Feline Infectious Peritonitis (FIP)

Introduction: Feline infectious peritonitis is a chronic progressive fatal infectious disease of cats caused by feline coronavirus. The disease has two forms: exudative (wet type) and non-exudative (dry type). The former is characterized by fluid accumulation in the body cavity (especially the abdominal cavity), while the latter is characterized by granulomatous lesions in various organs and related clinical symptoms. . Supportive care is usually used, with immunosuppressive and anti-inflammatory drugs.

Feline infectious peritonitis (FIP) is a chronic progressive fatal infectious disease of cats caused by feline coronavirus (FCoV).

There are two forms of the disease: exudative (wet type) and non-exudative (dry type):

  • The exudative type is characterized by fluid accumulation in body cavities, especially the peritoneal cavity;
  • The non-exudative type is characterized by granulomatous lesions in various organs and associated clinical symptoms.

The disease was first discovered in the 1960s, but the prevention, control and treatment of the disease are still unresolved.

Feline Infectious Peritonitis (FIP) Imaging

Feline Infectious Peritonitis (FIP) Imaging
X-rays showed accumulation of stool in the colorectum, a small amount of gas accumulation, and liquid secretion in the abdominal cavity. Source: Internet.


FCoV is a member of the Coronaviridae family Coronaviridae. It may be derived from the same virus as porcine transmissible gastroenteritis virus and canine coronavirus (CCV), and it is a mutant strain of cross-infection between species.

CCV can experimentally infect cats and cause pathological changes typical of exudative peritonitis. There is also evidence that attenuated CCV vaccine can induce FIP-like lesions in dogs.

FCoV has 2 biotypes, feline infectious peritonitis virus (FIPV) and feline enteric coronavirus (FECV).

  • The two are different in biological characteristics, but they are the same in morphology and antigenicity;
  • FIPV usually causes infectious peritonitis;
  • FECV causes only self-resolving mild enteritis.

FIPV may be a mutant strain of FECV, derived from FIPV and FECV from the same group of cats, and the genetic background of the two is more closely related than FIPV and FECV isolated from different regions.

After FECV is mutated into FIPV, FIPV can replicate in macrophages, so that FIPV can escape from the intestinal tract and cause infectious peritonitis.

It is generally believed that there are two serotypes of FCoV.

  • Serotype I infection is more common clinically. The virus cannot be neutralized by CCV antiserum and does not grow well on cells.
  • Serotype II may be a virus produced by recombination between serotype I and CCV.

FCoV virus resistance is weak, but it can maintain infectivity on the surface of external environment objects for more than 7 weeks.


FCoV infection spreads worldwide and cats of all ages are susceptible. Studies have shown that 75% to 100% of breed cats and about 25% of domestic cats are positive for FCoV antibodies.

Maternal antibodies of kittens usually disappear at 8 weeks of age, and can be infected and seropositive at 10 weeks of age. FIP is common in young cats. Kittens aged 6 months to 2 years are most susceptible, and cats over 13 years old are also susceptible.

The mortality rate of domestic cats is very low, about 1/5000. In some countries, the mortality rate of sick cats is relatively high, and the mortality rate of purebred cats is about 5%. The mortality rate of sick cats in cat farms at the initial stage of infection can even reach 40%.

Young cats under stress conditions such as pregnancy, weaning, and moving into a new environment, as well as infected cats’ own diseases and feline immunodeficiency diseases are important factors that promote the pathogenesis of FIP.

The disease can be transmitted through contact with sick cats, and clinically healthy carriers are also one of the important sources of infection. Mother cats can infect their kittens. Experiments have confirmed that infected cats mainly transmit the virus through feces, because the intestinal tract is the main site of FECV persistent infection and replication.


Enteritis caused by FECV infection is usually mild and subclinical infection, mainly in post-weaning kittens. In some cases, temporary vomiting and mild or severe diarrhea may occur for about 1 week.

In the initial stage of peritonitis cases, the symptoms of wet and dry type peritonitis are similar, including fever, depression, loss of appetite, lethargy, and sometimes diarrhea, followed by typical symptoms.

Wet peritonitis develops more rapidly than dry peritonitis, and both types of peritonitis present with chronic, fluctuating fever, loss of appetite, and weight loss. However, in dry-type FIP cases, the fever may last longer and the diseased cats may be stunted in growth.

With the development of the disease, 75% of the sick cats developed ascites, 25% of the sick cats developed pleural effusion, and pericardial fluid increased, resulting in dyspnea and dull heart sounds. The male cat’s scrotum becomes enlarged, and the liver is involved in the later stage of the disease, resulting in jaundice.

Granulomas appear in various organs of cats with dry-type FIP disease, and corresponding clinical symptoms appear:

  • Among them, abdominal organs such as liver and mesenteric lymph nodes are most seriously affected.
  • Kidney disease
  • Other affected parts include the central nervous system and eyes. The cats showed behavioral changes such as ataxia and mild paralysis, disorientation, nystagmus, seizures, sensory hypersensitivity, and peripheral neuritis. Typical eye diseases cause uveitis (eg, iritis, anterior uveitis), choroid and retinitis. Sometimes eye lesions are the only clinical manifestation of the disease.


It is difficult to diagnose mild temporary diarrhea caused by feline enteritis coronavirus (FECV) infection, mainly because there are many reasons for mild diarrhea, and the pathogenic virus can be detected by electron microscope observation.

The clinical diagnosis of FIP is histopathological examination.

Wet-type FIP is easier to diagnose than dry-type FIP. Typical pleural and peritoneal effusions appear in wet FIP. The effusion is light yellow, viscous, high in protein content, foams easily when shaken, and can coagulate when left standing. It contains a moderate amount of inflammatory cells, including macrophages and neutrophils etc.

Cats with central nervous system and ocular lesions have increased protein levels in the cerebrospinal and aqueous fluid.

Serological detection of animal antibodies and PCR detection of FCoV RNA, although specific, can only be used as one of the auxiliary methods for diagnosis because healthy cats also carry positive serum and virus.


Currently, there is no effective treatment. Supportive care is usually used, with immunosuppressive and anti-inflammatory drugs. Corticosteroids, cyclophosphamide, and some immunomodulatory drugs such as interferon have been tested for the treatment of this disease. It has been proved that these drugs can prolong the life of sick cats to a certain extent, but they cannot be cured.

Both conventional and recombinant vaccines for FIP have been ineffective. The main reason is that FIPV infection has the phenomenon of antibody-dependent enhancement (ADE), that is, when there are anti-FCoV antibodies in cats, vaccination with strong virus can promote the occurrence of FIP. However, there is still a lack of direct evidence whether this phenomenon exists in naturally infected cats.

The mechanism of ADE is relatively complicated. A large number of antibodies produced by infected cats can neutralize the virus in cultured cells. However, in cats with FIP, after the antibody binds to the virus, it can promote the ingestion of the virus into the macrophage by binding to the Fc receptor on the macrophage. At the same time, the complement-fixing receptors are also involved, the complement system is activated, and blood coagulation cytokines are released, resulting in peritonitis and exudation of body fluids.

Recently, it was found that the temperature-sensitive mutant strain prepared by serum type II DF2 strain can prevent the occurrence of this disease through intranasal inoculation. The vaccine proliferates only in the upper respiratory tract, can induce strong local mucosal immunity (IgA) and cellular immunity, and does not induce ADE. The vaccine has been shown to be safe and effective in both laboratory and field trials, and it has been sold in many parts of the world. This vaccine is recommended for kittens over 16 weeks of age.