Original article: 2021 - FIP IN AGED CATS
Niels C. Pedersen, DVM, PhD, Professor Emeritus, University of California School of Veterinary Medicine, Davis; 2/11/2021
Facts: The relationship between age and the incidence of FIP has already been discussed in the literature and has even been the subject of research . 29% cases of FIP occur in kittens up to 0.5 years, 50% up to 1 year, 80% up to 3 years of age and 96% up to 8 years of age (Table 1). The incidence of FIP between the ages of 7 and 11 is extremely low. At least two studies, one from the United States in 1976  and the other from Europe in 2021  [Table 1], confirm a real but less dramatic increase in the incidence of 3% in older cats. The purpose of this article is to discuss the causes of this increase and its implications for the diagnosis and successful treatment of FIP in old cats.
Source of FIP exposure: FIP is the result of FECV exposure  and the same applies to older cats. However, there are several unique situations associated with such exposure. The first scenario is analogous to that which occurs in younger cats, ie mass exposure of younger cats. However, it is common for older cats to mate as kittens and live their lives together in relative isolation. This leads to another interesting scenario in which one cat in a pair is likely to die sooner than the other, leaving her without a companion. You will then get a new companion, most often a kitten, from a rescue organization, shelter or kennel. The chances of the kitten excluding FECV from these sources are high. There is also a second source of FECV exposure that does not require cat-to-cat contact. It is known that FECV can be transmitted from one cat to another through human clothing. FECV is present in high concentrations in litter dust, especially in young cats and kittens, and can survive for many days in the environment. Therefore, the owner's contact with younger cats away from home is another source of FECV infection.
Unique aspects of FIP in old cats: Experience with the treatment of old cats GS-441524 leads to three possible factors that should interest us: 1) misdiagnosed FIP, 2) the existence of FIP in connection with one or more other disorders common in the aging cat and 3) treatment of FIP in connection with weakened immune system.
Incorrect FIP diagnosis: Observations made in cats treated with GS-441524 suggest that FIP in older cats is more prone to misdiagnosis and more difficult to treat. The misdiagnosis of FIP in old cats can best be explained by a simple probability. For example, one study found that FIP is the most common cause of spinal disease in cats less than two years old, while cancer was the most common single disorder in the 2-8 age group . Therefore, the first consideration for a spinal cord disorder in a young cat should be FIP, while the first consideration for an old cat should be cancer. The incidence of many non-retroviral cancers is also on the rise, starting at around 7-9 years of age, accounting for about one-third of deaths in old cats. Chronic kidney disease also begins to manifest clinically at about the same age and accounts for about one-third of deaths in old cats. Remaining deaths in old cats include diabetes and hyperthyroidism and numerous musculoskeletal, cardiovascular, neurological and gastrointestinal disorders. Older cats, like older people, tend to have a decrease in immune function, which is often manifested by increased serum immunoglobulin levels and diseases associated with relative immunodeficiency and autoimmunity. The clinical and laboratory symptoms of these aging disorders often mimic the symptoms of FIP, but the likelihood that aging cats will have FIP is much lower than in these other conditions. In contrast, a young cat with clinical and laboratory features compatible with FIP that has FIP is much higher than an old cat with similar findings.
FIP as a secondary disease: The second scenario is more common in many old cats with FIP suffering from other serious medical conditions. Chronic renal failure is the most common of these underlying conditions, with cancers such as lymphoma being uncommon. Older cats also suffer from an aging immune system, leading to a state of relative immunodeficiency, which is another predisposing factor for FIP.
FIP and immunodeficiency due to aging: The immune system is sensitive to the adverse effects of aging in all animal species, including cats [6, 7]. Decreased immune function in aging cats is associated with changes in B and T cell populations and increased levels of non-specific immunoglobulin. Therefore, older cats often have higher levels of serum protein and globulin than usual. Increases in total serum protein and globulin levels in younger cats are often considered symptoms of FIP, while their diagnostic value in older cats is less significant.
The relative immunodeficiency caused by aging makes it difficult to fight new infections and older infections that remain hidden or latent in the body for decades. The best studied examples of the impact of aging on resistance to new and latent infections come from humans. COVID-19 deaths in the elderly with complicating diseases such as diabetes and chronic lung disease are the best examples of the impact of aging and chronic degenerative diseases on resistance to infectious agents. It is well known that tuberculosis remains latent in the pulmonary lymph nodes for decades before reactivation. Therefore, TB is a particular problem in humans in facilities for the elderly and in individuals treated for autoimmune disorders with cytokine inhibitors. There are also indications that FIP in some cats may exist in a subclinical state for months to years before it becomes clinically evident. There are also cases of cats that live their entire lives in residential isolation until they develop FIP in old age.
Conclusion: Fortunately, FIP in old cats is unusual, but it has unique properties that affect diagnosis and treatment. Particular attention should be paid to verifying the diagnosis of FIP and identifying other health factors that either predispose to FIP or complicate its successful treatment. This is a major diagnostic challenge in older cats and other degenerative disorders common to this age group make treatment difficult. The FIP cure rate, which is higher than 80% in young cats, is not as high in old cats.
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