Of all of the infectious disease that cats can acquire, feline infectious peritonitis (FIP) is perhaps the most devastating. FIP is a viral disease caused by a type of virus called a coronavirus. Most cats are exposed to, and become infected with, the coronavirus as kittens.

At worst, kittens may get mild diarrhea. Many show no clinical signs at all.  The immune system makes antibodies against the virus but does not eliminate it, and the virus continues to reside in the intestinal tract usually causing no further problems for the cat.

Occasionally, for reasons not fully understood, the harmless intestinal coronavirus can change or mutate, gaining the ability to leave the intestinal tract and cause horrible problems. The immune system tries to defeat the virus, but the virus manages to evade it. This mutated intestinal coronavirus is now the evil FIP-inducing coronavirus.

The FIP virus causes damage to blood vessels, allowing fluid to leak through the vessel walls. Effusions (collections of fluid) can develop in the abdominal cavity, chest cavity, and pericardium (the sac around the heart) resulting in what is referred to as the “wet” form of FIP. Some cats develop nodular accumulations of inflammatory cells called granulomas throughout many of the body’s organs. This form of the disease is known as the “dry” form of FIP.

Diagnosis of the disease is difficult because clinical signs are vague and non-specific. Most cats are young (usually less than one year), and show lethargy, weight loss, poor appetite, and a fever that doesn’t respond to antibiotics. A serum chemistry panel often only shows elevated protein (mainly in the form of increased globulins) unless the virus has begun to affect the kidneys or liver, in which case the liver and kidney parameters may be abnormal. The wet form is easier to diagnose because the presence of fluid in the abdomen or chest is relatively easy to detect and fluid analysis can give additional information supporting the diagnosis. The dry form remains one of the more challenging diseases for veterinarians to diagnose.

There is no simple blood test for FIP. Many laboratories offer veterinarians an “FIP test”, but these tests only measure antibodies to coronaviruses in general. It cannot distinguish whether the antibodies are there due to the cat being infected with the harmless intestinal version of the coronavirus, or the deadly FIP version of the coronavirus. A positive test does not mean that the cat has FIP. Until a rapid reliable test is developed that allows veterinarians to make a diagnosis, biopsy of the affected organs or tissues remains the only way to definitively diagnose FIP, although Auburn University offers an assay that they claim is very good for diagnosing FIP.

A commercial FIP vaccine is available, however, the efficacy of the vaccine remains questionable. The American Association of Feline Practitioners and the Academy of Feline Medicine, in their regularly published guidelines for feline vaccination, currently do not recommend the FIP vaccine at this time.

FIP is progressive and fatal. Cats with FIP tend to succumb to the disease rather quickly, in a few days or weeks. The “wet” form of FIP tends to progress faster than “dry” FIP. Treatment is generally symptomatic and supportive. Nutritional support, antibiotics, and corticosteroids may produce a temporary alleviation of clinical signs, but the disease invariably progresses.

A few years ago an article appeared in the Journal of Feline Medicine and Surgery about a potential new treatment for FIP. This was very exciting for us feline practitioners, because FIP is a truly devastating disease that leaves us totally helpless. In the study, 12 cats with FIP were given prednisolone and recombinant feline interferon. Four of the cats died relatively quickly. Four cats survived for a short period of time – between 2 and 5 months. The last four, however, showed prolonged survival. This was an uncontrolled study, but it was really all that we had.

UPDATE: Human antiviral ‘GS-441524’ shows great promise against infectious disease in cats Feb2019

UPDATE: Clinical Trial Shows Promise for FIP Treatment Dec2017

Not long after the article came out, I diagnosed a cat with FIP. I mentioned the new study to the client, and they were interested. A problem: the interferon that we use in veterinary practice is recombinant human interferon. Recombinant feline interferon is not available in the United States. It’s available in the European Union. Well… I managed to score some. I used it on the cat, but to no avail. The cat (a kitten actually) succumbed to the disease. Not long after that, a controlled study of the use of recombinant feline interferon in over 50 cats with FIP was published. The study concluded that recombinant feline interferon was ineffective in treating FIP. Darn it.

An article was published in the November 2009 Journal of Feline Medicine and Surgery on the use of polyprenyl immunostimulant for the treatment of FIP. Polyprenyl Immunostimulant is an investigational veterinary drug. The drug is given orally and it has low toxicity. In the report, three cats with the dry form of FIP were treated with this drug. Two of the three cats are still on treatment and are alive and well. Once cat was treated for only 4.5 months; that cat survived 14 months, which is pretty long. (He might have survived longer if the owners had continued treatment.)

Once again we have a small, uncontrolled case study. The authors, however, are university professors who are very well known in the veterinary community. Of course, we’ll have to wait for a larger, controlled study before we can make any conclusion regarding whether this is truly is a good treatment option. It’s gratifying to see that there are veterinarians working hard to try to find some type of treatment… any type of treatment… for this terrible disease.

SEE ALSO: Amy Lowe, DVM wrote a great succinct FIP knowledge sheet