Feline Infectious Peritonitis is caused by a class of viruses called Coronaviruses. The Feline Infectious Peritonitis Virus (FIPV) is just one of many coronaviruses which are capable of infecting cats. Others have long names and are usually designated by letters. These include CCV, TGEV, HCV. All of these coronaviruses have similar genetic material and cannot be differentiated from each other, even at the protein level. Feline Enteric Coronavirus (FECV) is a different coronavirus of cats that is thought to cause a mild intestine infection in kittens. It cannot grow in white blood cells (wbc's) while FIPV can grow in wbc's.
FIPV is actually several strains of coronavirus which vary in ability to cause disease from one strain to the next. FIPV is capable of producing FIP, intestinal inflammation (enteritis) or both in kittens and cats. It is capable of growth in wbc's. Body tissues which are infected by the FIPV are liver, spleen, lymphnodes, eyes and the central nervous system (CNS). There are two types of feline coronavirus that are recognized based on laboratory tests and are termed FCoV-1 and FCoV-2. There are FIPV and FECV strains of virus in both groups. Based on the laboratory tests, 85% of the infections with FIP in cats in the U.S. is caused by FCoV-1. With present tests used in veterinary practices, it is not possible to distinguish clinically between these two types of FCoV. The prevalence of FIPV in the general cat population is unknown but is usually estimated at < 2%. In larger cat households or, in endemic catteries, it may be as high as 10%. The incidence of FIP is greatest in cats less than 5 years old or in cats greater than 10 to 11 years old. Pure bred cats are at higher risk, probably because of their cattery life style. Catteries, multi-cat households, and outdoor cats are at higher risk than solitary, indoor cats. The virus is easily inactivated with common household detergents.
The primary route of transmission is still subject to debate but is either through the oral or nasal route. Most researchers believe that the primary mode of transmission is ingestion of virus following contact with an acutely infected cats who are shedding the virus. Cats are thought to be the only natural host for infection. Although there is no recognized environmental reservoir for the virus, a study at Cornell University indicated that under favorable conditions the virus may survive in the environment for up to 7 weeks. Cats infected with FIPV can harbor the virus for weeks to years without showing clinical signs.
Sensitization is thought to occur in some cats with preexisting antibody to FIPV. This preexisting antibody has been shown experimentally to accelerate the disease process resulting in more serious disease as compared to cats without preexisting antibody. Antibodies to FIPV do not seem to kill the virus and may actually promote and accelerate the disease process.
There are three proposed mechanisms for the role of antibody (the body's response) in FIP: 1. Complexes of the virus and the antibody made by the body (Ag-Ab) form on infected cells which leads to damage to local tissues. 2. Antibody may coat free virus particles leading to increased uptake by wbc's and thus spread of virus. 3. Inflammation of blood vessels occurs due to (Ag-Ab) complex deposition in small to medium sized blood vessels (vasculitis) . Vascular damage fluid to leak into the space around body cells causing the characteristic FIP effusion. FIPV preferentially replicates in a type of wbc known as a macrophage. The spread of infection is determined by resistance of the individual cat's macrophages to FIPV. The macrophage then serves to spread FIPV throughout the body. During clinical disease, infected macrophages in release a variety of chemicals that worsen the body's inflammatory response.
In general, the clinical signs of FIP are nonspecific and vague. Symptoms include unexplained loss of appetite, depression and weight loss. A chronic fluctuating fever that is unresponsive to antibiotics or corticosteroids is usually present. Reproductive failure and early kitten death are known to occur. The duration of clinical illness prior to death is usually 3-to-6 weeks in the form of FIP in which fluid fills the body cavities ("wet FIP" of FIP). Wet FIP results in fluid build-up in the abdomin in about 85% of cases. Effusion is detected on physical examination, or using radiography or ultrasound. Peritoneal inflammation may involve the GI tract, hepatobiliary system, or pancreas with the resulting clinical signs of vomiting, diarrhea, and icterus. Exudative pleuritis may occur in one-third of cats with effusive FIP with clinical signs of dyspnea and exercise intolerance. On physical examination these cats have muffled heart and lung sounds. Pericardial effusion may also be present. In general, the fluid of from FIP infection is viscid, and egg-white in consistency. Flecks of fibrin may appear floating in the fluid, and the fluid will clot on standing. The fluid generally has a high specific gravity, ranging from 1.018 to 1.030. In the "dry form" of FIP, prolonged illness is possible and has been reported as long as several years after initial infection. The incubation period can vary from several days to several months before the onset of disease. Dry FIP (noneffusive FIP) creates small abscesses (pyogranulomas). Effusion is minimal or absent. These pyogranulomas may involve any tissue but are most commonly seen in the liver, kidney, and the lining of the abdomen (peritoneum). Less frequently lesions may be seen in the CNS and eyes. Disease manifestations in the eyes include: anterior uveitis (iridocyclitis) characterized by a small pupil (miosis), increased protein in the fluid in the front of the eye (aqueous flare), wbc's in the front of the eye (hypopyon), bleeding in the front of the eye (hyphema), wbc accumulations on the inside of the cornea (keratic precipitates), adhesions of the iris (colored portion of the eye) to the lens (synechia) and inflammation of the back of the eye (posterior uveitis or chorioretinitis).
The CNS form results in diffuse pyogranulomatous inflammation of the brain (menigoencephalitis). Signs include paralysis of the hind legs, weakness, trembling, vertigo, seizures, and personality changes.
While newer information and tests allow more accurate diagnosis of FIP, it is still very difficult to make an accurate clinical diagnosis in many cases, especially in the dry form of the disease. No single laboratory test will confirm the diagnosis short of pathology or special stains (immunohistochemistry) on tissues removed by biopsy or autopsy. The clinical diagnosis of FIP is like putting together a puzzle. No one piece of the puzzle will provide the picture--it is only after many pieces of the puzzle are in place that an accurate diagnosis of FIP can be made. It is also known today, based on immunohistochemistry analysis, that FCoVs can cause more disease than just the traditional "wet" or "dry" FIP. The cat's history along with characteristic clinical signs are important in making a diagnosis. The age of the cat, the source of the cat, and the opportunity for exposure to FCoV are important in this regard. Laboratory findings are variable and nonspecific. Analysis of the blood are variable and has a low predictive value for diagnosis of FIP. Anemia is only 39% predictive for a diagnosis of FIP (predictive value of a positive test), and low wbc count (leukopenia) has only a 35% predictive value. Low blood protein (hyperproteinemia) with a high serum globulin to albumin ratio may be present. Evaluation of fluids from body cavities reveals a fluid which is yellow, sticky, with high specific gravity, protein, fibrin, inflammatory cells, and no bacteria. Serologic tests indicate only that a cat has been infected with a coronavirus. This could be FECV, FIPV or any of several other non-feline coronaviruses which can infect cats. A "positive" titer is consistent with FIP, but is still not diagnostic. A negative titer does not mean that a given cat does not have FIP. More specific tests are definitely needed. Histopathology by biopsy or necropsy is the only definitive means of diagnosing FIP. Findings of pyogranulomatous inflammation or pyogranulomas are suggestive of FIP.
Treatment of FIP is usually unrewarding and virtually all cats die from infection once clinical signs have developed. Cats with disease localized to the eyes may live for months to years. For that reason, eye conditions should be treated to help the cat feel better.
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