Updated: Feb 3, 2021
What is Feline infectious peritonitis (FIP)?
Feline infectious peritonitis (FIP) is an immune-mediated disease triggered by infection with a feline coronavirus (FCoV). Coronavirus-specific antibodies are present in the majority of cats. However, <5% of FCoV-infected cats develop FIP in multicat households. , 
Where is FIP Virus found in nature?
FCoV infection and FIP occur worldwide with similar prevalence and are found in both domestic and wild cats. There are two strains of FCoV, FCoV serotype I and FCoV serotype II. These subtypes are dependent on their relationship to canine coronavirus (CCoV) antigenically, and their proportions vary in different countries. Most cats with FIP have been infected with FCoV serotype I. However, both serotypes can cause FIP, and both can cause clinically inapparent FCoV infections.
FCoV belongs to the same cluster of coronaviruses found in pigs, dogs, and humans coronaviruses. Coronaviruses have a relatively restricted organ preference, mainly infecting the respiratory and/or gastrointestinal tracts. In cats, however, coronavirus infections can, in certain circumstances, involve multiple organs.
How does a cat get (FIP) virus?
FIP is a sporadic disease caused by viral variations that develop within each specific cat. The pathogenesis (development) of FIP is unclear, but there are two main theories.
The “internal mutation theory."
This theory states that cats are infected with the primarily less potent Feline Coronavirus (FCoV) that reproduces in cells within the intestines. A change then occurs in a certain region of the FCoV genome that creates a new phenotype with the ability to replicate within other cells called macrophages. Viral reproduction in macrophages is the defining event in FIP, a critical contributing factor for the development of FIP. Macrophages replicate the mutated coronavirus and carry it to target tissues such as the abdominal cavity, thorax, kidneys, eyes, and the nervous system, resulting in widespread immune-mediated inflammation of blood vessels. .
The existence of distinct circulating virulent and avirulent strains in a population, and exposure to the pathogenic strain theory
The viral load and the cat’s immune response determine whether FIP will develop. It is likely both viral genetics and host immunity play a role.
In both theories, the key infective event in the development of FIP is the massive replication of FCoV in macrophages. If the cat does not eliminate macrophages infected with replication-competent virus early in the infection, the presence of the virus within circulating macrophages initiates an ultimately fatal immune-mediated reaction, which defines FIP.
What factors contribute to the development of FIP in cats?
Factors that increase FCoV replication in the intestines (and increase the probability of the mutation) include:-
■ Young age,
■ Breed predisposition,
■ Immune status,
■ Corticosteroid treatment,
■ Dosage and virulence of the virus and
■ Reinfection rate in multicat households.
■ Crowded environments, such as catteries or shelters, also may increase stress and exposure to FCoV.
Whenever FCoV infection exists, so does the potential for the development of FIP.
How is FIP transmitted?
FCoV and FIP are a major problem in multicat households where many cats are kept together in a confined space (e.g., catteries, shelters, pet stores). FCoV is found less commonly in free-roaming community cats because they do not typically use the same locations to bury their faeces; shared litter boxes are a significant source of transmission in multicat households.
A cat becomes infected with FCoV via a faecal-oral transmission (or less commonly, inhalation). The main site of viral multiplication is the intestinal lining. Replication of FCoV in the intestinal cells leads to diarrhoea in some cats. In many cats, the infection persists for weeks to months in the absence of clinical signs. These cats shed FCoV either intermittently or continually and act as a source of infection for other cats. FIP is a complex immune disease involving viral antigen, antiviral antibodies, and complement. Within weeks after macrophage invasion and replication, virions are found in the cecum, colon, intestinal lymph nodes, spleen, and liver after distribution by macrophages in the whole body, including the CNS.
Most commonly, kittens are infected at the age of 6–8 wk at a time when their maternal antibodies wane, mostly through contact with faeces from their mothers or other FCoV-excreting cats.
FCoV is a relatively fragile virus that is destroyed by most household disinfectants and detergents. However, it may survive in cold or dry conditions (e.g., in carpet) for as long as 7 wk outside the cat. Indirect fomite transmission is possible, and the virus can be transmitted for a short time via clothes, toys, and grooming tools.
What are the clinical signs of FIP?
■ Progressive distention of the abdomen due to fluid buildup
■ Lung involvement and difficulty breathing
■ Fever lasting 2 to 5 weeks
■ Loss of appetite
■ Weight loss
■ History of vague illness, including longterm fever, malaise, weight loss, and occasionally major organ system failure (kidneys, liver)
■ Eye and central nervous system signs may occur simultaneously or independently.
■ About 50% of all cats have signs related to the involvement of abdominal organs (kidney, liver, spleen, pancreas, lymph nodes)
NB: Reproductive disorders, neonatal deaths, and fading kittens are not usually associated with FIP.
How is FIP Treated?
Treatment of cats with FIP remains frustrating and is limited to the cases that respond favourably within the first few days. The prognosis for a cat with FIP is very poor. Cats that show no improvement within 3 days after treatment initiation are unlikely to show any benefit from therapy and euthanasia should be considered. Longer survival or remission from clinical signs is rare.