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Nature of the disease
Feline infectious peritonitis (FIP) is a disease of cats caused by a coronavirus infection. 

Many different strains of coronavirus are capable of infecting cats, but most do not produce serious disease. FIP-producing strains are distinguished by their ability to invade and grow in certain white blood cells. 

The infected cells transport the virus throughout the cat's body. An intense inflammatory reaction occurs in the tissues where these virus-infected cells locate. It is this interaction between the immune system and the virus that is responsible for the disease.

SPC List D disease
Susceptible species
FIP is a disease of cats. Young cats and very old cats (14 years and older) appear most susceptible. FIP tends to be more prevalent in cats bred or raised in catteries or maintained in multiple-cat environments.
FIP has been reported from the UK, Canada, USA, South Africa, Japan, Europe, Australia and New Zealand. It is probably widely distributed in cat populations, worldwide.
Clinical signs 
Initial exposure to the FIP virus usually results in no obvious clinical disease, although some cats may experience a mild upper respiratory disease (sneezing, watery eyes, and watery nasal discharge), or mild intestinal disease. Most cats completely recover, although some of them may become virus carriers.
A small percentage of exposed cats develop the lethal disease weeks, months, or perhaps years after primary infection. The major forms of lethal FIP are effusive (wet) FIP, and non-effusive (dry) FIP.
Effusive form
There is accumulation of substantial quantities of fluid in body cavities. Some of these animals appear profoundly ‘pot-bellied’. With excessive fluid accumulation, it may become difficult for the cat to breathe normally.

Non-effusive form 
Onset is usually slower. Fluid accumulation is minimal, although weight loss, depression, anaemia, and fever are almost always present.
The most common clinical signs of FIP are non-specific and include:

  • Fluctuating fever
  • Intermittent inappetence
  • Lethargy
  • Weight loss
  • Rough hair coat

The type and development of disease is quite complex and, in large part, dependent on the status of the animal’s immune system. Signs of kidney failure (increased water consumption and urination), liver failure (jaundice), pancreatic disease (vomiting, diarrhoea, diabetes), neurologic disease (loss of balance, behavioural changes, paralysis, seizures), enteritis (vomiting, diarrhoea), or eye disease (inflammation, blindness) may be seen in various combinations.

Mortality from exposure to the virus is sporadic, even in populations of cats where FIP virus carriers are known to be present.

Post-mortem findings 
Cats will be in poor condition. In the effusive form, excessive fluid in the abdominal and chest cavities is the most notable feature.
There may also be evidence of kidney or liver failure e.g. uraemia, jaundice.
Differential diagnosis 
In both forms, clinical signs can be quite variable; virtually any organ or soft tissue system can become affected, thus mimicking many diseases.
Specimens required for diagnosis 
A presumptive diagnosis of FIP can usually be made on the basis of clinical signs, routine laboratory tests, and evaluation of abdominal or chest fluid. Some cases, however, present a diagnostic challenge, since the signs of illness are not distinct for FIP.

In the effusive form, body cavity fluid samples should be collected. When neurological abnormalities are present, cerebral spinal fluid should also be collected for analysis.

Tissue samples should be collected from affected organs for histological examination. This is the most specific of all the available tests for the definitive diagnosis of FIP. In the live animal tissue biopsies may be necessary.

Blood samples can be taken for serology. This is an area of considerable controversy. A high level of antibody to the FIP virus along with signs of the disease (which may or may not be specific) is not necessarily diagnostic for active FIP infection — both false positives and false negatives are common.

Infected cats shed coronavirus in their saliva and faeces. Most cats become infected by inhaling or ingesting the virus, either by direct contact with an infected cat, or by contact with virus-contaminated surfaces like clothing, bedding, feeding bowls, or toys. The virus can also be passed to unborn kittens or via the milk to newborns.

Some apparently healthy cats may carry the virus, which can be shed intermittently in bodily fluids or faeces, with faeces being the most common source and oral infection the most common route.
The virus may persist in the environment for several weeks, but is easily killed with common household disinfectants.

Risk of introduction   
FIP is most likely to be introduced with an infected cat. Some apparently healthy cats may carry the virus, which can be shed intermittently in bodily fluids or faeces.
Control / vaccines  
There is no specific treatment for FIP and the prognosis is poor once a positive diagnosis has been made. As there appears to be an immune-mediated component to the progression and severity of the disease, immune-suppressive and/or immune-modulating drugs have been tried. Some antiviral drugs alone or in combination with immune modulators are being investigated.

There is an intranasal live vaccine available for cats that are at least 16 weeks old. Initially, two doses are given separated by three weeks; thereafter an annual booster is required. According to the manufacturers, protection provided by the vaccine is 50-70%, but there are independent studies, which question whether the vaccine is in fact beneficial.

  • Feline Infectious Peritonitis, In Merck Veterinary Manual, National Publishing Inc. Eight ed, 1998, Philadelphia, p 551-555