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Plain Talk About FIP

By Barbara Redalia

Although there is still no reliable diagnostic test, no adequate vaccine for young kittens, and no effective treatment, there has been significant progress in recent years in our knowledge about FIP. It is the goal of this article to present information about FIP which would be useful to cat-owners. Much of this information was presented in scientific detail at the UC Davis FIP Workshop August 12-14, 1994. The author has attempted to summarize the main ideas which emerged from this workshop without the burden of scientific proof. Errors, if there be such, are those of the author rather than of the presenters at the conference. 


The virus of Feline Infectious Peritonitis (FIPV) probably arose as a mutation, or mutations, of the virus of Feline Enteric Corona Virus (FECV), which is commonly found in cats living in groups. In 1968 it was determined that the cause of FIP was a virus. Since 1970 it has been known that the virus was of the corona family. In 1972 we learned that there were two different forms of FIP; wet, and dry. In 1981, with the discovery of FECV, which could not be distinguished in the usual ways from the FIP virus, it became clear that although these viruses looked alike, and provoked similar antibody/antigen reactions there is a major biologic difference between them. That difference allows many cats to be positive for corona virus, and be healthy, but with only a mutation or two FECV becomes the deadly virus of FIP. The virus of FIP is a mutant form of FECV which has acquired the ability to make copies of itself in macrophages (white cells) and thus to spread throughout the animal. 

FECV and the FIP virus co-exist, environmentally, and cases of FIP have arisen, after several months, among cats which had been infected with FECV in an otherwise pathogen-free (disease germ-free) cat population. The genetic structure of such FIPVs has been found to be closer to the genetic structure of FECV in that locale than it was to an FIP virus from a distance. (Corona viruses in general, such as in mice and pigs, are known to mutate easily.)  Although it now appears certain that FIP is a mutant form of FECV, we do not know when the mutation occurs, that is, whether it causes disease in the same cat in which it originates, or does not cause FIP until it passes through the intestine of the cat and is ingested by a cat which is susceptible. 

Spread of Infection

FECV thrives in environments in which large numbers of cats share food, water and litter. In single-cat households only 25% of the cats test positive for antibodies to corona virus; in catteries from 75 to 100% of cats test positive.  The percentage of cats which test positive for FECV rises with increases in the numbers of kittens, movement of cats in and out of the cattery, and years involved in breeding cats. There was no clear relationship between attendance at cat shows and the incidence of FIP in a cattery. Cattery breeding practices did not appear to have a major influence on the presence of FIP in the cattery. However, permanent introduction of new cats did appear to increase the probability of FIP in a cattery.  Susceptible cats brought into an FIP endemic environment typically show antibody evidence of infection in 1 to 2 weeks. Nearly all cat shelters and catteries experience endemic corona virus infection, but relatively few report such losses. Why? Kitten deaths frequently are not diagnosed. Husbandry varies from one cattery to another, and can influence the replication of FECV and the mutation which allows it to make copies of itself (replicate) in macrophages. Other factors which can influence the probability of development of FIP in a particular animal are: existence of FIP in a littermate, stresses of various sorts, including surgeries, pregnancy in a young cat, weaning, sale, shipment, and adaptation to a new home.  Successful control of FIP depends upon prevention of transmission of the infection from infected animals to susceptible animals by means of the fecal-oral route. Infected animals probably remain carriers for a few weeks, or possibly months. In a cattery with endemic FIP you can delay breeding by six months to a year. Virus particles may survive several weeks in a litter box or on a porous floor. You can disinfect your floors and litter boxes frequently. Susceptible animals include kittens in the age range 6 to 16 weeks, susceptible cats brought from outside, or previously infected cats which have lost their immunity. If you suspect a queen is an FIP carrier you can prevent her from infecting her kittens if you wean her kittens by six weeks and isolate them from the mother and all other cats until they are 16 weeks old. If you have FIP in your cattery you can close the cattery to susceptible animals from the outside for any reason. 

Signs, Symptoms & Diagnosis

FECV infection may escape notice as it is characterized by a few days of diarrhea and sometimes vomiting. The duration of an FIP infection may vary from a few days to several months, proceeding more rapidly in wet FIP than in the dry form. Both wet and dry forms may be seen at different times in the same cat. Symptoms common to both wet and dry FIP include a fever which does not respond to antibiotics, lack of appetite, weight loss, and stunting of growth in kittens. Peritonitis and an excess of fluid in the abdomen are usual in wet FIP. 

Symptoms of the dry form of FIP may include: granulomas which can be felt or seen on ultrasound, a variety of irregularities in the eyes, and central nervous system signs, such as withdrawal, hiding, rage, or convulsions. Laboratory evidence from the examination of body fluids is important. A diagnosis of FECV cannot be made on clinical signs alone; there are too many other causes of transient diarrhea.  A diagnosis of FIP by serologic means is not possible because the available tests cannot distinguish between antibodies produced by FECV exposure and antibodies generated by FIP exposure. 


Cats with a positive FCoV titer (level of antibodies above 1:100) have probably had an exposure to a corona virus, but you don't know whether it was FECV or FIP from the titer alone. Whether the cat is sick or well, and the environment from which it came, help to interpret the titer. Cats should never be euthanized based only on a positive titer for the corona virus. Cats with higher FCoV titers (above 1:100) are more likely to be shedding virus in their feces, and thus to be contagious to other cats, including their own kittens from age 6 to 16 weeks. 

Prognosis and Treatment

Because there is no adequate diagnostic test to distinguish between FECV and FIPV we do not know what proportion of cats that are infected with FIPV survive. Cats that recover from FIP usually have not been noticeably ill. Among those cats which are obviously ill and are subsequently found to have FIP, fewer than 5% survive.  With FIP infection antibiotics are ineffective, anti-inflammatory drugs merely delay the inevitable, and immunostimulants have not been proved to be of ultimate benefit. It usually makes sense to support the cat which is active and eating well, but not to prolong the suffering of the animal with severe disease. FECV infection is usually mild and needs only supportive treatment and fluid replacement for several days. Kittens typically become susceptible when between 6 and 10 weeks of age, as they lose their maternal immunity. Although kittens may be infected that early the main losses occur between the age of 6 and 18 months. 

Prevention of All Coronavirus Infections

Because FIPV is but a mutation or two from FECV, prevention of FIP should be directed at prevention of all FECV infections. Husbandry can be of significant assistance in preventing fecal contamination of the environment, and in preventing susceptible animals from coming into contact with fecal contaminated litter, floors, food, water, and cages. This may require construction of special facilities for the cats, with separate quarters for each pregnant/lactating queen, and establishing practices which minimize fecal-oral contact, especially between virus-shedders and susceptible animals. It is possible to gain some idea as to which cats are likely to be shedding virus from the level of the cat's titer for coronavirus antibodies. FCoV (a general term including both FECV and FIPV) can be eliminated from catteries because; a) infected cats usually do not shed the virus for more than a few months, and b) kittens born to carrier queens seem to be immune for their first 6 weeks. There appears to be a proportional relationship between the queen's coronavirus titer and the probability that she is shedding virus. A titer below 1:25 might be used as an indicator that a queen could be safely bred or that her kittens might safely remain with her after 6 weeks of age.  Queens with a titer of 1:100 or above are often infectious. The kittens of such a queen are only protected by maternal immunity for the first 4 to 6 weeks. From that point on they need to be isolated from her and from all other cats until their own immunity kicks in at about 12 to 16 weeks. Kittens reared with a positive queen will usually test positive up to 4 to 6 weeks of age, and negative between 6 and 12 weeks of age. Thus, if kittens were tested at 6 to 12 weeks of age and tested negative, one might wrongly assume that they had never been exposed to the coronavirus. If tested again between 12 and 16 weeks one would expect they would again test positive for coronavirus antibodies. When a cat or kitten has been lost to FIP it is probably inadvisable to screen an entire cattery for FCoV antibody titers because there is no way to be sure when an individual cat was infected or whether a cat which tests positive will have FIP in the future. Coronavirus serology thus should not be used as the sole criterion in a culling program to rid a cattery of coronavirus. If 6-12 months is allowed for animals to stop shedding virus, and new infections are prevented, the virus can be eradicated. The success of such an effort requires great care to avoid the spread of infection on contaminated clothing, hands, litter boxes, cages, etc., and the greater the number of cats involved the more difficult it would become. 


The Primucell FIP vaccine has been demonstrated to be effective (50-75% efficacy) for cats that are 16 weeks of age or more. It has not proved to be effective in kittens less than 16 weeks or in cats which are FCoV positive. Effectiveness has not been proved when used on animals coming from households with endemic coronavirus probably because those animals are already exposed. 

Cats living in households of 1 to 3 cats, where the expected incidence of FIP is only about 1/5,000 would be the group expected to develop the best immunity from the Primucell vaccine, but the use of the vaccine in such environments would be quite inefficient in reducing the amount of FIP in the total cat population. For example, if the vaccine were 50% effective, one would have to vaccinate 10,000 cats to save one cat from infection. Some cattery owners have experimented with the use of Primucell FIP on 6-week-old kittens, the rationale being that since kittens are losing their maternal antibody at that age they may respond to vaccination and develop immunity to FECV, and that by preventing FECV we are also preventing them from its mutant form, FIPV. Thus far, experimenters have shown a reduction in virus-shedding in such vaccinated kittens but not a reduction in kitten losses. It remains to be seen whether the benefit outweighs the risk in vaccinating very young kittens. One of the possible risks involves the recombination of coronavirus variants to form new, even more virulent viruses. 

Control by Isolation & Early Weaning

In a study done by Addie in Scotland of 600 kittens born to FCoV positive cats no relationship could be found between the queen's titer, age, or breed and whether her kittens became infected. However, there was a strong relationship between the rearing practice and whether the kittens contracted FCoV. Of those kittens which mixed freely with other cats in the household 124 of 238 (52%) became infected; five died of FIP. Thirty-five of 114 (30%) kept with their mothers until sold became infected. None of the 46 kittens which were isolated with their mothers until 5-6 weeks and then weaned and isolated as a unit were infected.  It appears from the above that kittens cease to be immune from coronavirus infection at 5-6 weeks, and that if the queen is shedding virus they are likely to be infected if they remain with her after that age, or if they mix freely with other cats in the coronavirus positive household. If even two queens and their kittens were mixed together the isolation did not work. What could have happened if one queen was positive and one negative, was that the positive queen was shedding virus which infected the kittens of the negative queen, and they had no maternally derived immunity. When the immunity of the positive queen's kittens waned, after 6 weeks, they were infected by the kittens of the negative queen.  In this study the majority (6 out of 7) of coronavirus positive cats became negative within one year. Three of these cats later were reinfected and became seropositive, and again later became seronegative. It thus appears that cats typically shed coronavirus for less than a year. 

When a study was made to determine the antibody titer of the queens at the time that their litters were infected the antibody titers ranged from 80 to 1280, although most were 160. Apparently cats with antibody levels less than 100 could be shedding virus.  When a comparison was made between households documented to have had FIP and households thought to have had just an FECV coronavirus it was found that approximately 10% of kittens from all coronavirus positive households developed FIP. It was never safe to assume that cats were infected with an innocuous strain of coronavirus. The conclusion of this study was that no cat breeder whose cats have antibodies to FCoV can afford not to isolate and early wean kittens.  After coronavirus has been eradicated from a cattery it can be reintroduced by bringing in a new cat or kitten. Even a cat or kitten with a titer below 100 can be shedding virus. It is recommended that kittens and stud and queen cats from other catteries should have titers of 1:10 or less before being allowed into a coronavirus negative cattery. Clinical signs which can alert one to the presence of coronavirus in a cattery include diarrhea and slow growth of kittens. Regular antibody testing at 3 to 6 month intervals, though expensive, may give useful information. Some breeders may choose to test only kittens or animals they wish to breed shortly, or to sell. It would appear that experienced breeders with catteries of 8-20 cats or more (where there is the highest probability of FIP) would be well-advised to maintain their cattery numbers at a level which would allow the practice of early weaning and strict isolation of litters from all other cats from 6 to 16 weeks. This practice, though difficult, has proved to be life-saving.


We are grateful to Dr. Niels Pederson of the Center for Companion Animal Health, School of Veterinary Medicine at UC Davis, Davis, CA 95616, and to Joan Miller of The Winn Feline Foundation, P.O. Box 1005, Manasquan, NJ 08736, and to all of the participants in The FIP Workshop for their untiring efforts to teach us more about FECV/FIP. Your contribution to either organization can help us to learn more about FIP, and to reach the day when there is a reliable diagnostic test, and an effective safe vaccine for kittens.

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