How does FCoV cause FIP
Feline infectious peritonitis (FIP) is a viral disease of cats that occurs throughout the world and is almost invariably a fatal disease.
Causes
Feline Infectious Peritonitis is caused by infection with a virus known as feline coronavirus. Coronaviruses are a common group of viruses that often infect the upper respiratory tract or gastrointestinal tract in different animals. Different species of coronavirus are adapted to cause disease in different animals. In humans, coronaviruses are one of the frequent causes of the common cold.
It is important to note that the coronaviruses affecting cats are different from the coronavirus which causes COVID-19 in people. In very technical terms, coronaviruses that affect cats (called feline coronaviruses or FCoV) are alpha-coronaviruses and the current SARS-Cov-2 coronavirus which causes COVID-19 in people is a beta-coronavirus. These are different diseases which occur in different species caused by different types of coronavirus. The FIP-causing feline coronavirus cannot infect people and the COVID-19 causing coronavirus not so often cause infection in cats.
Infection with coronavirus is actually very common in cats, but most of the time it does not cause any problems, other than perhaps mild self-limiting diarrhea. There are some hypothesis, in particular of Prf. Pedersen, that the virus mutates (changes) to a strain of coronavirus which has the potential to cause disease. This mutated strain is the cause of FIP.
In an affected cat, the virus spreads throughout the body and can cause a wide range of different signs (including peritonitis with the accumulation of fluid in the abdomen, but in other cats, fluid may accumulate in the chest cavity; in others, the virus may cause inflammation affecting the brain, eyes, liver, kidneys or elsewhere).
How do cats get infected with coronavirus?
Coronavirus infection is extremely widespread in cats, especially where large numbers of cats are kept together. It is estimated that 25–40% of household pet cats have been infected with FCoV, but the infection rate increases to 80 – 100 per cent for cats kept in multi-cat households or colonies.
Cats are infected mostly by indirect transmission through sharing letter tray with cats shedding virus in their faeces: virus transmission is faecal - oral.
course bonus: video from Dr. Diane Addie of infection transmission
The virus mainly infects the intestinal tract, where it replicates. FCoV is shed in the faeces and may survive in the environment for a short while (several days or a few weeks), but is readily destroyed by common disinfectants. Infection is caused when a cat ingests the virus (e.g., through licking). The relationship between the virus and the cat is complex – some cats may remain persistently infected with FCoV and shed virus in their faeces the whole time; others may be infected, develop a strong immunity and be protected from future infections; and others may be infected and manage to eliminate the virus, but then get recurrent infections.