Feline Infectious Peritonitis (FIP): an in-depth look.
Please note: this section is intended for individuals who want to learn more and have a deeper understanding of feline coronavirus and FIP. Basic knowledge of veterinary/scientific terms is needed to fully comprehend this section which was created for information purposes ONLY.
If you are a veterinarian, veterinary student, vet tech or hold any other type of position in the veterinary field, please visit Dr. Diane D. Addie’s website catvirus.com.
If you are a pet parent with a sick cat, please consult a qualified veterinarian IMMEDIATELY.
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FCoV is a virus of the gastrointestinal tract, most infections are either asymptomatic or cause diarrhea, especially in kittens as maternally derived antibody wanes at between five and seven weeks of age. The virus is commonly referred to as feline enteric coronavirus (FECV), but this is a misleading name because all FCoVs got through a systemic phase. From the gut, the virus very briefly undergoes a systemic phase,  returning to the gut from where it is shed in the feces.
In brief, feline infectious peritonitis (FIP) is the result of an excessive inflammatory response to infection with feline coronavirus (FCoV). The pathogenesis of FIP is complicated: the reductionist view is that it is entirely due to mutation of the virus, enabling it to enter or replicate more successfully in monocytes.  While this internal mutation theory is the most widely accepted, no consistent mutation has been identified. The holistic approach is that FIP occurs as a result of a number of factors, including viral load (some strains are undoubtedly more virulent than others) and the immune status and general health of the cat (host).
The defining event in FIP developing is viral replication in macrophages, but it is not the virus that causes widespread damage in FIP.
The disease is a consequence of the cat’s immune reaction to the virus and any cat infected with FCoV has the potential to develop FIP.
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