Feline infectious peritonitis: what's new? (Proceedings)

Feline Infectious Peritonitis (FIP) represents the worst outcome of infection with a common group of viruses: the feline coronaviruses (FCoVs). Coronaviruses are common pathogens found in many mammals and birds. They have the largest RNA genome of any RNA virus. Because of this, errors in replication of genetic material will occur more commonly than in other smaller RNA viruses. Ultimately, this means that no two coronaviruses are identical! This realization has revolutionized our thinking about the spread of FIP.

Quoting from Dr. Marian Horzinek: "Although generally associated with acute, self-limiting enteric and respiratory infections, coronaviruses can persist in infected animals. We have shown this to be true for feline coronavirus, which normally cause only mild enteric infections, and which occur in almost any cattery in Western Europe and America. The low-virulence "enteric" FCoVs and the disease-causing FIPVs are closely related genetically; we think that the latter are virulent variants of the former, which arise in individual FCoV-infected hosts. This means that no two cases of FIP are caused by the same virus, and that horizontal transmission, i.e., cat-to-cat transfer is rather the exception than the rule.

Supporting this we see that serologic studies in the United States have shown that between 75 – 100% of purebred cats in catteries have FCoV antibodies. This compares to only 30% of non-purebreds living in pet households. Notably, despite these staggering statistics, only approximately 7-8% will develop FIP.

How do cats get it?

Available data supports a model in which chronic carriers maintain endemic infections in cat societies. Virtually every kitten born in a breeding facility becomes infected, probably from its queen, as soon as its maternal protection wanes. Once infected, the cats appear to resist superinfection by closely related FCoVs, with every cat carrying its private, harmless clan of variants.

FIP is a mutation of the benign, non-life threatening enteric coronavirus that causes a self-limiting diarrhea in most purebred kittens.

1. Almost all kittens will become infected with FCoV. Within 24 hours of ingesting FCoV particles, they can be found in tonsil and small intestinal tissues. Within the next 14 days, the cecum, colon, mesenteric lymph nodes and liver will become infected.

2. They will develop antibodies to FCoV within two to four weeks. But they may start to shed FCoV in their feces in as short a time as two days.

3. They develop diarrhea, which resolves.

4. In most cats, the FCoV remains in the enterocytes without causing further illness. These cats will continue to shed FCoV in feces, which can infect other cats and kittens. Virus is spread between cats from the feces or saliva to the oral or nasal tissues. In some cats FCoV strays from the enterocytes infecting circulating monocytes and tissue macrophages. Macrophages and monocytes cannot rid themselves of coronaviral infection.

5. In some cats, the virus will mutate and gradually become a unique virulent virus that can cause FIP.

6. In some of these cats, there will be no cell-mediated immunity (CMI) and the cat will develop "wet" FIP.

7. In some of these cats there will be a partially protective cell mediated immune response which will eventually allow the mutated virus to cause the "dry" form of the disease.

8. In some of these cats, it will be held in check by solid cell mediated immunity until either the CMI declines (e.g. old age, FeLV infection, FIV infection, chemotherapy, etc.)

9. In some of these cats, it may become a latent infection but these "immune" cats continue to shed FCoV to other cats.

10. Feral cats are at low risk for infection as roaming behaviour precludes fecal-oral transmission.

11. At highest risk are kittens raised "under-foot". Kittens raised in isolation with their queens are at next greatest risk. Early weaned and isolated kittens are at the lowest risk for coronaviral infection.

How is the diagnosis of FIP made?

Making the diagnosis of FIP is easiest in the effusive form as fluid cytology can be performed. The pathologist describes a non-septic exudate with high protein content and moderate cellularity (macrophages, non-degenerate neutrophils, lymphocytes, plasma cells and a few red blood cells). The protein is mostly globulins (immunoglobulins) rather than albumin. When the albumin: globulin ratio is < 0.4, the chances of the diagnosis being FIP increase. Because other conditions (congestive heart failure, neoplasia, liver failure, chylothorax, etc.) may also cause similar fluid accumulation in abdominal or thoracic cavities; fluid cytology is extremely helpful.

Blood tests will show non-specific changes including a mature neutrophilia, lymphopenia, non-regenerative anemia and a hyperglobulinemia. This last is due to uncontrolled inflammatory protein and antibody production. Protein electrophoresis will reveal a non-specific polyclonal gammopathy. Any severe inflammatory condition and some types of cancer will show a polyclonal gammopathy. Seventy percent of cats with the "dry" form of FIP have a polyclonal gammopathy; 30% don't even have increased proteins.

Serology is unfortunately not very helpful because it is unable to differentiate between coronaviruses. Some modified live vaccines used in cats may also cause false positive readings because of reaction to bovine serum in the vaccine. Certainly the FIP vaccine, Primucell-FIP™ will also cause false positive reactions. The biggest headache comes from the confusion that we have because there is no specific, unique genetic sequence in FIPV that can be used for serologic assay because there is no consistent FIP virus.

Problems interpreting FIP titres follow:

1) False positive tests occur when:

• A cat has been exposed to and made antibody against FCoV

• A cat has been exposed to and made antibody against CCoV, TGE, some feline vaccines, or Primucell-FIP™ vaccine.

2) False negative tests occur when:

• An infected cat (genetic testing by PCR) isn't making antibody. This cat may well still shed virus in feces, yet test negative on serology.

• A cat with FIP disease may test negative (or low) if either, they have such advanced disease that all antibody is bound in complexes with the viral antigen and therefore none is left in the serum available for the test

• A cat with FIP disease may be immunologically exhausted, protein malnourished from not eating and too sick to make antibody anymore.

• It is an insensitive test to low levels of antibody.

• If the infection is peracute, then the body doesn't have time to make antibodies.

3) There are many reasons why we can't compare titres from different laboratories.

• Different labs use different antigen: some use TGE, some use FCoV, some use different strains of FCoV.

• Different labs use different cut-off points for what supports a diagnosis of positive vs. what is likely negative for FIP.

• Serology is very sensitive to slight changes, thus even using the same serum sample one can end up with different results from the same lab on different days.

In summary, a "positive" coronavirus test does not confirm that a cat has FIP and a "negative" coronavirus antibody test does not rule out a diagnosis of FIP. Importantly, FCoV titers will not predict which cats will succumb to FIP, so testing housemates of a cat who died from FIP provides information that is impossible to interpret.

Polymerase chain reaction (PCR) is a very sensitive and specific way to detect small amounts of viral nucleoprotein. In other words, it is a good way to look for the "footprints" that the virus has taken up residence in the body's cells. It can identify nucleoprotein sequences that are specific/unique to a specific virus; trouble is, there are no sequences that are unique to "FIPV" because each one is different. Additionally, some FCoV can leave the intestinal tract and invade systemically, thus the presence of coronaviral protein sequences in systemic circulation does not confirm infection with a pathogenic coronavirus. PCR is an extremely sensitive test and, as it is so good at picking up tiny amounts of nucleoprotein, picks up stray RNA from the laboratory environment resulting in false positive results. This means that stringent quality control must be used and PCR must be run in a room vented separately, away from other samples and tests. Very few commercial labs are set up to provide this level of quality control. Future tests may be developed to detect immunological changes, such as cytokine profiling and assaying activated coronavirus-specific CD4+ and CD8+ and T cells.

The gold standard of diagnosis remains histopathological examination of biopsied tissues: characteristic changes include a perivascular orientation of pyogranulomas. Generally the ileum, colon, mesenteric lymph nodes and liver will hold the clues, as well as any other tissue that grossly looks abnormal to the surgeon.

How do we treat FIP?

Currently there is no effective treatment or cure for FIP. Traditionally corticosteroids are used along with cyclophosphamide, we can improve/minimize clinical signs and make the patient more comfortable. A promising option that is undergoing study currently is recombinant feline interferon therapy with glucocorticoids. The rationale for this is that cats with FIP fail to produce IFN-gamma. Virbagen Omega™ (Virbac), recombinant feline interferon omega (IFN omega), was first used in treatment of FIP by Ishida. The protocol he reported was IFN omega initially given subcutaneously at 1 MU/kg every other day, and then once every week for variable period if remission was seen. In addition, dexamethasone was given (at 1 mg/kg intrathoracically or intraperitoneally once) followed by oral prednisolone therapy at 2 mg/kg q24h, tapering to 0.5 mg/kg q48h after remission. In Ishida's study, 4 cats of 12 recovered completely and two others survived 4 and 5 months. The cats who recovered completely, had had the effusive form and were relatively older cats. Dr. Addie is seeing success in younger cats as well as cats with non-effusive FIP. Some cats are receiving oral IFN Omega at 50,000 units/day.

Diluting feline interferon to 50,000 units/ml: Virbagen Omega™ comes in vials of 5 or 10 million units. Reconstitute with 1ml of diluent (sterile water or saline). Put the reconstituted 5MU vial into 99mls diluent, or a 10MU vial into 198 mls diluent. Aliquot into 20ml syringes or tubes and freeze. Once reconstituted, IFN Omega remains potent refrigerated for only 3 weeks.

There are a few anecdotal reports of the benefits of pentoxyfylline (Trental™) 10-15 mg/kg PO q12h + prednisolone 1.1 mg/kg PO q24h + interferon alpha 150 units PO q24h early in the course of both wet and dry FIP. Pentoxyfylline is used for the treatment of vascular disease and cerebrovascular diseases where microvascular blood flow is impaired. No properly designed studies have been conducted using this agent in FIP therapy.

What can we do to prevent FIP disease?

A study performed in FIP-affected, coronavirus seropositive catteries compared the seropositivity rates of kittens raised "under foot" (allowed to mix with all other cats), kittens isolated with the queen, and kittens isolated from all adult cats, including the queen, from the age of 4-6 weeks. Of kittens raised "under foot" and kittens raised with the queen, 52% and 30% respectively were seropositive at 12-16 weeks of age. All kittens isolated from all adults in the household were seronegative at 16 weeks of age. This suggests that transmission of coronaviruses to kittens often takes place horizontally after birth by individuals other than the queen. Because coronaviruses can be transmitted indirectly, strict attention to hygiene is also essential to achieve success with this method. Control of FCoV is also related to good nutrition, good general health status, sanitation, and other cattery management procedures.

Both serology and PCR can be used to monitor the success of quarantine and "early weaning" programs and the coronavirus-free status of catteries. Especially, PCR could be useful for monitoring individual animals to be introduced into FCoV-free catteries. One approach based on isolation after early weaning was successful, but successful control could not be repeated by other workers. Another possible approach is the removal of strong shedders from a multi-cat society. These cats can be identified by testing four fecal samples collected at weekly intervals using a Taq-man PCR technique. Strong shedders can be identified and separated from the group, thereby decreasing infection pressure for the remaining cats. It remains to be shown whether this approach will work, however, in conjunction with other measures (keeping cats in small groups, without contact between groups, frequent cleaning of litter boxes, introduction of new cats only after quarantine and PCR testing etc.) the elimination of strong shedders might be useful.

Serology may be used to check exposure rate. Kittens born to FCoV-carrier queens acquire maternal antibodies within the first 24 hours of life. The maternal antibody titres wane over the next four to six weeks at which time if a kitten is no longer exposed to FCoV through lactation, the titres will decline and the kitten will be FCoV free. If titres continue to increase despite early weaning, this is a reflection of environmental exposure from carriers in the cattery.

New information suggests that the virus is more stable in the environment than previously believed. Dried virus in inaccessible areas may survive for several months.

It has been shown that one of the risk factors for development of pathogenic FIP is housing 6 or more cats together. Crowding is also a stress factor and needs to be avoided. Maintaining stable groups and avoiding disruption of introductions is a valuable goal.

Breed cats who produce cats who are resistant to the development of FIP and cull those who are less hearty.

Concurrent diseases and parasitism must be controlled.

Vaccine: A temperature sensitive strain of FCoV was developed which replicates well at the lower temperatures of the upper respiratory tract (31C/87F) but not at systemic temperatures (38-9C/100-102F). Administered intra-nasally, it stimulates local immune responses resulting in IgA antibodies, mimicking natural infection. The virus appears to be less efficacious than the original trials showed especially when one realizes that the vaccine was given to FCoV negative kittens. This, sadly, is not the case in the 'real world". The vaccine is licensed for use in kittens over the age of 16 weeks because it was not possible to show efficacy in kittens younger than this. This is problematic because maternal antibody protection wanes at four to six weeks, leaving the kittens exposed to becoming infected with the FCoV that is present in their environment. Finally, as previously mentioned, use of the vaccine can result in false positive FCoV titres even if the cat was FCoV negative at the time of vaccination.

New forms of vaccines are being researched. These include a DNA vaccine, a recombinant vaccine and a vaccine produced using a genetically modified spumavirus as a vector. Another vaccine based in using non-structural coronaviral genes is being studied in the Netherlands.

Breeders who have had a cat with FIP, should not be ostracized by other breeders, because development of FIP in a coronavirus positive environment is a matter of bad luck and time.