Feline Coronavirus (FCoV) and Feline Infectious Peritonitis (FIP): Towards a reliable diagnosis

Feline coronavirus (FCoV) is an RNA virus widely present in many cat populations worldwide. Primarily a virus affecting the digestive system, it generally causes no clinical symptoms or manifests itself only as mild enteritis, or even delayed weight gain. However, a small proportion of infected cats develop a serious disease, feline infectious peritonitis (FIP). FCoV genomes like all coronaviruses, exhibit high genetic variation due to the high rate of RNA polymerase errors, resulting in different types of mutations, such as point mutations, deletions, introduction of termination codons, and recombinations. Although FCoV can spread systemically in the monocytes of healthy cats, it is hypothesized that specific mutations, occurring in a given cat, allow the virus to switch its cellular target from enterocytes to monocytes, facilitating the emergence of a highly pathogenic form of the virus responsible for FIP. However, no specific mutation has yet been identified, and it is unlikely that such a mutation exists.

FCoV is a highly contagious virus. Feces are the main source of infection, with litter trays a major transmission factor within groups of cats. The infection is mainly transmitted orally, usually indirectly, after contact with objects contaminated with feces, such as litter trays, cleaning accessories (shovels, brushes, vacuum cleaners), and shoes. In addition, grooming contaminated paws after using the litter tray is also a route of contamination. As a result, transmission is mainly via the fecal-oral route.

FIP particularly affects cats of certain breeds, as well as youngsters under the age of two. In addition, a large number of cats developing FIP come from multi-cat households or have a history of being housed in such households. A recent stress factor, such as adoption, shelter stay, sterilization, upper respiratory disease, or vaccination, is frequently observed in their history. The most frequent clinical signs include effusions (usually abdominal and/or pleural, sometimes pericardial or scrotal), accompanied by fever, anorexia, and weight loss. Enlarged abdominal lymph nodes are also frequently observed, particularly in cats without effusions. Ocular symptoms (e.g. uveitis) and neurological symptoms (e.g. ataxia) may also occur.

When an effusion is present, its analysis (cytology, biochemistry, and FCoV antigen or RNA testing) is crucial to the diagnosis of FIP. In the absence of effusion, fine-needle aspiration (FNA) of affected organs for cytology and FCoV RNA analysis is also useful.