.g. Lagevrio) contain capsules containing 200 mg of the active substance, and re-encapsulation of the drug is necessary for use in the treatment of FIP. Of course, Chinese manufacturers also produce tablets intended for direct use in animals. Although molnupiravir is not strictly the drug of first choice in the treatment of FIP, it shows great potential in the treatment of FIP relapses, resistance to GS-441524, or can be an important part of FIP combination therapy.

FIP typeMolnupiravir
Wet FIP (abdominal effusion, without ocular and neurological symptoms)10-12 mg/kg twice a day
Dry FIP (without effusion, or with effusion in the chest cavity without eye and neurological symptoms)10-12 mg/kg twice a day
Ocular FIP (ocular symptoms - cloudy eye, blood in the eye chamber, etc.)10-12 mg/kg twice a day
Neurological FIP (neurological symptoms, eg anisocoria or mydriasis)12 mg/kg twice a day
Relapse of FIP (usually associated with neurological manifestations)15 mg/kg twice a day

With molnupiravir, as with GS-441524, neutropenia can often be observed after the end of treatment, especially at high dosages.


Mefloquine is an interesting substance that has its primary application in the treatment of malaria in humans. It is not effective as a monotherapy in the treatment of FIP, but it is shown that it can play an important role in the adjunctive treatment of FIP, for example with GS-441524. It mainly makes it possible to reduce the price of FIP treatment or reduce the risk of relapse. The normal length of treatment using GS-441524 is about 12 weeks, but in principle it is possible to shorten this treatment to about 8 weeks and use mefloquine for the next 4 weeks. In Europe, mefloquine is available under the trade name Lariam. One tablet contains up to 250 mg of active substance. In practice, the drug is administered at a dose of 12.5 mg/kg twice a week, or 62.5 mg twice a week for one cat. This corresponds to 1/4 tablet of Lariam twice a week (for example, Monday and Thursday). Lariam must be given with food, otherwise there is a higher probability of the cat vomiting.

I strongly reiterate the fact that mefloquine is not intended for the treatment of FIP as a monotherapy, and should always follow as a supplement after the treatment of FIP with one of the above-mentioned antivirals, or in combination with them.

FIP typeMefloquine (Lariam)
Wet FIP (abdominal effusion, without ocular and neurological symptoms)62.5 mg per cat twice a week
Dry FIP (without effusion, or with effusion in the chest cavity without eye and neurological symptoms)62.5 mg per cat twice a week
Ocular FIP (ocular symptoms - cloudy eye, blood in the eye chamber, etc.)62.5 mg per cat twice a week
Neurological FIP (neurological symptoms, eg anisocoria or mydriasis)62.5 mg per cat twice a week
Relapse of FIP (usually associated with neurological manifestations)62.5 mg per cat twice a week

Other antivirals

It turns out that there is actually up to several dozen of potentially suitable antivirals applicable to the treatment of FIP. Unfortunately, no clinical studies have yet been conducted for many of them, which are important not only for the verification of effectiveness, but mainly for the determination of cytotoxicity. The goal, of course, is to cure the animal without causing poisoning or other health problems that would lead to the cat's death.

Combined therapies

The currently used FIP treatment usually takes the form of monotherapy, that is, the drug contains only one active substance. Unfortunately, this approach has the disadvantage that it is only a matter of time before resistance to the used antiviral begins to manifest itself. The way out of this situation is combined therapy, when 2 or more antivirals are used simultaneously for the treatment of FIP. It is not an entirely simple issue, as in some combinations the therapeutic effect is significantly strengthened, but there are also combinations where, on the contrary, the therapeutic effect is weakened. Currently, the most likely drug combinations are the pairs GS-441524 and Molnupiravir, or GS-441524 and GC376. Regarding the second named combination, it has already taken place in China study, the result of which is really encouraging. In addition to curing all cats, the treatment time was reduced from 12 weeks to 4 weeks. It turns out that a dosage of GS-441524 5mg/kg/24h and GC-376 20mg/kg/12h could be used to achieve a therapeutic effect. However, this combination therapy still needs further independent verification of efficacy.

2023 – Neurological and ocular FIP

Original article: 2023 – NEUROLOGICAL OCULAR FIP
Published 1/4/2021, updated 2/10/2023, Translation update 3/8/2023

Basic facts

Dr. Pedersen

What is FIP? – FIP is caused by a common and mostly harmless enteric coronavirus, similar to those that cause the common cold in humans and diarrhea in foals, calves and poultry. Most cats are infected with feline enteric coronavirus (FECV) at around 9 weeks of age and may be reinfected before 3 years of age, when cycles of infection become less frequent. Specific mutations that allow FECV to escape from the cells lining the lower intestine and infect the most basic cell of the immune system, the macrophage, occur in about 10 % infections. However, this macrophage infection is eliminated in all but 0.3–1.4 % cats. Predisposing conditions that lead to disease in this small proportion of cats include young age, genetic susceptibility, sex, overcrowding, poor nutrition, and a number of stressful events in the environment. The site of initial onset of the disease is in the lymphoid tissue in the lower small intestine, cecum, and proximal colon. Infected macrophages leave these initial sites of disease and migrate locally and in the bloodstream to small veins in the lining of the peritoneal cavity, the uveal tract of the eye, the ependyma, and the meninges and spine. Symptoms of the disease appear within days, weeks, sometimes months, and rarely a year or longer. The form of the disease that manifests itself is simply referred to as wet (effusive) or dry (non-effusive). The two forms are easily distinguishable, although there may be intermediate forms between them. Some cats may have symptoms of dry FIP but later develop wet FIP, or vice versa. Overall, about two-thirds of cats have wet FIP and one-third have dry FIP. The duration of illness until death, usually by euthanasia, used to be only a matter of days or weeks. Fewer than 5 % diseased cats, especially those with milder forms of dry FIP, survive longer than one year with the best symptomatic care.

Manifestations and forms of FIP

Clinical manifestations of FIP – The clinical manifestations of wet (Table 1) and dry (Table 2) FIP differ depending on the site(s) in the body where the infected macrophages end up causing inflammation. The intensity and nature of the inflammation are responsible for the form of the disease. Wet FIP is a more acute and severe form of FIP and is characterized by the accumulation of inflammatory fluid in either the abdominal cavity and/or the chest cavity. Involvement of the central nervous system (CNS) and eyes is relatively rare in the wet form of FIP (Table 1). The dry form of FIP is not characterized by diffuse inflammation and fluid discharge, but rather by fewer and more tumor-like lesions (ie, granulomas) in organs (e.g., kidneys, cecum, colon, liver, lungs, lymph nodes) in the abdomen or chest cavity or in the eyes and brain (Table 2). While the brain and/or eyes are involved in only 9 % cases of the wet form, neurological and/or ocular disease is the main clinical sign in 70 % cats with the dry form of FIP.


Symptoms associated with:occurrence (%)
Peritoneal cavity58%
Peritoneal and pleural cavities22%
Pleural cavity11%
Peritoneal cavity, eyes2,8%
Peritoneal cavity, CNS *1,9%
Peritoneal and pleural cavity, CNS0,9%
Peritoneal and pleural cavity, eyes0,9%
Pleural cavity, CNS, eyes0,9%
Peritoneal cavity, CNS, eyes0,9%

* CNS - Central nervous system (brain, spine)


Symptoms associated with:occurrence (%)
Peritoneal cavity30%
CNS and eyes8%
Peritoneal cavity, eyes7%
Peritoneal and pleural cavities4%
Peritoneal and pleural cavity, CNS3%
Peritoneal and pleural cavity, eyes2%
Peritoneal cavity, CNS, eyes2%
Pleural cavity1%

Blood-brain and blood-eye barrier

Basic facts - The eye and central nervous system (CNS) are protected from harmful substances by blood-eye barriers (blood-eye barrier) and blood-brain (blood-brain barrier). These barriers are of great evolutionary importance because they protect brain and eye functions from the effects of systemic toxins and infectious agents. Such barriers have been developed over millions of years by positive selection of the most capable individuals. The blood-brain barrier in cats does not pass about 80% most drugs, while the blood-eye barrier about 70%. Therefore, if a given dose of a drug such as GS-441524 reaches an effective blood level (plasma) of 10 μM, the levels in the brain (cerebrospinal fluid) will be only 2 μM and the level in the eye (ventricular water) will only be 3 μM. However, higher levels are likely to be reached in inflamed tissues and will decrease as inflammation subsides. This may be one of the explanations for the rapid improvement that is often observed in the first days of treatment.

Several other aspects of these two blood barriers need to be considered. First, their impermeability of undesirable substances varies from individual to individual. Second, the effectiveness of this barrier decreases in inflamed tissues and increases as inflammation subsides. This is good for treatment in the early stages of the disease, but bad for treatment in the final stages when the inflammation disappears and only the virus remains. Third, there are no simple, safe or effective means of weakening these barriers, and the only way to increase the level of the drug in the brain or eyes is to increase their level in the blood plasma by administering a higher dose, either orally or parenterally.

How these barriers affect forms of FIP - Paradoxically, ocular and neurological forms of FIP are also a consequence of the same barriers, but in this case in neurological and / or ocular FIP, the main problem is the entry of antibodies and immune lymphocytes. The phenomenon of neurological disease after a common systemic viral infection is well known in humans and animals. A typical example is polio-encephalomyelitis in humans and canine distemper in dogs. Poliomyelitis virus (polio) is a common intestinal pathogen and usually causes a mild or mild intestinal infection. However, in some people, the virus also penetrates the brain and spinal cord. Humans develop a strong systemic immune response to the polio virus, which is highly effective in eliminating the virus in all parts of the body, except the nervous system, where the limits of the blood-brain barrier are an obstacle to immunity. These unfortunates develop a classic neurological form of infection. A similar phenomenon occurs in canine distemper. Canine distemper virus, which is closely related to the human measles virus, causes an acute respiratory infection in young dogs, which manifests 7-14 days after exposure and lasts one to two weeks. Most of these dogs recover completely, but some develop neurological disease in three or more weeks. This highly lethal secondary form of canine distemper is caused by a virus that has escaped from the body to the brain and spinal cord during the respiratory phase of the infection and is protected from the host's immune system by the blood-brain barrier.

The distribution of the disease between the CNS and other parts of the body may also explain why blood tests are rarely abnormal in cats with primary neurological disease or in those who have relapsed to these forms during or after treatment with non-neurological forms of FIP. It appears that inflammation at privileged sites such as the CNS may not elicit a systemic inflammatory response and may not cause significant changes in hematology, nor an increase in total protein and globulin, and a decrease in albumin to globulin A: G ratio.

Preliminary diagnosis of ocular and neurological FIP

Preliminary diagnosis – Eye and neurological diseases are much less common in cats with wet than with dry FIP (Tables 1, 2). They also occur in primary and secondary forms. Primary disease accounts for approximately one-third of cases of dry FIP (Table 2), and lesions outside the eyes and central nervous system (CNS) are either absent or not readily discernible. Secondary neurological and ocular forms of FIP become much more common as a result of antiviral therapy and occur either during the initial treatment of the common extra-ocular/CNS forms or as a relapse during the 12-week post-treatment observation period.

The initial suspicion of neurologic and/or ocular FIP is based on age, origin, and presenting clinical signs. FIP occurs mainly in cats under 7 years of age, three-quarters of them under 3 years of age and with the highest incidence between 16 weeks and 1.5 years. Common symptoms in both ocular and neurological FIP were stunted growth in kittens and adolescent cats, weight loss in adults, and vague signs of ill health often associated with fever.

It is believed that the diagnosis of FIP, especially the dry form, is difficult. However, a preliminary diagnosis is relatively easy to establish due to stereotypic signaling, clinical history and physical findings, and the rarity of disease confusion in the group with the highest risk of FIP. Neurological and/or ocular forms of FIP can be confused with systemic feline toxoplasmosis, so many cats with these forms of FIP are tested for toxoplasmosis and treated with clindamycin. However, systemic toxoplasmosis is an extremely rare disease in cats, especially compared to FIP. FIP is easily distinguished by the cat's origin (breeding station, foster/rescue station, shelter), signaling (age, sex, breed) and basic blood test results. Deep fungal infections (coccidioidomycosis, blastomycosis, histoplasmosis) can cause ocular and sometimes neurological symptoms similar to FIP, but are still rare even in their endemic areas. Lymphoma can also be a differential diagnosis of dry FIP, but this disease is usually sporadic and occurs in older cats. A number of congenital disorders can also present with progressive neurological signs, but these occur mainly in younger cats and are not associated with the inflammatory manifestations of infectious diseases such as FIP, toxoplasmosis or deep mycoses.

Symptoms of ocular FIP - Ocular disease occurs as the sole or primary symptom in about one-third of cats with dry FIP and in two-thirds of cases associated with extra ocular lesions (Table 2). Eye disease is an unusual manifestation in cats that initially had wet FIP (Table 1). The initial clinical manifestation is unilateral or bilateral anterior uveitis, manifested by a change in iris color, turbidity and remnants of flocculant in the anterior chamber, keratic clots on the back of the cornea, and anisocoria (unequal pupil size). In some cats, retinitis (inflammation of the retina) is an accompanying feature, and is manifested by focal wallpaper hyporeflectivity associated with local inflammation and microhemorrhage (minor bleeding) of the retinal vessels. Less than one-third of cats with ocular FIP also show indeterminate or overt neurological symptoms (Table 2). In some cases, glaucoma, usually unilateral, and panopthalmlitis (inflammation of all layers of the eye) occur, which can lead to enucleation (removal of the eye).

Symptoms of neurological FIP - the same prodromal signs have often been observed in cats with neurological signs, but include vague signs of dementia, aggressive behavior, compulsive licking of inanimate objects and other cats, reluctance to jump to high places, spontaneous muscle twitching, abnormal swallowing movements and occasional seizures. Later symptoms include posterior ataxia, inability to jump to high places, physical and auditory hyperesthesia, hyperreflexia, and cerebellar-vestibular signs (cruciate extensor reflex, loss of conscious proprioception), seizures, and increasing incoordination and dementia. Symptoms of spinal involvement often include fecal and/or urinary incontinence, paralysis of the tail and hind limbs, pain in the lower back. Catastrophic decerebral symptoms are also associated with sudden and severe herniation of the brain into the spinal cord.

Confirmatory tests of ocular and neurological FIP

Basic facts - The definitive diagnosis of FIP is based on the identification of the presence of viral antigen or RNA in macrophages in typical effusions or lesions by PCR or immunohistochemistry (IHC). Definitive diagnosis can be a difficult and expensive process in many cats, and PCR / IHC can be false negative in up to 30% samples. In most cases, however, it is not necessary to go that far because of the diagnosis. A comprehensive set of historical, physical, and less direct laboratory abnormalities may be sufficient to make a diagnosis.

Laboratory symptoms - The diagnosis of ocular and neurological FIP can usually be made by combining characteristic changes in cerebrospinal fluid (CSF) and aqueous humor (high protein, high cell counts, neutrophils, lymphocytes, macrophages) with significant abnormalities in history and history, physical examination, CBC ), biochemistry, or MRI. Total protein concentration is often increased (mean, 9.4 g / l; median 3.6 g / l; range 0.85-28.8 g / l) as well as increased erythroblast (NRBC) count (mean 196 / μL median 171 / μL; range 15–479 / μL). Neutrophils are the dominant inflammatory cell in most cats, while lymphocytes and a mixture of neutrophils and lymphocytes are observed in a smaller proportion.

MRI is a useful tool for diagnosing neurological FIP, especially in combination with routine signaling / history, typical clinical signs, and CSF analysis. MRI identified three different clinical syndromes in 24 cats with an autopsy confirmed by neurological FIP (Rissi DR, JVDI, 2018.30: 392–399): 1) T3-L3 myelopathy, 2) central vestibular syndrome, and 3) multifocal CNS disease. In all cases, MRI abnormalities were found, including increased meningeal contrast, increased ependymal contrast, ventriculomegaly, syringomyelia, and foramen magnum herniation. 15 cases showed hydrocephalus (10 cases), cerebellar herniation through the foramen magnum (6 cases), swelling of the brain with flattened gyri (2 cases) and fibrin accumulation in the ventricles (2 cases) or leptomening (1 case). Histologically, 3 main different distributions of neuropathological changes were observed, namely periventricular encephalitis (12 cases), rombencephalitis (8 cases) and diffuse leptomeningitis with superficial encephalitis (6 cases).

In one study, the most useful anti-mortem indicator of neurological FIP was the positive titer of IgG anti-coronavirus antibodies in the CSF. Antibody titers in CSF 1: 640 or higher were found only in cats with FIP and RT-PCR was always positive. Initial studies indicated that the antibody present in the CSF was produced, at least in part, in the CNS. However, in another study, the antibody was detected only in cats with serum titers of 1: 4096 to 1: 16384, and the researchers concluded that the antibodies in the CSF were obtained passively. In another attempt to measure local CNS antibody production in cats with FIP, the albumin quotient and IgG index were measured to determine if the proteins in the CSF were of blood origin or of local origin. Neither the albumin quotient nor the IgG index identified a pattern consistent with intrathecal IgG synthesis in cats with the CNS form of FIP. In conclusion, anti-coronavirus antibodies appear to enter the CSF at high levels, when they are also at high serum levels. Indeed, IFA serum coronavirus antibody titers in cats with ocular and neurological FIP are among the highest in any form of FIP.

PCR test performed from a sample of CSF and aqueous humor with a higher number of proteins and cells is highly sensitive and specific for ocular and neurological FIP. However, it is recommended that only a PCR test targeting the FCoV 7b gene be used, and no less sensitive PCR to FIPV specific mutations in the S gene. This FCoV gene is often used for PCR because it is the most abundant viral transcript and is therefore likely to that it will be detected. In some PCR assays, the FCoV M gene was targeted because it is highly conserved in all isolates, but transcripts are less numerous than in the 7b gene.

Immunohistochemistry on cells collected from spinal fluid is as sensitive and specific as PCR on samples with higher protein and cell counts. The antigen is localized specifically to macrophage-like cells.

The rapid FIP response to GS-441524 is being used as a confirmatory test increasingly. However, it should only be used in cases where there is other supporting evidence for a diagnosis of FIP. However, the truth is that there are probably no other simpler or cheaper means available at the moment to facilitate the diagnosis.

Treatment of neurological and ocular FIP

Difficulties in obtaining authorization for veterinary use of medicinal products for human use – Pharmaceutical companies such as Gilead Sciences and Merck have refused to compromise the development and approval processes of their promising anti-coronavirus drugs such as GS-5734 (Veklury®/Remdesivir) and EIDD-2801 (Molnupiravir®) or their respective biologically active forms GS-441524 and EIDD -1931. Out of desperation, cat owners around the world have turned to the Chinese black market for drugs like GS-441524. This black market was not entirely motivated by profit – China's FIP problem also grew at the same time as the domestic cat population. Moreover, even if Gilead Sciences had approved the use of GS-441524 in animals, the immediate need for an effective treatment for FIP has overtaken the official approval and commercialization process, which takes many years. Chemical companies and a dozen or more vendors of injectable and oral products have been able to satisfy the demand for GS from tens of thousands of desperate cat owners around the world. Veterinarians have been reluctant to pressure human pharmaceutical companies like Gilead to license their promising antiviral drugs for use in animals, but they are increasingly involved in helping owners with treatments. It therefore appears that the unapproved use of human drugs such as GS-441524, which are also desperately needed in veterinary species, will be the norm for many years to come.

(This paragraph comes from the original article from 1/4/2021.)

Virus-specific inhibitors – Inhibition of viral genes regulating specific stages of infection and replication has become the mainstay of treatment for chronic RNA virus infections in humans, such as HIV and hepatitis C virus. Currently, two classes of antiviral drugs have been shown to be effective against FIP. The first class consists of RNA synthesis inhibitors and includes the nucleoside analogs GS-441524 (the active ingredient in Remdesvir) and EIDD-2801 (molnupiravir). The second class of drugs consists of viral protease inhibitors, such as GC376 (prodrug of GC373) and Nirmatrelvir (prodrug of nitrile modification of GC373). Protease inhibitors are much less effective at crossing the blood-brain and blood-ocular barriers than nucleoside analogues and are not recommended for the treatment of neurological or ocular FIP.

Treatment with GS-441524 – GS-441524 has become the drug of first choice for the treatment of cats with all forms of FIP, and both injectable (SC) and oral forms are available in the off-label Chinese market. However, oral absorption is less than 50 % effective compared to injection, thus requiring twice the dosage of oral GS-441524. Suppliers of oral GS-441524 almost never disclose the actual concentration of GS-441524 in tablets or capsules, but rather label them as an equivalent injection dose. There is also an upper limit to the absorption efficiency of oral GS, making it difficult to achieve the higher blood levels needed to reach sufficient amounts of the drug in the brain and eyes. Therefore, if cats with ocular and neurological disease fail despite high equivalent doses of oral GS-441524, a switch to injectable GS-441524 should be considered before switching to a drug such as molnupiravir is considered.

The starting dose for cats with wet or dry FIP and no signs of ocular or neurological disease is 4-6 mg/kg daily for 12 weeks, with younger and wet cases tending towards the lower end and dry cases towards the higher end. Cats with eye lesions and no neurological signs are started at 8 mg/kg daily for 12 weeks. Cats with neurological signs are started at 10 mg/kg daily for 12 weeks. If cats with wet or dry FIP initially develop ocular or neurological signs, they are switched to the appropriate ocular or neurological doses. The dose of GS is adjusted weekly to account for weight gain. Weight gain can be huge in many of these cats, either because they are in poor condition to begin with or because their growth has been stunted. If the cat does not gain weight during treatment, this is considered a bad sign. The initial dosage is not changed unless there are serious reasons for this, such as ineffectiveness of treatment or improvement in blood test values, improvement is very slow, low activity level, initial clinical symptoms have not resolved, or the disease form has changed with the appearance of ocular or neurological symptoms. If there are good reasons to increase the dosage, it should always be from +2 to +5 mg/kg per day and for at least 4 weeks. If these 4 weeks exceed the original 12-week treatment time, the treatment time is extended. A positive response to any increase in dosage can be expected, and if you don't see an improvement, it means that the dosage is still not high enough, drug resistance is emerging, the GS mark is not what it should be, the cat does not have FIP, or there are other diseases that confuse the treatment.

One of the most difficult decisions is determining when to stop treatment. Although some cats, often younger with wet FIP, can be cured as early as 8 weeks and possibly earlier, the usual treatment period is 12 weeks. Some cats may even require dose adjustments and even longer treatment periods. Critical blood levels such as hematocrit, total protein, albumin and globulin levels and absolute lymphocyte counts usually return to normal in curing cats after 8 to 10 weeks, when there is often an unexpected increase in activity levels. We believe, but there is no evidence yet, that after 8-10 weeks, the cat will have its own immune response against the infection. This is a situation that occurs in the treatment of people with hepatitis C, which is also a chronic RNA virus infection that often requires antiviral treatment for up to 12 weeks or more.

Cats with ocular disease and no neurological impairment show a rapid response to GS, and complete recovery of vision with minimal or no residual damage is expected in as little as two weeks. Cats that develop neurological abnormalities, develop neurological disease during the treatment of other forms of FIP, or develop neurological symptoms during the 12-week post-treatment observation period also improve rapidly, but the dose is much higher, the duration of treatment often longer and the cure rate slightly lower. Treatment failures in cats with neurological FIP are due to either insufficient dose or the development of drug resistance.

Unfortunately, there is no simple blood test that can determine when a cat with neurological impairment has fully recovered. Many cats with neurologic FIP show minimal blood abnormalities, especially those with primary neurologic FIP, and the abnormalities often disappear by the end of treatment, even though residual sites of inflammation remain in the brain or spinal cord. In addition, some cats that recover from the infection will have mild to moderate neurological deficits that are residual effects of the previous illness. These facts make it difficult to use blood test results or residual neurological deficits as indicators of cure or undertreatment. Although a thorough eye examination can clearly rule out active signs of disease, the true state of the disease in the brain and spinal cord can only be determined by an MRI, ideally together with an analysis of the cerebrospinal fluid. These procedures are expensive, not available to everyone, and may not provide definitive proof that the infection in the CNS has been cleared.

Fear of relapses means that many people involved in GS treatment are too cautious about a single blood parameter that is slightly abnormal (eg, slightly high globulin or slightly low A: G ratio), or final ultrasound results suggesting suspiciously enlarged abdominals. lymph nodes, small amounts of abdominal fluid or blurred irregularities in organs such as the kidneys, spleen, pancreas or intestines. It should be borne in mind that the normal range of blood values applies to most animals, but it is a bell-shaped curve, and that there are a few non-standard patients who will have values at the edge of these curves. Ultrasonographers must consider the degree of pathology that can occur in the FIP of the affected abdomen and how scars and other permanent consequences can change the normal appearance of successfully treated cats. In situations where such questions arise, it is better to focus in more detail on the overall picture and not just on one small part. The most important outcome of treatment is a return to normal health, which has two components - external signs of health and internal signs of health. External signs of health include a return to normal activity levels, an appetite, adequate weight gain or growth, and coat quality. The latter is one of the best criteria for cat health. Internal health symptoms include the return of certain critical values to normal based on periodic complete blood count (CBC) monitoring and serum chemical profiles. The most important values in CBC are hematocrit and relative and absolute total white blood cell, neutrophil and lymphocyte counts. The most important serum values for chemical analysis (or serum electrophoresis) are total protein, globulin, albumin and A: G ratio levels. Bilirubin is often elevated in cats with effusive FIP and may be useful in monitoring the severity and duration of inflammation. There are many other values in the CBC panels and serum, and it is not uncommon for some of them to be slightly higher or lower than normal, and it is better to ignore these values unless they are significantly elevated and associated with clinical signs. For example, high BUN and creatinine, which is also associated with increased water consumption, excessive urination, and urinary abnormalities. The number of machine-counted platelets in cats is notoriously low due to the trauma of blood collection and platelet aggregation and should always be verified by manual examination of blood smears. The final decision to discontinue or extend treatment when faced with unclear doubts about different testing procedures should always be based on external manifestations of health than on any single test result.

(This paragraph comes from the original article from 1/4/2021.)

Relapses usually refer to infections that have escaped into the central nervous system (brain, spine, eyes) during treatment for wet or dry FIP that are not accompanied by neurological or ocular symptoms. Doses of GS-441524 used to treat these forms of FIP are often insufficient to effectively cross the blood-brain or blood-ocular barrier. The blood-brain barrier is even more efficient than the blood-ocular barrier, which explains why eye lesions are easier to heal than brain and/or spinal cord infections. Post-treatment relapses involving the eyes, brain, or spine are usually treated for at least 8 weeks at an initial daily dose at least 5 mg/kg higher than the dose used during primary treatment (eg, 10, 12, 15 mg/kg per day). Cats that fail to clear the infection at doses up to 15 mg/kg per day are likely to have developed varying degrees of resistance to GS-441524. Partial resistance may allow suppression of disease symptoms but not cure, while complete resistance is manifested by varying severity of clinical symptoms during treatment.

Different groups focused on the treatment of FIP have made various modifications in the treatment protocols. Some groups will treat with an extremely high dose of GS from the beginning and not increase the dose when indicated, or will recommend discontinuing or extending the high dose for the last two weeks in the hope that this will reduce the risk of relapse. In addition to GS, systemic prednisolone is often prescribed, but should only be used temporarily to stabilize serious illness. Systemic steroids reduce inflammation but tend to mask the beneficial effects of GS, and if used for an unreasonably long time and in high doses, can interfere with the development of immunity to FIP. Restoration of immunity to FIP is thought to be an important part of successful GS treatment. Therefore, some people advocate the use of interferon omega or non-specific immunostimulants to further stimulate the immune system, and some come up with other modifications. There is no evidence that using an extremely high dose will improve the cure rate. Also, interferon omega and non-specific immunostimulants have not been shown to have beneficial effects on FIP, whether given as a single treatment or as an adjunct to GS. The practice of adding another antiviral drug, the viral protease inhibitor GC376, to the treatment of GS in cats that develop resistance to GS is also emerging, but this still requires further research. Finally, it is common for owners, treatment groups and veterinarians to add many supplements, tonics or injections (eg B12) to increase hematopoiesis or to prevent liver or kidney disease. However, such supplements are rarely necessary in cats with pure FIP.

Molnupiravir (EIDD-2801) – Molnupiravir is very similar to GS-441524, but is a cytidine rather than an adenine nucleoside analog. It is widely used as an oral treatment for early cases of COVID-19 in humans, but in the last 1-2 years it has been increasingly used to treat cats with FIP. Due to the toxicity observed in cats at higher doses and as yet unknown chronic side effects, it is most often recommended for cats that developed resistance to GS-441524 during primary treatment or relapsed with neurological/ocular signs after treatment with high doses of GS- 441524. Fortunately, molnupiravir has a different resistance profile than GS-441524.

The safe and effective dosing of molnupiravir in cats with FIP has not been established in properly controlled and monitored field studies such as those performed for GC376 and GS-441524. However, the estimated starting dose of molnupiravir in cats with FIP was derived from published EIDD-1931 and EIDD-2801 in vitro cell culture studies and other laboratory and experimental animal studies. Molnupiravir (EIDD-2801) has an EC50 of 0.4 µM/µL against FIPV in cell culture, while the EC50 of GS-441524 is approximately 1.0 µM/µL. Molnupiravir begins to show cellular cytotoxicity at concentrations of 400 µM or higher, while GS-441524 is non-toxic at 400 µM. Both have similar oral absorption of around 40-50 %. The current recommended starting dose of molnupiravir for neurologic and ocular FIP is 8–10 mg/kg orally every 12 hours for 84 days. Depending on the response to treatment, it may be necessary to increase it to a maximum of 15 mg/kg orally every 12 hours. At higher doses, molnupiravir toxicity is likely to occur as indicated by changes in the complete blood count.

Causes of treatment failure

Incorrect dosage adjustments - It is important to start treatment with the appropriate dosage and to monitor it closely with regular checks on temperature, weight and external signs of improvement. The CBC and serum chemical analysis panel, which contains baseline protein values (total protein, albumin, globulin (TP - albumin = globulin) and A: G), should be performed at least once a month. with GS-441524 Expensive serum protein electrophoresis does not provide much more valuable information.

Low quality GS-441524 - GS-441524 is not approved for marketing in any country and is sourced from a small number of Chinese chemical companies which sell it to distributors as pure powder. Vendors dilute it into injectable solutions or prepare oral forms for sale under their trade names. There is no independent mechanism to ensure the quality of the final product sold to cat owners. Nevertheless, the main providers of dilute forms for injectable solutions and / or oral preparations are surprisingly honest, and some even offer limited guarantees if treatment with some of their products does not cure the disease. However, the batches sold by some providers appear to be counterfeit and some are not in the specified concentration. There may also be differences between batches, probably due to occasional problems with the supply of raw GS by retailers or problems with meeting the needs and expectations of the cat owner. Various groups of FIP Warriors have good information about the most reliable brands.

Drug resistance - resistance to GS-441524 may already exist at the time of diagnosis, but this is unusual. It occurs more frequently during treatment and is initially only partial and requires only higher doses. In some cats, it may become complete. Resistance is the biggest problem in cats with neurological disease, or they develop brain infections during treatment or within a few days or weeks after stopping treatment. Many cats with partial drug resistance may be "treated" for their symptoms, but they relapse as soon as treatment is stopped, as is the case with HIV treatment, for example. There are cats that have been able to partially or completely treat the symptoms of FIP for more than a year, but without a cure. Resistance eventually worsens and the symptoms of the disease worsen, treatment difficulties become unbearable for the owner or the owner's financial resources run out.

GS side effects

GS-441524 treatment is incredibly free of systemic side effects. It can cause mild kidney damage in cats without significant kidney damage, but does not lead to latent disease or kidney failure. Systemic drug reactions such as vasculitis have been observed in several cats and can be confused with injection site reactions. However, these drug reactions are in places where injections are not given, and often stop on their own or respond well to short-term low-dose steroids. The main side effect of GS treatment is pain at the injection sites, which varies from cat to cat and according to the abilities of the person giving the injections (usually the owner). Swelling or ulcers at the injection site sometimes occur in owners who do not change the application site often enough (do not stay between the shoulder blades) and do not inject into the muscular and nervous layers under the skin. I recommend choosing places starting one inch behind the shoulder blades, down from the back to 1 to 2 inches in front of the root of the tail and one third to half way down to the chest and abdomen. Many people use gabapentin to relieve pain before injections. Swollen spots and ulcers at the injection site should be stripped of surrounding hair and gently cleaned 4 or more times a day with sterile cotton swabs soaked in homemade hydrogen peroxide diluted 1: 5. They usually do not require any more complicated treatment and will heal in about 2 weeks.

Prognosis of treatment with GS441524

Exact cure rate data with GS-441524 are not yet available, but it seems possible to cure more than 80% cats with confirmed FIP. Treatment failure is due to misdiagnosis of FIP, inadequate treatment monitoring and dose adjustment, complicating diseases, poor GS, resistance to GS, or economic difficulties. The cure rate is slightly lower in cats with neurological forms of FIP and in older cats. Older cats are more susceptible to other chronic diseases, which either predispose cats to FIP or complicate overall health.

Cats with neurological FIP may suffer permanent residual symptoms of the disease. This is especially true for cats with spinal involvement and urinary and/or fecal incontinence or hind paralysis. Hydrocephalus and syringomyelia are common complications of neurological FIP and often persist to some extent after the infection has cleared. Fortunately, most cats with neurologic FIP recover normal or near-normal function despite persistent traces of hydrocephalus and syringomyelia.

Legal treatment for FIP?

We hope that the legal form GS-441524 will be available soon. The drug, called Remdesivir, is the greatest hope of the present because Remdsivir breaks down into GS when given intravenously to humans, mice, primates and cats. Remdesivir (Veklury®) has been fully approved by the US FDA and similar approval is likely to follow in other countries. If so, it can be prescribed by any licensed human physician as well as veterinarians. However, the use of Remdesivir in the United States was initially limited to a specific subset of patients with Covid-19 and only under controlled conditions and with ongoing data collection. Until all restrictions are lifted, it will not be easily accessible for human use. We have no experience in treating cats with Remdesivir instead of GS-441524. The molar basis of Remdesivir is theoretically the same as GS-441524. GS-441524 has a molecular weight of 291.3 g / M, while Remdesivir is 442.3 g / M. Therefore, 442.3 / 291.3 = 1.5 mg of Remdesivir would be required to obtain 1 mg of GS-441524. The diluent for Remdesivir is significantly different from the diluent used for GS-441524 and intended for intravenous use in humans. How diluted Remdesivir will behave when given by subcutaneous injection over 12 weeks or more is not known. Mild signs of hepatic and renal toxicity were observed in humans. GS-441524 causes mild and progressive renal toxicity in cats, but without apparent hepatic toxicity. It is uncertain whether renal toxicity observed in humans receiving Remdesivir is due to its active substance (ie GS-441524) or to chemical agents designed to increase antiviral activity.

The GC376 approval process for cats (and humans) is ongoing at Anivive, but will take two or more years. GC376 is a viral protease inhibitor and, unlike GS-441524, which inhibits the initial stage of viral RNA replication, GC376 prevents viral replication in the final stage of its replication process. Therefore, it is unlikely to have a significant synergistic viral inhibitory effect and its use in combination will be much more important in inhibiting drug resistance (e.g. in combination antiviral therapy for HIV / AIDS).

Improper use of GS-441524

Some veterinarians, in collaboration with major Chinese supplier GS-441524, have advocated its use to eliminate feline enteric coronavirus (FECV) infection. The reason is to prevent the occurrence of a mutant virus causing FIP (FIPV) and thus prevent FIP. This approach was supported by limited and highly controversial studies with shelter cats, which were naturally exposed to the FECV. Although this approach is attractive at first glance, it is a very incorrect use of GS-441524 in cats. FECV infection originally occurs in kittens and is not associated with any significant symptoms of the disease. Elimination lasts for weeks, months, and in some cases indefinitely, but in most cats, it eventually stops when immunity develops. Most cats over the age of three will no longer shed the virus. GS-441524 treatment is highly unlikely to result in more permanent immunity than is observed in nature and to eliminate cycles of infection and reinfection in younger cats.

Although our current knowledge of FECV infection seriously challenges this approach, there are even more compelling reasons why we will not treat healthy cats GS-441525 or other antiviral agents in the future. We already know from published studies that some primary strains of FIPV are resistant to GS-441524 (and GC376). We also know that drug resistance has become a long-term problem in cats with long-term treatment for GS-441524, especially in neurological forms of FIP. Therefore, the use of drugs such as GS-441524 in a large population of healthy cats will undoubtedly lead to widespread resistance to enzootic FECV. This resistance will also manifest itself in FIP-causing FECV (FIPV) mutations from these populations, making it impossible to use GS-441524 in more and more FIP cats. Unfortunately, veterinary medicine does not have the means of human medicine, it is not stimulated by potential benefits, which would lead to the discovery, testing and approval of more and more antiviral drugs to circumvent either natural or acquired drug resistance, which is already the case in HIV / AIDS treatment. achieved (at least on time).

(This part comes from the original article from 1/4/2021.)

Niels C. Pedersen, DVM PhD
Distinguished Professor Emeritus
UC Davis, Center for Animal Health Companion January 4, 2021
, updated February 10, 2023

Current information about the treatment of FIP in the UK

Original article: An update on treatment of FIP in the UK (1.2.2022)

dr. Sam Taylor BVetMed(Hons) CertSAM DipECVIM-CA MANZCVS FRCVS Prof. Séverine Tasker BVSc BSc DSAM PhD DipECVIM-CA FHEA FRCVS, Prof. Danielle Gunn-Moore BSc(Hon), BVM&S, PhD, MANZCVS, FHEA, FRSB, FRCVS Dr. Emi Barker BSc BVSc PhD PGCertTLHE DipECVIM-CA MRCVS, Dr. Stephanie Sorrell BVetMed(Hons) MANZCVS DipECVIM-CA MRCVS

Given the current situation, Sam Taylor, Séverine Tasker, Danièlle Gunn-Moore, Emi Barker and Stephanie Sorrell discuss treatment protocols to help doctors manage this viral disease.


Figure 1: Remdesivir intended for intravenous or subcutaneous administration

In August 2021, remdesivir (Figure 1) became legally available to UK vets to treat FIP in cats. Since then, many cats and kittens have been and are still being successfully treated. As with any new product, protocol modifications are adopted with experience, and in light of the recent release (November 2021) of oral GS-441524 (50 mg tablets) from a specialist UK manufacturer (Figure 2), this article has been drafted to support general practitioners in the use of remdesivir and GS-441524 in the treatment of FIP. It should be kept in mind that treatment may need to be tailored to the individual cat based on the client's response, compatibility and financial capabilities. The specific protocols below may help veterinarians and their clients, but will not be appropriate for all cases.

Treatment protocols (updated November 2021)

The dosage of the drugs has been increased compared to previous recommendations based on the experience of our Australian colleagues, who have treated more than 600 cats so far. Although some cats responded to previously recommended lower doses, they found that relapse was possible at or near the end of the 84-day (12-week) treatment period, leading to the need to extend treatment with a higher daily dosage. This was ultimately more expensive than starting treatment at a higher dosage.

Figure 2: GS-441524 oral tablets

With the use of remdesivir and/or GS-441524, treatment options are now available including a 12-week course of injectable remdesivir, switching from injectable remdesivir to oral GS-441524, or an exclusively oral GS-441524 protocol.

Suggested dosing, benefits, and limitations of each protocol are listed below. Remdesivir cannot be taken orally. The recommended dosage of drugs (Table 1) depends on the clinical picture - ie whether there is an effusion or not and whether there is eye and/or neurological involvement - this is due to differences in drug penetration into tissues. In case of doubt, it is more appropriate to use a higher dosage.

Please note that these dosages of oral GS-441524 are higher than reported in some publications - this is because these publications used so-called black market preparations of GS-441524 in which the amount of active ingredient administered to cats was not confirmed. The dosages given in this article are based on experience using an oral formulation of known GS-441524 that is legally available in the UK and Australia. Therefore, extrapolation cannot be used for other oral preparations for which the active ingredient and/or its concentration is not known or is not indicated by the manufacturer.

Combined injection and oral treatment protocols

The decision when to switch from injectable remdesivir to oral GS-441524 may depend on tolerability of injections (or oral tablets), differences in product cost (including cost of needles, syringes, sharps disposal, losses), owner preference, and finances.

Experience suggests that this transition may occur between days 7 and 14 after initiation of intravenous or subcutaneous remdesivir therapy. The change can be made directly; remdesivir is given for one day and GS tablets are started the next day.

The protocol chosen depends on the severity of the FIP disease in the cat. Dosage is shown in Table 1.

Serious condition

If the condition is severe (anorexia, dehydration, the cat is usually hospitalized):

  • Initial treatment with remdesivir given once daily intravenously (Table 1) for three to four days – ie days 1, 2, 3 and/or 4. This will achieve the loading dose of the drug. Each day, dilute the required dose of remdesivir to a total volume of 10 mL with saline and administer slowly over 20 to 30 minutes by hand or pump.
  • Subsequently, administer SC remdesivir once daily at the same dose (Table 1) until days 7 to 14.
  • On days 8-15, switch to oral GS-441524 once (or twice) daily (Table 1) and continue until at least day 84.

Table 1: Overview of dosing recommendations for remdesivir and GS-441524

Clinical presentationRemdesivir - by injectionGS-441524 – oral
Cats with effusion and no ocular or neurological signs10 mg/kg once a day10-12 mg/kg once a day
No effusion and no ocular or neurological symptoms12 mg/kg once a day10-12 mg/kg once a day
Ocular symptoms present (effusive and non-effusive FIP)15 mg/kg once a day15 mg/kg once a day
Neurological symptoms (effusive and non-effusive FIP)20 mg/kg once a day10 mg / kg twice daily (ie 20 mg/kg in divided doses)
Translator's Note: the injectable form of GS-441524 is not used for legal treatment in the UK. Given that the molecular weight of remdesivir is approximately 2x higher than the molecular weight of GS-441524, the recommended dosage of remdesivir is approximately 2x higher than that of GS-441524. Coincidentally, the bioavailability of GS-441524 when administered orally is about 50%, so the dosage for tablets with the stated real GS content from the manufacturer BOVA is practically identical to the dosage for remdesivir injection.

Less serious condition

Regarding a less severe condition (normal hydration, food intake):

  • Initial treatment with remdesivir SC 1x a day (Table 1) until the 7th to the 14th day.
  • Change to 1x (or 2x if a very high neurological dose is required) daily oral administration of GS-441524 (Table 1) on days 8-15 and continue until at least day 84.

An exclusively oral protocol

In the event that injectable treatment is not tolerated/financially feasible, only the oral GS-441524 treatment protocol is recommended:

  • 1x (or 2x if a very high neurological dose is required) daily oral GS-441524 (Table 1) for at least 84 days.

Possible side effects of remdesivir:

Remdesivir appears to be well tolerated. However, the following side effects have been reported:

  • Transient local discomfort/stinging on injection (see prevention later).
  • Development/worsening of a pleural effusion (not always proteinaceous) during the first 48 hours of treatment, sometimes requiring drainage.
  • Cats may be depressed or nauseous for several hours after intravenous administration.
  • An increase in the activity of the enzyme alanine aminotransferase has been reported (whether due to the underlying disease of FIP or an adverse effect of the drug is unclear).
  • Mild peripheral eosinophilia has been reported.

A note on weighing cats

During treatment, it is very important to weigh cats weekly using an accurate scale - with successful treatment, kittens will gain weight and/or grow, which will require a dose increase to ensure that the dose of antiviral given is still appropriate for the type of FIP being treated.

Options for clients with a limited budget

Please note that ideally, treatment should be administered using the recommended preparations and dosage for as long as possible (up to 84 days) to increase the likelihood of a cure.

Use the options below only when absolutely necessary, as a relapse may occur, which then requires longer treatment, leading to increased costs:

  • Administer oral treatment with GS-441524 only for 84 days as above.
  • Administer injectable remdesivir or oral GS-441524 for as many days as the owner can tolerate, then switch to oral mefloquine 62.5 mg two to three times weekly (in large cats three times weekly) or 20 mg to 25 mg orally once daily (if possible to change the composition of the tablets - for example, Novalabs) to complete the 84-day treatment protocol; mefloquine is less expensive than remdesivir and GS-441524, but further research is needed to assess its effectiveness in this setting.
  • If it is necessary to increase the dose of remdesivir (for example, due to a neurological disease that appears during treatment), but it is not possible to afford it, mefloquine treatment can be added as an adjunctive treatment, because it is cheaper than remdesivir, although it is necessary to assess the effect of this combination further research.
  • Feline interferon omega has also been used in the post-remdesivir/GS-441524 treatment period, but further research is needed to assess whether this combination is necessary.

Is the oral treatment given with or without food?

  • GS-441524 is administered on an empty stomach (with some water) - food may be administered 30 minutes after administration of the drug.
  • Mefloquine is given with food, otherwise vomiting often occurs.

Remember to support clients when giving oral medications, as this can also be challenging for them. Direct clients to the website iCatCare, where you can find information and videos.

How can I help owners with remdesivir SC application?

Remdesivir injection may cause temporary local discomfort. The following measures can help reduce discomfort and improve cooperation:

  • Make sure owners use a new needle each time to withdraw medication from the vial (this will reduce the risk of bacterial contamination of the vial, as well as rubbing the top of the reusable seal vial with alcohol before inserting the needle).
  • Make sure owners change the needle after removing the medicine from the bottle and before giving the injection (puncturing the reusable seal will blunt the needle).
  • Needle size preferences vary; some prefer a 21G needle to make the injection faster; others find the finer 23G needle better tolerated, so it may be worth trying both if you have problems.
  • Alternate injection sites.
  • Allow remdesivir to warm to room temperature before administration.
  • Oral gabapentin (50 mg to 100 mg per cat) and/or intramuscular or SC buprenorphine given at least 30 to 60 minutes before remdesivir injection may be useful to induce mild sedation/analgesia.
  • The area to be injected may also be shaved to help owners locate a suitable injection site and to allow topical EMLA cream to be applied 40 minutes prior to injection, although superficial desensitization may not help as discomfort is usually caused by remdesivir under the skin.
  • Ensure that the full injection dose is always administered and encourage owners to report any failures as this may influence decisions in case of relapse.
  • Cats will need several weeks of treatment. Encourage owners to make the injection more enjoyable by using treats at the time of the injection or by petting, combing or playing with the cat if it is less motivated to eat. Suggest that owners spend time with their cat in a positive way each day to avoid any damage to the cat-owner relationship that may reduce cooperation.

What can I expect during treatment?

  • During the first two to five days, you should see an improvement in behavior, appetite, resolution of pyrexia, and a decrease in abdominal (Figure 3) or pleural fluid if an effusion is present (please note that in some cases pleural fluid may be transient in the first few days worsen - if the cat is at home, advise the owner to measure the resting respiratory rate and respiratory effort) - the effusion usually subsides within two weeks.
  • If discharge is still present after two weeks, consider increasing the dosage.
  • Serum albumin increases and globulin decreases (that is, normalizes) within one to three weeks, but note that globulins may initially increase when a large volume of effusion is absorbed.
  • The resolution of lymphopenia and anemia may take longer, up to 10 weeks.
  • Mild peripheral eosinophilia is a common finding and may be a favorable marker for disease resolution, similar to that seen in patients with COVID-19.
  • The size of the lymph nodes will decrease within a few weeks.
  • If progress is not as expected, consider reassessing the diagnosis (see below) and/or increasing the dosage.
Figure 2. Cat with FIP and ascites. Effusions should begin to subside within three to five days of starting treatment.

What should be observed during treatment?

  • Ideally, serum biochemistry and hematology after two weeks and monthly thereafter.
  • For clients on a limited budget, monitor only weight/behavior/effusion/neurological signs/key biochemical abnormalities (for example, measuring only globulin and bilirubin).
  • Note that the activity of the enzyme alanine transaminase (ALT) may increase - it is not clear whether this is due to the pathology of FIP or a reaction to the drug, and it is not usually a reason to stop treatment. It is not known whether the addition of hepatoprotective treatment (eg S-adenosyl-L-methionine) helps in these cases.
  • Ultrasonography in the outpatient clinic to monitor the resolution of the effusion and/or the size of the lymph nodes.

If I observe a positive response to the treatment, when should I stop the treatment?

  • Not earlier than after 84 days (12 weeks).
  • Verify the disappearance of previous abnormalities (clinical, sono, biochemical and hematological examination).
  • Discontinue treatment only after the cat has been normal (clinically, biochemically and hematologically) for at least two weeks (ideally four weeks).

What should I do if I have no or only a partial response to treatment?

  • Make sure the cat actually has FIP - reevaluate the diagnosis, look for other pathologies, consider repeated sampling (eg, external laboratory analysis of any fluid; cytology or lymph node biopsy).
  • If biochemical abnormalities (especially hyperglobulinemia and albumin-to-globulin ratio) remain after 6 to 8 weeks, increase the dosage as for relapse (see below) by 3 mg/kg to 5 mg/kg daily and continue treatment without stopping until parameters do not normalize for at least 2 weeks, as mentioned above in the section "when to stop treatment?". – This may also mean extending the treatment for more than 12 weeks.

What should I monitor after treatment?

  • Advise the owner to monitor the cat closely for recurrence of the clinical condition - this monitoring should continue for 12 weeks after the end of treatment.
  • Ideally, repeat serum biochemistry and hematology two weeks and one month after stopping treatment (to detect any changes that might indicate an early relapse).
  • Note that relapse may occur with clinical symptoms but without any significant biochemical/hematological abnormalities.


In case of relapse – e.g. recurrence of effusion, pyrexia, development of ocular or neurological symptoms, or return of hyperglobulinemia:

  • Make sure the cat has FIP - reassess the diagnosis, consider other pathologies, consider repeat sampling (for example, external laboratory analysis of any fluid, cytology or lymph node biopsy).
  • If relapse occurs during treatment, increase the dose of remdesivir or GS-441524 and monitor treatment as before, making sure that treatment is not stopped before the cat has been normal for at least two weeks. The increased dosage depends on the dosage the cat is receiving at the time of the relapse, the nature of the relapse and the financial possibilities, but can be up to the recommended dosage for neurological FIP (see above).
  • If relapse occurs after stopping treatment, restart remdesivir or GS-441524 at a higher dose (usually 3 mg/kg to 5 mg/kg daily higher than previously used doses) and continue treatment for an additional 12 weeks. The increased dosage used depends on the dosage the cat was receiving at the time of the relapse and the nature (eg severity and/or development of neurological signs) of the relapse, but may be up to the dosage recommended for neurological FIP (20 mg/kg - see Table 1). It is possible that some cats will respond to a shorter treatment, but ideally relapse treatment is continued for the full 12 weeks after treatment has been completed to prevent relapse.
  • If the dose of remdesivir or GS-441524 cannot be increased (for example, the highest neurologic dose of 20 mg/kg is already being used), consider mefloquine as adjunctive therapy (see above) while continuing remdesivir or GS-441524 at the same dose.

Castration and routine measures during the treatment of FIP

  • If the cat responds to treatment, castration is ideally carried out a month after its completion. However, if leaving an unneutered cat causes a lot of stress - for example, attempts to escape or stress when mothers are in heat, it is advisable to prioritize neutering during treatment. If the second option is necessary, neutering should ideally be done at a time when the cat is coping well with the treatment and has at least two weeks of treatment left after neutering (so antiviral treatment takes place at a time of potential "stress" after neutering).
  • There is no contraindication for routine deworming and flea treatment in cats treated with remdesivir or GS-441524.
  • No information is available on vaccination of cats treated for FIP. If the cat is well during treatment, it should be vaccinated as usual, as it is still likely that the vaccination will have a protective effect. For cats that have completed the initial round, consider giving a third dose of vaccine after completing FIP treatment (see WSAVA Vaccination Guidelines).
  • If veterinary procedures are required, the clinic stay should be minimized and protocols and handling should be implemented according to Cat Friendly Clinicto avoid stressing the cat.

Complementary treatment

  • If a cat is receiving prednisolone, it should be discontinued during administration of remdesivir or GS-441524, and then discontinued completely, unless needed for short-term treatment of a specific immune-mediated disease resulting from FIP—for example, hemolytic anemia.
  • Supportive therapy such as antiemetics, appetite stimulants, fluid therapy, and analgesics may be given along with remdesivir or GS-442415 as needed.

Possible future updates

We are constantly learning during treatment with these drugs, and recommendations may change over time. Other substances have been tested in cats, such as protease inhibitors (such as GC376) and other nucleoside analogues (such as molpurinavir), but these are not currently commercially available. How these agents and other immunomodulatory agents (such as polyprenyl immunostimulant) will fit into future protocols is currently unknown.

Translator's Note: The original article was published and updated in February 2022, since molnupiravir officially became available for the treatment of COVID-19 in humans, and there is also the possibility of its use in the treatment of FIP.


We thank Richard Malik and Sally Coggins for their advice in the preparation of this article.

dr. Richard Malik DVSc MVetClinStud PhD FASM graduated from the University of Sydney in 1981. He is a specialist in small animal internal medicine with a special interest in infectious diseases of dogs and cats. She works at the Center for Veterinary Education and helps organize CPD.

dr. Sally Coggins BVSc (hons I) MANZCVS (Feline Medicine) she graduated from the University of Sydney in 2007 with first class honours. Sally is currently investigating novel antiviral therapeutics for the treatment of feline infectious peritonitis and is conducting clinical trials open to national recruitment.

FIP advisory line

The above experts have teamed up to launch an email address “FIP advice” ( where they volunteer to answer questions about the new treatment and spread the word among vets and vet nurses in the UK. So far they have answered more than 150 emails on the advice line.

FIV – Testing and treatment?

As the name suggests, this is not about FIP, but it's important to know. I registered the strange information that when treating FIP with molnupiravir, it was also possible to cure FIV. And that cats that were positive before treatment were negative for FIV after treatment. And that the tests were not quick tests, but tests from Laboklin...

Apparently, many of you have the misconception that if something comes from a lab, that automatically means there is a clear answer. But it is a huge mistake. Let's talk about the principle of FIV and FeLV testing. Rapid tests (snaptests) are antibody-based for FIV and antigenic for FeLV. And here is the basic stumbling block. Antibodies, even if there is some miraculous cure, do not disappear after treatment. Antibodies are proteins produced by the immune system and their purpose is to identify and neutralize foreign objects in the body. So the very negative result of the proilase test after the treatment, before which the test was positive, means only one thing - One of the two tests was false positive (or false negative) and therefore defective.

It is for this reason that it is strongly recommended, especially in the case of a positive FIV or FeLV test, to perform a confirmation test using another laboratory method.

And why did I say that the fact that something is done in the laboratory does not necessarily mean anything? Simply. If you have the FIV and FeLV test done by a laboratory and do not specify the method, it is very likely that the laboratory will do a SNAP test or an ELISA (EIA) test. You can tell by the price of the test, but also by what is on the report. The image below shows that this was a test FIV AK, but what does it mean? antibody test (AK=Antikörper). In parentheses is EIA, which stands for “enzyme-linked immunosorbent assay”. The abbreviation ELISA is also used. You should know that the gold standard of the confirmation test for FIV is the method test Western Blot... In that case, it would be mentioned in the report like this. Although the WB test is also an antibody test, it works on a completely different principle. For FeLV, the standard PCR is used as a confirmatory test. And something else. Why do you think you will test positive for FCoV antibody after treatment for FIP? Exactly for the reason I wrote about above. Antibodies remain in the body after treatment for FIP and this is completely natural. Even after you are cured of the much-maligned Covid, you will still have antibodies. Otherwise, it would be very bad for you. And ask yourself why FCOV antibodies would remain after treatment and FIV antibodies would disappear? Antibodies remain in the body for several months after treatment, and in some cases or for incurable diseases such as FIV, even for years.

In the picture you can see an FIV antibody test with a negative result, which led the cat's owner to the fantastic but unfortunately premature conclusion that the FIV was cured by treating the FIP.

For the sake of completeness, I am also attaching an FIV test using the Western Blot method for my cat, which unfortunately confirmed that it is FIV positive. And we even had a few snap tests done before (one even in the laboratory), some of which were negative and some were positive.

Please stop jumping to conclusions and tame the euphoria about the FIV cure. The result of two antibody tests with a conflicting result does not mean that a cure has occurred, but that one of the tests showed a faulty result.

In addition to the above information, you should also be aware that after vaccination based on the principle of an inactivated virus, it is no longer possible to use antibody tests for the diagnosis of the given disease, because the vaccination serves precisely to make the body create antibodies.

Antibody tests can come out positive even in the case of young kittens (under 20 weeks), when they can have maternal antibodies from breast milk and subsequently the tests can be negative.

Regarding PCR testing for FIV, I would add that you should read the article, where the basic principles of FIV tests and their reliability are presented. You will learn, for example, that the error rate of negative PCR tests is really very high.

FIV treatment ???

The FIV virus is a retrovirus related to the virus that causes HIV (AIDS). The main problem is that the virus is "built-in" into the host's genome, and that is why such a disease is not curable. Of course, this does not mean that the life expectancy of an affected individual cannot be extended with the use of symptomatic therapy. If an FIV cat is affected by an infection, antivirals can help, if a bacterial infection appears, ATB is used... Thus, accompanying diseases and infections are dealt with, and with this treatment, the FIV disease itself is kept under control, but it is not cured. To be sure, I also asked those actually called about the possibility of FIV treatment with molnupiravir. Answers by Danielle-Gunn Moore - professor of feline medicine from the University of Edinburgh and Yunjeong Kim - professor at Kansas State University, who together with Dr. Pedersen is behind the discovery of the treatment of FIP using GS-441524, hopefully they will convince those who got "drunk" on the croissant and succumbed to the vision of treating FIV with molnupiravir.

Translation: “Retroviruses such as FIV or HIV (AIDS) are not treatable with antivirals because the viruses are embedded in the host's genome. If there is a good antiviral for FIV, it can help the cat stay symptom-free for a longer period of time (similar to HIV drugs in humans), but I don't think there is any evidence that molnupiravir is effective against FIV.”
Translation: "It was pointed out to me that molnupiravir, because it works on the replicating virus, will never cure FIV, just like the effectiveness of these HIV drugs, because the proviral non-replicating virus can never be their target. Even HIV always requires 3-4 drugs, occasionally 2 – just to keep it under control, so really no chance of a cure.”

Christmas greetings from SOCK FIP

with current information from Dr. Brian Murphy and Patty Pesavento

Original article: Season's Greetings from SOCK FIP

Funding from the SOCK FIP program has made great progress this year in our clinical trials at the UC Davis School of Veterinary Medicine. Drs Brian Murphy, Amir Kol and Krystle Reagan are leading three new clinical trials. Their goal was to identify FIP treatment strategies that are highly effective, legally available, and affordable. GS-441524, a drug closely related to remdesivir, has been found to be safe and effective in treating cats with naturally occurring FIP, but is not licensed in the United States. However, the FDA may soon grant full approval to remdesivir, allowing it to be legally sold for veterinary use. The study will include two treatment groups – one receiving oral GS-441524 and the other receiving oral remdesivir for comparison. Cats must be diagnosed with the wet form of FIP to be included in the study. A third clinical trial is trying to see if antiviral drugs combined with mesenchymal stem cells can improve the response to FIP treatment. The goal of the study is to determine whether cell therapy can enhance the natural antiviral immune response and promote the regeneration of lymphatic tissue after infection. The two treatment groups in this study receive either oral GS-441524 along with stem cell infusion or oral GS-441524 with placebo. Cats must be between 3 and 12 months of age and suffer from the wet form of FIP. Owners interested in enrolling their cats in these clinical trials must reside near Davis, California due to the need for repeat follow-up visits. For more information about these tests, call the Clinical Trials Office at 855-823-1390. Although we do not currently have the final results of these treatment studies, field cases that have been assigned to different treatment groups are generally doing very well.

The Center for Companion Animal Health (CCAH) at the University of California, Davis, also used donations from the SOCK FIP to support Drs. Patty Pesavento and Terza Brostof's fourth project. They lead a team of biophysicists, immunologists and vaccinologists in the development of an mRNA vaccine against feline enteric coronavirus (FECV) and its mutant biotype FIP virus (FIPV). This vaccine, based on the technology used to produce the modern mRNA vaccine for COVID-19, will hopefully lead to the generation of sufficient immunity to reduce the incidence and negative consequences of diseases such as FIP and FECV infections. Work on this vaccine has progressed rapidly and significantly over the past year. They developed the first mRNAs to be tested and the nanoparticle molecules that coat the mRNAs so that they can be safely administered. They are currently working on methods to scale up production to begin laboratory and field testing. Since the actual virus is not present in the cat's body, there is no risk that the cat will develop the disease. This makes mRNA vaccines a potential breakthrough in the fight against feline diseases that have so far been very difficult to prevent, including FIP.

Status of FIP treatment and prevention in 2022

Niels C. Pedersen, DVM PhD, November 28, 2022

Original article: Status of FIP treatment in 2022

I am pleased to announce that I have ended my advisory role at SOCKFIP and have officially become a member of the SOCKFIP Board of Directors. It reflects my transition from university to private life, but will not affect my commitment to FIP research. I hope that this more direct involvement will help SOCKFIP transition to a broader role in cat health issues beyond FIP. FIP research continues at the University of California, Davis, as well as at other institutions around the world. Research projects related to FIP at UC Davis are summarized in ” Best regards SOCK FIP” of 2022. SOCKFIP continues to provide financial assistance for such studies through public donations, and I will provide scientific knowledge whenever needed.      

I wish there was a licensed antiviral treatment for FIP in cats, but even the efforts of many individuals and groups have not been able to change the current reality. Therefore, it is questionable whether legal antivirals for FIP will reach the market in the next 2 to 5 years, even if the obstacles are removed immediately. Fortunately, restrictions on the general use of closely related human medicines for COVID-19 are being eased worldwide, allowing them to be prescribed by all doctors and used more widely in the field. Full human approval allows their use in animals, provided the drug needed is derived directly from the actual human product. This would allow drugs made for humans, such as remdesivir and molnupiravir (EIDD-2801), to be used legally in animals, albeit at the cost of human drugs. The goal should still be for drugs specifically licensed for animals and available at a veterinary rather than a medical price. 

More and more veterinarians are already helping owners with treatment. However, it still saddens me that some vets have not heard of effective treatments for FIP, believe that published treatment reports are bogus, or that obtaining drugs from unapproved markets is so scary that they can't even help with treatment once an owner buys it. I commend those veterinarians who accept the reality of treatment and work with owners and their cats with FIP.  

The most significant discovery after GS-441524 is the use of molnupiravir (EIDD-2801) (Merck) as a second effective treatment for FIP. Molnupiravir is also extremely effective in treating cats that have developed resistance to GS-441524, which are the most common cats that develop neurologic FIP during or after treatment with GS-441524. Reports of its use in cats are just beginning to emerge and are being posted on the SOCK FIP website. 

I believe that there are several areas of research that veterinary researchers should consider. One area concerns the safety and efficacy of EIDD-1931 (beta-d-N4-hydroxycytidine), which is the biologically active component of molnupiravir, just as GS-441524 is the active component of remdesivir. This orally administered drug has been the subject of research for almost half a century and should no longer be patent protected. Preliminary research at the University of California, Davis suggests that it may be even more effective and safer than molnupiravir. I also believe that the oral protease inhibitor (nirmatrelvir) component of Paxlovid (Pfizer) should be tested for non-ocular/non-neurological cases of FIP. Nirmatrelvir is broken down into a simple chemical modification of GC373, the active form of GC376. Paxlovid is widely available and can be easily prescribed by both pharmacists and doctors for general treatment of COVID-19. This should make it widely available for use by veterinarians. I also believe that further research should be pursued to find ways to limit FECV infection and to understand the factors that suppress the natural normal protective immunity against FECV mutants. At this point, it is clear that most healthy cats have strong natural and acquired immunity to FIP viruses. What is this immunity and how can this knowledge contribute to strengthening immunity against FIP?

Unlicensed molnupiravir is an effective rescue treatment after failure of unlicensed GS-441524 therapy in cats with suspected FIP

Meagan Roy 1, Nicole Jacque 2, Wendy Novicoff 3, Emma Li 1,Rosa Negash 1 , Samantha JM Evans 1 *

  1. Department of Veterinary Biosciences, College of Veterinary Medicine, The Ohio State University, Columbus, OH 43210, USA
  2. Independent Researcher, San Jose, CA 95123, USA
  3. Departments of Orthopedic Surgery and Public Health Sciences, School of Medicine, University of Virginia, Charlottesville, VA 22903, USA
  4. * Author to whom correspondence should be addressed.

Academic editors: Alessia Giordano and Stefania Lauzi
Pathogens 2022, 11(10), 1209;
Received: 19/09/2022 / Revised: 9/10/2022 / Received: 19/10/2022 / Published: 20/10/2022
(This article is part of a special issue of Advances on Feline Coronavirus Infection)

Original article: Unlicensed Molnupiravir is an Effective Rescue Treatment Following Failure of Unlicensed GS-441524-like Therapy for Cats with Suspected Feline Infectious Peritonitis


Feline infectious peritonitis (FIP) is a complex and historically fatal disease, although recent advances in antiviral therapy have revealed treatment options. A newer therapeutic option, unlicensed molnupiravir, is used as first-line therapy for suspected FIP and as salvage therapy for cats that have persistent or recurrent clinical signs of FIP after treatment with GS-441524 and/or GC376. Treatment protocols for 30 cats were documented based on owner-reported data. 26 cats treated with unlicensed molnupiravir as rescue therapy were treated with a mean starting dose of 12.8 mg/kg and a mean final dose of 14.7 mg/kg twice daily for a median period of 12 weeks (IQR = 10-15). A total of 24 of the 26 cats were still living without signs of disease at the time of writing this report. One cat was euthanized after treatment due to persistent seizures and the other cat underwent retreatment due to relapse of clinical signs. Few adverse effects have been reported, with the most prominent - drooping ears (1), broken whiskers (1) and severe leukopenia (1) - occurring at doses above 23 mg/kg twice daily. This study provides proof of principle for the use of molnupiravir in cats and supports the need for future studies to further evaluate molnupiravir as a potentially safe and effective therapy for FIP.

Keywords: FIP; coronavirus; antiviral drug; EIDD-2801; black market

1. Introduction

Feline infectious peritonitis (FIP) is a complex and historically fatal disease caused by mutation of the ubiquitous feline enteric coronavirus (FECV) [1]. Recent advances in feline and antiviral medicine have revealed potential treatment options for FIP. The 3C-like protease inhibitor GC376 was the first targeted antiviral therapy used against this disease [2]. GC376 was highly effective in improving clinical signs of FIP in 19 of 20 naturally infected cats, but showed limited ability to manage long-term disease [2]. Pedersen et al. continued to investigate the antiviral compound GS-441524, a nucleoside analog and active metabolite of remdesivir (GS-5734). GS-441524 demonstrated superior ability to treat and control disease in naturally infected cats compared to GC-376, with 25 of 31 cats disease-free at the time of writing [3].
Since these discoveries, cat owners worldwide have obtained these mostly unlicensed drugs to treat their FIP cats with remarkably high success rates [4]. Legal FIP treatment is in high demand in the United States due to ethical and legal concerns regarding the unlicensed drugs GC376 and GS-441524. In addition, some cats with FIP have exhausted all current treatment options due to disease relapse and/or treatment failure after GS-441524, GC376 and/or combination therapy. Therefore, an effective and legal treatment option for FIP is urgently needed.
In connection with the recent outbreak of SARS-CoV-2, a number of new antivirals have entered the market. Molnupiravir (EIDD-2801), manufactured by Merck, is currently available under an emergency use authorization (EUA) from the FDA for the treatment of COVID-19 in adults [5]. It is an oral prodrug of the nucleoside analog BD-N4-hydroxycytidine, which increases guanine to adenine and cytosine to uracil nucleotide transition mutations in coronaviruses [6]. This mechanism increases the rate of mutations above the accepted limit, which in turn inactivates the virus [7]. Molnupiravir has been found to be safe and well tolerated at doses up to 800 mg twice daily in patients with COVID-19 [8]. Some studies have reported significant reductions in hospitalizations and deaths in mild-to-moderate COVID-19 patients, although efficacy appears to be lacking in severe COVID-19 patients [7].

Because of molnupiravir's strong potential to treat other coronavirus infections, cat owners have begun using unlicensed molnupiravir (or its active metabolite EIDD-1931) purchased over the Internet to treat FIP. However, the use of molnupiravir for the treatment of FIP is currently not documented in any scientific literature. Unlicensed molnupiravir can be used as first-line therapy for suspected FIP, but also as rescue therapy to treat cats that have persistent or recurrent clinical signs of FIP after GS-441524 and/or GC376 therapy. The aim of this study is to document this use and provide proof of principle for molnupiravir as a potential treatment for FIP based on owner-reported data.

2. Materials and methods

The survey was conducted using the Qualtrics XM program (Qualtrics Version May-August 2022, Provo, UT, USA) under license from Ohio State University. The survey (Supplementary Data S1) was written in English and consisted of 94 multiple-choice and free-response questions asking about FIP diagnosis, clinical signs, initial therapy (used before molnupiravir), molnupiravir treatment, adverse events, duration of treatment, and remission time. The number of free-response questions was limited to limit recall bias. The survey also allowed owners to upload relevant documents (eg veterinary medical records and laboratory results). The survey was formatted using questions from previous studies to maintain consistency of language and style, as well as newly developed questions specific to the experience of molnupivir treatment. The logic of the survey dictated that some questions appeared only after a particular answer was selected, while others were skipped when a particular answer was selected. This conditional logic was used to reduce questionnaire completion bias and questionnaire fatigue. The survey took approximately 20-30 minutes to complete and could be saved and completed later if needed. This study was approved by the Ohio State University Institutional Review Board (Protocol No. 2021E0162).

The survey was distributed to participants individually by email and data were collected from June to August 2022. Participants were selected from a subset of owners seeking molnupiravir therapy for their cat with suspected FIP through popular FIP therapy and social media support groups. Inclusion criteria were surveys of cats suspected of having FIP based on veterinary diagnosis, failure to respond to initial therapy, or recurrence of clinical signs after completion of initial therapy other than molnupiravir (eg, GS-441524 or GC376) and completion of 8–10 weeks of oral molnupiravir therapy (or those who subsequently died or were euthanized during therapy). This study also included a small group of cats that received molnupiravir for 8-10 weeks as initial and sole therapy, which will be referred to as first-line therapy in the rest of this paper, when FIP is suspected. Exclusion criteria were surveys with incomplete data or cats not diagnosed with FIP by a veterinarian.

3. Results

3.1 Demographic data

A total of 80 potential participants were identified through the FIP social media support group, and 37 questionnaire invitations were sent to those participants with available contact information. A total of 33 questionnaires were sent and follow-up emails were sent to 21 participants in order to obtain complete data from the questionnaires. Seventeen owners attached relevant documents to the sent questionnaires, and two other owners sent relevant documents to the study e-mail address, which included veterinary medical records, laboratory results and diagnostic images. These listed documents were used to document adverse reactions reported by one participant. One response was refusal to participate. Two cases were excluded because the cats did not have a veterinarian diagnosis of FIP (one was reportedly diagnosed based on the loss of a sibling to FIP, and the other was examined by a veterinarian who concluded that blood tests were not consistent with FIP). Thus, a total of 30 cats with suspected FIP were included in this study, 4 of which received no treatment prior to molnupiravir administration. These four cats were enrolled as a separate small cohort for first-line molnupiravir treatment. A block diagram of these cases is shown in Figure 1. The countries of origin represented were the United States (25), Germany (2), Poland (2), and Sweden (1). The sex/neuter status of the cats at the time of diagnosis was 40 % neutered males, 40 % spayed females and 20 % non-neutered males. The average age at diagnosis was 9.7 months, with a range from 1 month to 6 years. Most cats were of mixed or unknown breed (70 %); among them were seven purebred cats and two special crossbred cats (eg, a cross between a Balinese and ragdoll cat and a Siamese cat). Responses identifying the cat as "American Shorthair" or "American Longhair" were instead categorized as mixed breed, given the commonly reported confusion among American owners regarding the breed's nomenclature.

Figure 1. This flowchart represents the number of cases in each treatment block.

Regarding comorbidities, feline leukemia virus was reported in only one cat and calicivirus was reported in one cat. Several cats also had a history of external and/or internal parasitic infections (3), conjunctivitis/ocular infections (2), and bacterial skin infections (pyoderma) (1). A total of 16 cats had neurological signs of FIP. Three cats had both neurological and ocular manifestations of FIP, and two cats had only ocular manifestations of FIP. Of the remaining cases, seven were effusive, while five cases were non-effusive. The full breakdown of FIP types is shown in Table 1.

CatAge at diagnosis (months)Sex/castration status at diagnosisTribePrevious medical conditionsCountry of originFIP formDuration of initial treatment (weeks)Disease-free periodSecond therapyDuration of the second therapy (weeks)Disease-free periodThe third therapyDuration of the third therapy (weeks)Disease-free period
14MaleEuropean shorthairparasitic infections, URI at an early ageGermanyneurologicalinjectable oral GS-4415248noneinjectable and oral GS-44152415none
215neutered catBurmesenoneSwedeneffusive, non-effusive, neurologicalinjectable GS-44152412less than 4 weeksinjectable GS-4415241417 daysoral GS-4415245 weeksnone
39neutered catBritish ShorthairnonePolandeffusive, neurological, ocularinjectable GS-44152413less than 2 weeksinjectable GS-44152412more than 6 months, less than 1 year
45neutered catAbyssinianoneUSAeffusiveinjectable GS-44152412less than 2 weeksinjectable GS-44152414less than 4 weeks
54neutered catBalinese/Ragdol mixcalicivirus, conjunctivitis, giardiasis, tapeworm, URIUSAnon-effusiveinjectable GS-44152413less than 8 weeks
67neutered catSiamesenoneUSAneurologicalinjectable and oral GS-441524, injectable GC, injectable and oral molnupiravir12none
77neutered catAmerican ShorthairnoneUSAnon-effusiveinjectable and oral GS-4415245none
86neutered catAmerican Shorthair/Siamese mixtapeworm, FCoVUSAeffusive, neurologicalinjectable and oral GS-4415245none
94neutered catHomemade mixedbroken pelvisUSAeffusiveinjectable and oral GS-44152414less than 6 monthsoral GS-44152413less than 4 weeksoral GS-441524/injectable GC6 weeks in combination then 6 weeks of oral GSnone
104neutered catHomemade mixednoneUSAeffusiveinjectable GS-44152423less than 4 weeks
1172neutered catHomemade mixedFeLVUSAnon-effusiveoral GS-44152412less than 6 months
125MaleHomemade mixednoneUSAnon-effusive, neurological, ocularinjectable and oral GS-44152417none
1301.VMaleSavannahnoneUSAeffusive, neurologicalinjectable and oral GS-44152424less than 6 monthsinjectable and oral GS-44152412less than 4 weeks
144neutered catHomemade mixedSkin and eye infections, fleasPolandnon-effusive, neurologicalinjectable GS-44152412less than 2 weeksinjectable GS-44152417less than 4 weeks
1512neutered catAmerican ShorthairnoneUSAeffusiveinjectable GS-441524/GC01.Vnone
165neutered catHomemade mixednoneUSAeffusive, neurologicalinjectable GS-44152412less than 4 weeks
174MaleAmerican longhairnoneUSAocularinjectable and oral GS-441524, GC37613none
186neutered catHomemade mixednoneUSAeffusiveinjectable GS-44152412none
1912neutered catHomemade mixednoneUSAnon-effusiveinjectable and oral GS-44152412less than 2 weeksinjectable GS-44152412none
206neutered catUnknownnoneUSAnon-effusive, neurologicalinjectable GS-4415244noneoral GS-4415243none
214neutered catNorwegian forestnoneUSAneurologicalinjectable GS-44152412less than 6 monthsinjectable GS-44152401.VnoneMolnupiravir, GS-441524, GC12 weeksnone
226neutered catHomemade mixednoneUSAneurological, ocularoral GS-4415243none
2312neutered catUnknownnoneGermanyneurologicalinjectable GS-44152416less than 6 months
243MaleHomemade mixednoneUSAneurologicalinjectable GS-44152412less than 6 months
256neutered catAmerican ShorthairnoneUSAeffusiveoral GS-44152413less than 1 week
261MaleUnknownnoneUSAnon-effusiveinjectable GS-44152412less than 1 week
277neutered catHomemade mixednoneUSAnon-effusive, neurologicalMolnupiravir12less than 1 week*Molnupiravir
2824neutered catHomemade mixednoneUSAeffusiveMolnupiravir
2912neutered catHomemade mixednoneUSAnon-effusive, ocularMolnupiravir
3024neutered catHomemade mixednoneUSAneurologicalMolnupiravir
Table 1. Signaling and initial therapy characteristics of all 30 cats treated with unlicensed molnupiravir for suspected FIP.

3.2. Initial treatment before initiation of molnupiravir

A total of 26 of 30 cats received initial treatment for suspected FIP with unlicensed GS-441524 or a drug combination containing unlicensed GS-441524 as the main base drug (GS-441524-based). Half (13) of the cats were treated with injectable GS-441524. Only three cats were treated with oral GS-441524, while the other seven cats were treated with a combination of injectable and oral GS-441524 throughout the treatment period. Two cats were treated with a combination of the unlicensed drug GS-441524 and the unlicensed drug GC376. Cube no. 6 was treated with all previously mentioned drugs along with molnupiravir for 12 weeks of a very complicated regimen (Supplementary Data S2). Dosing of combination drugs used as part of primary therapy (eg, GC376 and molnupiravir) was not determined. Reported initial doses of the unlicensed GS-441524 ranged from 2 mg/kg to 10 mg/kg; the most frequently reported dosages were 5-6 mg/kg (eight cats) and 10 mg/kg (seven cats). Most (21) cats received a dose once a day. Only four were dosed twice daily, and one cat was dosed twice daily for one week at first, then switched to once daily dosing. The median duration of treatment based on GS-441524 was 12 weeks (IQR = 12-13). In fifteen cats, a change in daily doses was reported during treatment. For several cats, the daily dose was increased by body weight to maintain the same dosage in mg/kg. Others increased the mg/kg dosage because of insufficient clinical response or a change in route of administration (eg, from injectable to oral GS-441524). No participant reported dose reduction during treatment.

A total of 6 of 26 cats completed a shorter than average 12-week treatment with GS-441524 due to insufficient clinical response and were immediately started on another treatment. Two of the six cats initiated a different route or dose of unlicensed GS-441524 treatment as shown in Table 1. One cat switched from injectable to oral GS-441524 treatment on the second treatment. In the second cat, the dose of GS-441524 was simply increased during the second treatment. The remaining four cats started treatment with unlicensed molnupiravir at this time, as shown in Table 2. Of the 20 cats that completed at least 12 weeks of treatment with GS-441524, 16 experienced clinical remission. All 16 were in remission for less than 6 months, with 2 cats in remission for less than a week before clinical signs returned. All 16 started a second round of treatment, with 10 receiving a second round of GS-441524-based treatment and 6 starting molnupiravir at this time. Four cats that completed treatment with GS-441524 but did not achieve clinical remission were immediately started on molnupivir. A total of 26 cats received primary treatment with GS-441524 and all 26 relapsed or did not respond adequately. A total of 10 of 26 completed a second round of GS-441524-based treatment and 16 started molnupivir treatment.

CatClinical symptoms at the beginning of treatmentBrand nameInitial dosage and frequencyFinal dosage and frequencyDuration of treatment (weeks)Time to improvePersistent clinical symptomsThe resultAdverse effects
1diarrhea, vomitingAura Plus11 mg/kg twice daily11 mg/kg twice daily12less than 1 weeknoneclinical remissionnone
2none reportedAura12 mg/kg twice a day12 mg/kg twice a day12uncertainnoneclinical remissionnone
3anisocoria, colored spots in the eye, polydipsia, pica, weight lossAura 280128 mg/kg twice daily14 mg/kg twice a day12within 2 weeksnoneclinical remissionnone
4anorexia, lethargy, weight lossEIDD7 mg/kg twice a day7 mg/kg twice a day12less than 1 weeknoneclinical remissionnone
5colored spots in the eye, diarrhea, hiding and lack of socializationAura 28016 mg/kg once daily13 mg/kg once daily10within 2 weeksnoneclinical remissionnone
6anisocoria, constipation, anorexia, fecal and urinary incontinence, lethargy, paralysis, seizures, pale gums, weight lossAura 280120 mg/kg twice a day20 mg/kg twice a day11less than 1 weeknoneclinical remissionnone
7anorexia, difficulty walking, hiding, lack of socialization, jaundice, lethargyCapella EIDD9 mg/kg twice daily13 mg/kg twice a day10less than 1 weeknoneclinical remissionnone
8anorexia, difficulty walking, urinary incontinence, paralysisAura 280117 mg/kg twice a day17 mg/kg twice a day15less than 1 weekdifficulty walking persisted for 2 months, still not normal but has a normal lifeclinical remissionnone
9cough, anorexia, hiding, lack of socialization, polydipsia, weight lossAura 280112 mg/kg twice a day16 mg/kg twice a day13within 2 weekspolydipsia persisted for 1 weekclinical remissionnone
10anorexia, lethargy, weight lossAura 280112 mg/kg twice a day12 mg/kg twice a day16within 2 weeksnoneclinical remissionnone
11anorexia, lethargy, URI, weight lossAura 193112 mg/kg twice a day12 mg/kg twice a day12within 2 weeksnoneclinical remissionnone
12blindness, head bobbing, difficulty walkingAura 280110 mg/kg twice a day14 mg/kg twice a day12within 3 weeksnoneclinical remissionnone
13difficulty walking, hiding, lack of socialization, polyuria, lethargy, anorexia, paralysis, tremorsAura 280112 mg/kg twice a day12 mg/kg twice a day12less than 1 weeknoneclinical remissionnone
14anorexia, heavy walking, hiding, lack of socialization, lethargy, unusual timidityAura 280111 mg/kg twice daily16 mg/kg twice a day18more than 4 weeksnothing physical but the MRI is still not normalclinical remissionnone
15blindness, constipation, anorexia, diarrhea, enlarged abdomen, hiding, lack of socialization, lethargy, pale gums, weight lossAura 280116 mg/kg twice a day16 mg/kg twice a day12less than 1 weeknoneclinical remissionnone
16anorexia, difficulty walking, lethargy, seizures, tremors, weight lossAura 280114 mg/kg twice a day14 mg/kg twice a day12less than 1 weeknoneclinical remissionnone
17cough, anorexia, difficulty breathing, hiding, lack of socialization, lethargy, vomiting, weight lossAura 2801 and Aura 193112 mg/kg twice a day17 mg/kg twice a day20within 3 weeksanorexiaclinical remissionnausea/vomiting, anorexia
18constipation, anorexia, difficulty walking, hiding, lack of socialization, weight lossAura 280112 mg/kg twice a day12 mg/kg twice a day8within 2 weeksnoneclinical remissionnone
19lethargy, anorexiaAura 280112 mg/kg twice a day12 mg/kg twice a day7within 2 weeksnoneclinical remissionnone
20trembling/shakingAura 280110 mg/kg twice a day23 mg/kg two to three times a day10less than 1 weekin remission about 1 1 weeks before the onset of seizureseuthanasiadecreased appetite when dosed three times a day, severe leukopenia, loss of beard, scaly skin on ears
21difficulty walking, fecal incontinenceAura 2801 and Aura 193113 mg/kg twice a day30 mg/kg twice a day14less than 1 weekdifficult walking, difficult jumping, fecal incontinence persisted during the study (1 week post treatment)relapse and euthanasiadrooping ear tips, muscle weakness
22colored spots in the eye, anorexia, difficulty walking, hiding, lack of socialization, lethargyAura 280116 mg/kg twice a day19 mg/kg twice a day9within 2 weeksnoneclinical remissionnone
23difficulty walking, anorexia, loss of balanceEIDD aura12 mg/kg twice a day15 mg/kg three times a day10within 2 weeksheavy walkingclinical remissionnone
24blindness, colored spots in the mouth, anorexia, difficulty breathing, difficulty walking, enlarged abdomen, urinary incontinence, jaundice, lethargy, paralysis, tremorsAura 280115 mg/kg twice a day15 mg/kg twice a day16less than 1 weeknoneclinical remissionnone
25difficulty breathing, difficulty walking, hiding, lack of socialization, lethargy, URIAura 28017 mg/kg twice a day7 mg/kg twice a day16within 2 weeksnoneclinical remissionnone
26lethargy, anorexiaAura 280114 mg/kg twice a day14 mg/kg twice a day15less than 1 weekneurological twitches, elevated liver enzymesclinical remissionnone
Table 2. Treatment and outcome characteristics of 26 cats receiving unlicensed molnupiravir as rescue therapy.

3.3. The second round of treatment before the initiation of molnupiravir

Overall, 10 of 26 cats that received initial GS-441524 treatment and subsequently relapsed were reported to have received a second round of unlicensed GS-441524 treatment prior to initiation of molnupiravir. Again, most cats received injectable GS-441524 (6), with two receiving oral GS-441524 and two receiving both injectable and oral GS-441524. Reported dosages ranged from 4-5 mg/kg to 15 mg/kg; the most frequently used dosages were 7-8 mg/kg (two cats) and 15 mg/kg (two cats). Most cats were dosed once daily (seven cats), one cat was dosed twice daily and one cat was dosed three times daily. In most cats, the dose was varied during treatment. The two doses were weight-adjusted to maintain the same dosage in mg/kg. Dosing in mg/kg was increased in five cats that did not respond adequately or developed new clinical signs (eg, neurological signs).
The median duration of treatment was 12.5 weeks (IQR 9.75–14.25). Only two cats did not undergo at least 12 weeks of therapy. One of the two added GC376 and molnupiravir to current GS-441524 therapy, and the other started molnupiravir as sole therapy. Of the eight cats that completed at least 12 weeks of GS-441524 therapy, two did not achieve clinical remission. Both cats started treatment with unlicensed molnupiravir at that time. The remaining six cats were reported to achieve clinical remission after a second round of treatment with GS-441524. Five of the six cats were in remission for less than 4 weeks, with the exception of one cat that was in remission for more than 6 months but less than a year. Seven out of ten cats started taking unlicensed molnupiravir at this time.

3.4. The third round of treatment before starting molnupiravir

The remaining three cats received a final round of GS-441524-based treatment before switching to molnupiravir. Cat no. 2 received oral GS-441524 for 5 weeks prior to initiation of molnupiravir. Cat no. 9 was treated for 6 weeks with oral and injectable GS-441524 and then continued for 6 weeks with oral GS-442524 alone. Dosing and frequency in both cats are unknown, as the survey collected data on only two therapies prior to molnupivir. Cat no. 21 received a combination of GS-441524, GC376 and molnupiravir for 12 weeks. The dosage, frequency and duration of each varied radically over the course of 12 weeks (Supplementary Data S3). All three cats started treatment with molnupiravir without clinical remission from this third round of treatment.

3.5. Molnupiravir as rescue therapy

Of the 26 cats receiving unlicensed molnupiravir as rescue therapy, most were using the Aura brand, with only 2 cats using a different brand of molnupiravir. More than 81 % cats (18) were treated with Aura 2801, 1 cat was treated with Aura 1931, and another 2 cats were treated with both Aura preparations. The mean initial dosage was 12.8 mg/kg twice daily. One cat was dosed only once a day and two cats were dosed 2 to 3 times a day. The most commonly used initial dosage was 12 mg/kg twice daily. Dosage ranged from 6 to 28 mg/kg twice daily. 11 dosage changes were reported, all but one being an increase in dosage. Reduction of dosage in cat no. 3 was not explained in any way. The mean final dosage was 14.7 mg/kg twice daily, with the same three cats differing in dosing frequency. The most common final dosage was also 12 mg/kg twice daily. The dosage range was 7 to 30 mg/kg twice daily.

Median duration of treatment was 12 weeks (IQR 10-15). Overall, a wide range of 7-20 weeks was reported. Only eight cats were treated for less than 12 weeks. A cat that completed only 7 weeks of treatment was reported to have discontinued treatment due to achieving clinical remission. All 26 cats completed treatment at 7 weeks or longer and all 26 cats survived. No cases of missed doses of molnupiravir have been reported.

Owners reported improvement in clinical signs in more than 92 % cats within three weeks of initiation of molnupiravir treatment, with 84.6 % cats showing improvement within two weeks and nearly half (46.2 %) within one week. Only two cases were reported differently, with one cat showing no signs of improvement for up to 1.5 months, and the owner of the other cat being unsure of the timescale and degree of improvement in clinical signs. A total of seven cats with persistent clinical signs of FIP were reported. In one of them, the disappearance of clinical symptoms was reported after one week of the observation period. Others are thought to have had residual symptoms such as difficulty walking or jumping, tremors, MRI changes and fecal incontinence. The full range of persistent clinical signs is shown in Table 2. Only three cats reported adverse reactions in response to molnupiravir, including nausea/vomiting, anorexia, drooping ear tips (Figure 2), brittle whiskers, leukopenia, scaly skin and muscle wasting. At the time of publication, 24 of 26 cats are living in clinical remission of FIP after oral molnupiravir treatment. One cat reportedly died 1 week after discontinuation of molnupiravir due to a prolonged seizure, and the other cat (No. 21) was disease-free 4 weeks before relapse. Cat no. 21 then started a second round of molnupiravir at the same dose, but was subsequently euthanized due to insufficient response to treatment.

Figure 2. Dropped ear tips were reported as an adverse effect of unlicensed molnupiravir treatment in cat no. 21.

In cat no. Severe leukopenia was reported in 22 cases. Through veterinary records, it was found that cat no. 22 has moderate panleukopenia with lymphopenia, neutropenia, and normal hem and thrombograms on 4 of 5 sequential complete blood counts, which were confirmed through veterinary records of sequential complete blood counts. The initial white blood cell count recorded was 10,700 cells per microliter (reference range 3,500–16,000 cells per microliter). Four more complete blood tests showed white blood cell counts ranging from 1,200 to 1,900 cells per microliter. The initial neutrophil count was 8560 cells per microliter (reference range 2500-8500 cells per microliter). The other four neutrophil counts ranged from 696 to 1292 cells per microliter. The initial lymphocyte count was 1177 cells per microliter (reference range 1200-8000 cells per microliter). The other four lymphocyte counts ranged from 330 to 532 cells per microliter.

3.6. Molnupiravir as primary therapy

A small group of four cats were treated with unlicensed molnupiravir as sole therapy for suspected FIP, as shown in Table 3. Three of them reportedly chose molnupiravir over the unlicensed counterpart GS-441524 due to financial constraints. Cat no. 29 received 12 weeks of oral molnupiravir 12 mg/kg twice daily prior to the treatment shown in Table 3. This cat was disease-free for less than one week prior to restarting oral molnupiravir 19 mg/kg twice daily for 10 weeks.

CatClinical symptoms at the beginning of treatmentBrand nameInitial dosage and frequencyFinal dosage and frequencyDuration of treatment (weeks)Time to improvePersistent clinical symptomsConclusionAdverse effects
* 27Hiding, lack of socialization, lethargy, anorexia, URI, vomiting, weight lossAura 280119 mg/kg twice a day19 mg/kg twice a day10less than 1 weeknoneclinical remissionnone
28Anorexia, difficulty walking, distended abdomen, hiding, lack of socialization, lethargyAura 28018 mg/kg twice a day8 mg/kg twice a day13in two weeksnoneclinical remissionnone
29Anisocoria, blindness, eye color changes, anorexia, hiding, lack of socialization, urinary incontinence, lethargy,Aura 280110 mg/kg twice a day10 mg/kg twice a day13in two weeksnoneclinical remissionnone
30Hiding, lack of socialization, lethargy, pale gums, weight lossAura 280110 mg/kg twice a day12 mg/kg twice a day10in two weeksnoneclinical remissionnone
Table 3. Treatment and outcome characteristics of 4 cats receiving unlicensed molnupiravir as primary therapy.* They received two rounds of molnupiravir treatment; the first round is documented in Table 1.

All four cats were treated with oral molnupiravir Aura 2801 at a mean starting dose of 11.75 mg/kg twice daily (range 8-19 mg/kg) and a mean final dose of 12.25 mg/kg twice daily (range 8-19 mg/kg ). The median duration of treatment was 11.5 weeks (IQR 10-13), with two cats treated for 10 weeks and two cats treated for 13 weeks. A Mann-Whitney test was performed and no significant difference was found between the median duration of molnupivir as rescue therapy (12) and the duration of molnupivir as initial therapy (11.5) (p = 0.692). All owners reported seeing clinical improvement within two weeks and one cat showed improvement within one week. All cats survived the treatment, were disease-free at the time of publication, and no adverse effects of the treatment were reported.

3.7. Molnupiravir by type of FIP

The above information was collected for all 30 cats and then further divided according to the clinical forms of FIP. First, 16 cats with a reported neurological form of FIP were evaluated. Subsequently, the other cats were divided according to ocular (2), effusive (7) and non-effusive (5) forms. The mean starting dose of molnupiravir in the neurological form of FIP was 14.4 mg/kg twice daily, with two cats treated 2-3 times daily. The mean final dosage was 16.4 mg/kg twice daily, with two cats treated 2-3 times daily. The most commonly used initial and final dosage was 12 mg/kg twice daily. Median duration of treatment for neurological FIP was 12 weeks (IQR 10-12,641).

In the two remaining cases of ocular FIP, the mean initial dose was 11 mg/kg twice daily and the mean final dose was 13.5 mg/kg twice daily. The treatment lasted an average of 16.5 weeks. Seven cases of effusive disease were treated with a mean initial dose of 10.5 mg/kg twice daily and a mean final dose of 11.1 mg/kg twice daily. Treatment lasted an average of 13 weeks (IQR 12–16). Five non-effusive cases were treated with a mean initial dose of 10.6 mg/kg twice daily and a mean final dose of 12.8 mg/kg twice daily. One cat was treated once a day. The average duration of treatment was 10 weeks (IQR 8.5-13.5).

3.8. Costs and owner satisfaction

The majority of cats in this study were switched to unlicensed molnupiravir due to treatment failure/relapse or insufficient response. In addition to cats that relapsed or did not respond to unlicensed GS-441524-based treatment, one cat was intolerant to the injectable form of GS and three owners were cost-restricted. Owners were not required to disclose the financial costs of treatment; this information was provided on a voluntary basis only. In addition, “0” responses that were reported were not included in the calculation of the following averages due to the inability to distinguish whether “0” means no cost or unknown cost. The mean reported cost of the first round of GS-441524-based treatment was $3448.83, and similarly the mean reported cost of the second round of GS-441524-based treatment was $3509.09. Only 4 owners reported paying for molnupiravir treatment, while 16 others reported “0” (or no cost/cost unknown). The overall mean for the 20 owners who responded to the financial cost survey question (including “0” responses) for molnupiravir was $209. The average cost of the four owners who did not answer “0” was $1045. While 90 % owners reported being "very" or "somewhat" satisfied with their cat's experience of treating their cat with molnupiravir, three were "very dissatisfied" with their experience. Unfortunately, no explanation was provided for the reported dissatisfaction.

4. Discussion

In this work, we describe the first known use of unlicensed molnupiravir for the treatment of suspected FIP in cats based on owner-reported data. For the treatment of cats using unlicensed molnupiravir as primary therapy for suspected FIP, the combined data from this study suggests that dosing at 12 mg/kg twice daily for approximately 12 weeks is effective in achieving clinical remission. For the treatment of cats receiving molnupiravir as rescue therapy when failing or relapsing after GS-441524-based therapy, the combined data from this study suggests that dosing at 12-15 mg/kg twice daily for 12-13 weeks is effective in achieving clinical remission. However, when broken down by clinical form of FIP, it was found that neurological cases of FIP were generally treated with a higher dosage than the average for all types of FIP. Ocular, effusive and non-effusive cases were treated with a dosage of around 12 mg/kg twice daily, with some variations. Therefore, dosing of 15 mg/kg molnupiravir twice daily for 12 weeks appears to be effective for neurological cases of FIP. For ocular, effusive, and non-effusive cases, 12 mg/kg molnupiravir twice daily for 12–13 weeks appears to be effective.

These data are somewhat inconsistent with the proposed treatment protocol of the company producing unlicensed molnupiravir under the trade name HERO Plus 2801. The recommended dosage in the package insert is 25 mg/kg once daily for effusive and non-effusive FIP, 37.5 mg/kg once daily for ocular FIP and 50 mg/kg once daily for neurological FIP [9]. The package leaflet of HERO Plus 2801 also includes the preliminary results of the study "Effect of treatment with oral nutrition on survival time and quality of life in feline infectious peritonitis", which includes 286 cats with a diagnosis of FIP. According to this package insert, 28 cats were cured after 4 weeks of treatment and 258 cats were cured after 8 weeks of treatment, with no deaths at the time of reporting [9]. Data from this study have not yet been published in the scientific literature.

However, the cats in this study were using molnupiravir from a different supplier, Aura, which did not provide specific treatment recommendations. The treatment protocols used were therefore based on advice and information shared in groups on social networks, worksheets published on the Internet [10,11] and information on possible adverse effects contained in information published as part of human drug approval applications [12].

The molnupivir treatment protocol derived from this study more closely matches an independently designed protocol [10] published on the Internet. Based on data from in vitro cell cultures of EIDD-1931 and EIDD-2801, laboratory and field studies of GS-441524, and human pharmacokinetic studies, these authors extrapolated the effective dosage of oral molnupiravir [10]. Their calculations suggested a dosage of 4.5 mg/kg every 12 h for effusive and non-effusive FIP, 8 mg/kg every 12 h for ocular FIP, and 10 mg/kg every 12 h for neurological FIP [10]. Although the dosage in this study was generally higher than the dosage suggested by the cited authors, the high survival rate and low relapse rate at the time of the study termination suggest that the manufacturer's unlicensed recommendations may not represent the lowest effective dosage. Ultimately, controlled scientific experiments are greatly needed to evaluate the lowest effective dosage of molnupiravir in cats with suspected FIP.

Several cats were treated with Aura 1931, which is the active metabolite of molnupiravir, EIDD-1931. The reported dosages used were in a similar range to those reported for molnupiravir. Nominally, because the molecular weight of EIDD-1931 is lower than that of EIDD-2801, these cats received more active drug than cats using molnupiravir. However, a previous study showed decreasing oral bioavailability with increasing doses in mice. Therefore, the difference in bioavailability may not be proportional [13]. Pharmacokinetic studies of both molnupiravir and EIDD-1931 in cats are unfortunately unknown.

No adverse effects were reported in the package insert for HERO Plus 2801, which is contrary to what was reported in this study. Among the reported side effects of molnupiravir, the most prominent were drooping ears, hair loss, and severe leukopenia. No skin or follicular lesions have been reported in the human medical literature to match the whisker shedding or ear folding reported here. However, it should be noted that the cats that experienced these side effects received the two highest doses of molnupiravir shown in this study: 23 mg/kg three times daily and 30 mg/kg twice daily.

Severe bone marrow toxicity was reported in dogs during a 28-day study that was discontinued due to severe drug effects [12]. At the dosage of 17 mg/kg/day and 50 mg/kg/day, all hematopoietic cell lines were affected [12]. Cat no. 22 received a maximum dosage of 23 mg/kg three times daily, which was much higher than the toxic dosage in dogs of 17 mg/kg once daily. In the study group with a dose of 17 mg/kg, the possibility of reversibility was noted when the treatment was stopped [12].

There are concerns about the content of unlicensed brands of molnuviravir, as these brands are not currently regulated and often do not list the actual ingredients. The Hero brand (same manufacturer as HERO Plus 2801) shown in Figure 3 was analyzed by our group in December 2021 through Toxicology Associates Inc. (Columbus, OH). It was found to contain 97.3 % of molnupivirus, with no other contaminants detected. The Aura 2801 product used by the majority of participants in this study was analyzed in September 2022 by the same laboratory. It was found to contain 96.8 % of pure molnupivirus. A more controlled assessment of the actual content and purity of the unlicensed preparations of both GS-441524 and molnupiravir is of great interest to the veterinary community and is an active research topic in our group.

Figure 3. Images of Hero brand unlicensed molnupiravir packaging.

Some limitations of this study result from the retrospective nature and legality of the therapies used. First, all data used in this study were obtained based on owner reports. Working closely with the owners and administrators of the social media websites that supported this group enabled a better understanding and interpretation of many of the survey responses. Due to the lack of a definitive ante-mortem diagnosis of FIP available for practical use, it was also not possible to confirm that the cats included in this study had FIP. In addition, the data are likely to be biased toward positive outcomes and may be burdened by recall error. During the distribution phase, a potential study participant responded by requesting to be removed from our email list and stating that he did not wish to participate in the study. Their cat did not respond to molnupiravir treatment and was eventually euthanized. We assume that others may have had the same feeling, since three other potential participants did not respond to the invitation to the study. This may have narrowed the number of participants with an adverse outcome and falsely inflated apparent survival rates. Therefore, the data presented here are intended to serve as evidence of the feasibility of using molnupiravir as primary or rescue treatment for FIP, not as an indication of the true rate of efficacy.

In cats using unlicensed molnupiravir as rescue therapy, the cause of failure to respond or relapse after GS-441524-based therapy was not determined. It could be related to the quality of the drug, the resistance of the virus or another factor. As there is currently no testing or regulation in the US, unlicensed versions of GS-441524 or GC376 may be of insufficient purity or concentration, leading to treatment failure. Another possible cause is natural or acquired resistance to GS-441524. These two causes may also be linked, as acquired resistance may be promoted when an insufficient amount of antiviral is used in treatment, for example with low-quality drugs.

A recent paper found no drug-induced viral mutations of SARS-CoV-2 during molnupivir treatment [14]. This suggests that SARS-CoV-2 is unlikely to develop resistance to molnupiravir. Therefore, treatment with molnupiravir may be similarly unlikely to induce FIPV resistance, making it an attractive therapeutic option.

However, there is clearly a need for (1) a legal (in the United States and elsewhere) alternative to unlicensed treatment with GS-441524 and (2) the availability of alternative rescue drugs, either alone or in combination, after failure of GS-441524 treatment. Molnupiravir has the potential to fill both of these gaps, and this is the first known report of its use in cats in the literature. Nevertheless, unlicensed preparations may continue to be used for the treatment of FIP given the cost and the widely established networks available for their acquisition.

In conclusion, based on owner-reported data, unlicensed molnupiravir appears to be an effective treatment for suspected FIP as both first-line and salvage therapy. At a dosage of 12-15 mg/kg every twelve hours, minimal side effects are reported and it provides survival with clinical resolution of FIP symptoms. Although the experiences of these owners in treating and apparently curing cats from FIP are unconventional and potentially illegal, they are undeniably remarkable and we can learn a lot from the experiments these "citizen scientists" are conducting. By reporting these experiences, we aim to provide a starting point for investigating molnupiravir for use in cats with suspected FIP and to document a "herd health" phenomenon that our profession should not ignore.

Supplementary materials

The following supplementary information can be downloaded at Supplementary Data S1: retrospective review of molnupiravir trials; additional data S2: abbreviated diary of clinical history cat. no. 6; supplementary data S3: Cat #21 abbreviated clinical history log.


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Tablets GS-441524 vs. Molnupiravir (EIDD-2801)


The development of FIP treatment with GS441524 was such that, since its discovery, it started with the use of an exclusively injectable form, to which a tablet form of the drug was later added. However, the existence of two forms using the same active substance brought a bit of confusion, which is related to the difference in absorption (biological availability - usability) of the active substance during injection and oral use. While almost 100% of the drug is used during injection, it is only about 50% for GS when administered orally. In order to be able to use the same dosage for both injectable and tablet forms of medicine, most manufacturers who produce both forms of medicine list the so-called equivalent GS content, which already takes into account the reduced availability of the drug administered orally. In this way, it is possible to easily switch from injections to tablets and vice versa. So, for example, if a tablet from the well-known manufacturers Lucky, Spark, or Hero etc... is equivalent (corresponding to the injectable form) let's say 16mg of GS, in fact it contains double the amount of GS, i.e. 32mg... But we can safely use 16mg in the calculation and follow the dosage recommendations for injectable use. However, due to the fact that many different manufacturers want to grab their piece of the GS pie on the market, many of them often state the real GS content for marketing reasons, because of course such a tablet looks financially more attractive than a tablet with the stated equivalent GS content... For such tablets but you really have to to know for sure from the manufacturer, what GS content it actually states, because if it states, for example, 40mg real content of GS in the tablet, when calculating the dose according to the generally valid recommendations for injections, you only have to use half tablet content… In other words, you use a 40mg GS tablet in your calculations as a 20mg tablet…

At this point, I would like to emphasize that GS-441524 is proven and safe by clinical studies and hundreds of thousands of cured cats worldwide, with minimal side effects, and is therefore used as a medicine first choice.

Recommended approximate dosage of GS-441524 at injection application or for tablets with the stated equivalent containing GS441524. The stated dosage applies to 1kg / 24h:

  • 6 mg/kg q24h – Wet FIP
  • 8 mg/kg q24h – Dry FIP
  • 10 mg/kg q24h – Ocular FIP
  • 12 mg/kg q24h – Neurological FIP
  • 15 mg/kg q24h – Neurological relapse

Recommended approximate dosage of GS-441524 for tablets with the stated real containing GS441524. The stated dosage applies to 1kg / 24h:

  • 12 mg/kg q24h – Wet FIP
  • 16 mg/kg q24h – Dry FIP
  • 20 mg/kg q24h – Ocular FIP
  • 24 mg/kg q24h – Neurological FIP
  • 30 mg/kg q24h – Neurological relapse

When you buy tablets yourself, pay extra attention to find out the information about whether the indicated content of the active substance in the tablet is real, or so-called. equivalent.

Note: Given that there is a reasonable assumption of an absorption limit of GS441524 in the digestive tract, it is recommended for dosages in equivalent 10mg/kg or more when taken orally, divide the dose into 2x a day.


In connection with the treatment of Covid-19, an antiviral called Molnupiravir appeared on the market, which is just a more comprehensible-sounding name for the substance labeled EIDD-2801. Unlike GS-441524, this antiviral is used exclusively in oral form. Even though its bioavailability (absorption capacity) in the digestive tract is similar to that of GS441524, i.e. around 50%, due to the absence of an injectable form, no "equivalent" content of the active substance is used in the recommended dosage by the manufacturers, and practically all state its real content. Thus, no conversion to 50% content is used. A 40mg tablet is simply 40mg, and this must also be taken into account when calculating the dose based on the recommended dosage. And there is one more very important thing... The pharmacokinetics of Molnupiravir is different from that of GS, and therefore Molnupiravir must be administered 2 times a day.

Recommended approximate dosage of EIDD-2801 tablets at 1kg / 12h:

  • 6 mg/kg q12h – Moist FIP
  • 8 mg/kg q12h – Dry FIP
  • 10 mg/kg q12h – Ocular FIP
  • 12 mg/kg q12h – Neurological FIP
  • 15 mg/kg q12h – Neurological relapse

Although at first glance this dosage may seem the same as that of GS, do not forget the essential difference. This is a dosage of 12 hours (as opposed to 24 hours for GS441524).

Given that no official clinical trial has yet been conducted for the use of Molnupiravir in the treatment of FIP (one is currently underway at UC Davis), its use is recommended only in cases of apparent resistance to GS441524, thus Molnupiravir will find application mainly in severe neurological relapses. Keep in mind that the side effects of Molnupiravir are not yet accurately mapped and one of the most feared is the potential mutagenic effect leading to cancer. There is no need to panic, but it is necessary to realize that in the current state of knowledge it is better to use Molnupiravir only in cases where it is really necessary. Time will tell if the possible side effects are a real threat or will never be confirmed.

Legality vs. Illegality of treatment

An interesting paradox occurred in connection with the drugs GS441524 and Molnupiravir (EIDD-2801).

Efficacy and safety of GS441524 in the treatment of FIP was indeed confirmed clinical study, but due to the licensing policy of the patent owner (Gilead company) there is no legal the source of medicines and practically all production is concentrated in China.

On the other hand, the efficacy and safety of Molnupiravir in the treatment of FIP has not yet been confirmed no official clinical study, it exists but is legally available a form of medicine primarily intended for the treatment of Covid-19. In the Czech Republic/SR, it is distributed under the name Lagevrio in packs of 40 capsules, each containing 200 mg of the active ingredient, which is too much. Therefore, for use in cats, the drugs must be decapsulated. Of course, EIDD-2801 is already produced by several Chinese companies that also produce GS441524. Since the drug is not officially intended for the treatment of FIP, it is used off-label.

EC22-007: EIDD-2801 (Molnupiravir): determination of oral dosage and confirmation of its efficacy in cats with FIP

Original article: EC22-007: EIDD-2801 (Molnupiravir): establishing an oral dose and evidence for efficacy in cats with FIP

Feline infectious peritonitis (FIP) is a highly fatal viral disease of cats with a worldwide incidence. Several antiviral agents have recently been shown to be promising in the treatment of cats with FIP. Despite these findings, there is currently no legal FIP treatment approved by the Food and Drug Administration (FDA), so many owners turn to unlicensed antivirals purchased online. Due to the ongoing COVID-19 pandemic, several antiviral drugs have recently been approved for oral use in human patients. One of these antivirals is the nucleoside analog EIDD-2801 (molnupiravir). EIDD-2801 causes hypermutation of developing viral RNA, effectively terminating the viral replication of many different RNA viruses. This antiviral agent has been developed for oral use in human patients at home. The use of EIDD-2801 in human medicine may facilitate veterinary use. An orally administered antiviral compound would be advantageous for the treatment of cats with FIP administered to a client. We will determine the effective oral dosage of EIDD-2801 and evaluate the efficacy of this compound in the home treatment of cats owned by a client with naturally occurring effusive FIP.

(Bria Fund)

Grant ID: EC22-007
Status: Active
Year: 2022
Grant amount: $33,550
Investigator: Brian Murphy, DVM, PhD, Dip ACP; Krystle Reagan, DVM, PhD, Dip ACVIM (SAIM); University of California, Davis