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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.
History
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.
Serology
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.
Vaccination
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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