Feline Health
Caring for Cats
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A Winn Feline Foundation Health Article On ...
Feline Leukemia Virus
by Susan Little, DVM, Diplomate ABVP (Feline)
Feline leukemia virus (FeLV) is a retrovirus, a family of viruses which
has many members that infect cats and cause disease and death in cats
around the world. Another well-known feline retrovirus is the feline
immunodeficiency virus (FIV). FeLV and FIV can be found together in the
same cat. The prevalence of FeLV in single-cat households is about 3% and
can be as high as 11% in stray cat populations. In large multi-cat
households and in households where cats roam freely outdoors, the
prevalence can reach as high as 70%. Cats roaming in urban areas are more
likely to be exposed to FeLV (40%) than cats roaming in rural areas (6%).
FeLV has been studied for over 30 years, both for its relevance to the cat
population and because it serves as an animal model for some human
diseases. Research has established key characteristics of FeLV: it is
contagious, it directly causes both fatal cancerous and non-cancerous
diseases, it can lie dormant in the bone marrow for a long time, and it
can be protected against by vaccination. FeLV is not transmissible to
humans or animal species other than the cat family.
FeLV is a fragile virus that does not survive in the environment. Ordinary
household detergents and bleach effectively kill this virus. There is
therefore no danger that cats can be exposed to FeLV in veterinary clinic
waiting rooms or exam rooms, or in cages, or at cat shows unless direct
contact is made with a positive cat who is shedding virus. Transmission of
FeLV requires intimate moist contact. The most common route is contact
with infected saliva through grooming, licking, biting and shared dishes
and litter pans. FeLV can also be transmitted through a blood transfusion,
so all cats who are blood donors are screened for FeLV. Kittens can be
infected by their mother before birth or during nursing after birth.
When a cat is exposed to FeLV, there are four possible outcomes. In about
30% of cats, an effective immune response is produced and the infection is
resisted. These cats then become naturally immune to FeLV infection for an
unknown period of time. In about 40% of cats, the virus is successful and
the cat eventually becomes persistently infected and excreting virus in
its saliva. Another 30% of cats do not produce immunity but also do not
become persistently infected immediately. In these cats, the virus hides
in the bone marrow for up to 30 months. Eventually, these cats either
overcome the virus or become persistently infected. Finally, some cats can
develop latent or sequestered infection. This probably happens to less
than 5-10% of cats. These cats, whose virus is hiding in sites such as the
bone marrow, will rarely be contagious and are unlikely to develop
illness. They will not test positive on routine testing. In general, young
cats, especially those under 4 months of age, have the least ability to
mount an effective immune response and so are most susceptible to FeLV.
FeLV is capable of producing a wide variety of associated diseases and
symptoms. Degenerative diseases, such as anemia, liver disease, intestinal
disease and reproductive problems can be seen. In other cats, the virus
produces cancerous diseases, such as lymphosarcoma and leukemia. Many cats
suffer from suppression of the immune system and other illnesses,
depending on which organ is involved. Cats whose immune systems are
depressed by FeLV are susceptible to a wide variety of infectious diseases
and other problems, such as chronic respiratory infections, chronic
gingivitis and stomatitis, feline infectious peritonitis, poor healing of
wounds and abscesses and chronic generalized infections.
Testing is the basis for diagnosing and managing FeLV infections. The most
common screening test for FeLV is the ELISA, while the immunofluorescent
antibody test (IFA) is the most common confirmatory test. Vaccination for
FeLV does not affect test results since the tests look for viral antigens,
not antibodies. Kittens can be tested at any age because maternal immunity
does not interfere with testing. The ELISA is the preferred test for
screening cats since it is quick and readily available in vet clinics. It
should be performed on a blood sample, since ELISAs done on tears or
saliva have been shown to be unreliable. Any positive or equivocal ELISA
test results should be confirmed using the IFA test, usually done at a
commercial laboratory. It is possible to have results on ELISA and IFA
which do not agree for a variety of reasons, and there is a testing
protocol to follow in order to determine the status of such cats.
The American Association of Feline Practitioners (AAFP) have published
recommendations for FeLV testing. The guidelines state that the FeLV
status of all cats should be known because FeLV is responsible for the
illness and death of more cats than any other disease condition. Testing
and identifying positive cats is the mainstay of FeLV control and is not
replaced by vaccination. Cats who have had a recent exposure to a known
FeLV positive cat should be tested as should any cats who are ill. Any new
kitten or cat should be tested before being added to a household with
resident cats. Even if the household does not already have resident cats,
new pets should be tested because the emotional bond that forms between
owners and pets justifies knowing any future threats to the pet’s health.
Cats who test positive for FeLV may live for months to years. Euthanasia
of positive cats must be addressed on an individual basis in consultation
with the veterinarian. In many cases, it is possible and feasible to keep
a positive cat and ensure good quality of life through the combined
efforts of the owner and the veterinarian. Positive cats are capable of
transmitting their infection to other cats, so they should not live with
other cats nor should they be allowed to roam outside. This not only
protects other cats from FeLV, but protects the positive cat against the
many diseases and illnesses they may contract due to their increased
susceptibility.
The mainstays of treating FeLV positive cats are protecting them from
exposure to other diseases, ensuring good nutrition, giving regular
vaccinations with killed virus vaccines (but not with a feline leukemia
vaccine), reducing stress, controlling parasites, and early and aggressive
treatment of any symptoms that appear. There is no specific treatment for
FeLV and no known cure. A large number of therapies have been investigated
for FeLV positive cats, but most have not shown encouraging results.
Anti-viral drugs, such as AZT, show some promise, but are associated with
many side effects. A drug that stimulates the immune system, interferon,
can be given orally to cats without side effects and may be helpful in
many cases. Experimental protocols have been developed that combine AZT
with interferon or interferon and another immunostimulant,
Propionibacterium acnes (Immunoregulin by ImmunoVet Inc, Tampa FL).
Specific cancers associated with FeLV have their own chemotherapy
treatment protocols. However, cats with cancer associated with FeLV have
an average survival time of 6 months even with aggressive chemotherapy.
Drugs that are being developed to treat AIDS in humans are often tested in
cats first, so that studies on new drugs for AIDS may produce drugs we can
also use to treat FeLV in cats.
The best protection against any infectious disease is eliminating possible
exposure. The FeLV test and removal program was developed to remove
infected cats from infected households. Using this program, no new cats
are added to the household and all resident cats are tested by IFA every 3
months. Any cats with positive tests are removed from the household. When
every cat tests negative by IFA for two tests in a row, the household is
declared free of FeLV. Any new cats are not admitted to the household
without a 3-month waiting period in which they must have two negative IFA
tests. This program has proven to be very effective for multi-cat
households and catteries.
There are presently a number of companies who make and sell vaccines
against FeLV. Vaccines may be against FeLV only, or they may combine FeLV with other
components. Many trials have been conducted to compare the effectiveness
of the various vaccines, but unfortunately, the studies remain hard to
interpret, largely due to inconsistencies in study designs. On average,
FeLV vaccines are able to prevent infection in about 80 to 90% of cats.
All the vaccines are recommended to be given as a two-dose regimen spaced
2-4 weeks apart, staring with kittens 8 to 9 weeks of age. Thereafter,
annual boosters are recommended.
Recently, the American Association of Feline Practitioners has released
feline vaccination guidelines. They divided vaccines into core and
non-core groups. Core vaccines are those felt to be necessary for all cats
and non-core vaccines are felt to be necessary only for those cats at
realistic risk of the disease. FeLV vaccines are designated as non-core
vaccines. Vaccination is recommended only for those cats whose lifestyle
places them at risk for FeLV. This includes outdoor cats or those that are
indoor/outdoor, feral cats, cats in open multi-cat households, cats in
FeLV-positive households, and cats in households where the FeLV status of
all resident cats is not known. Since young cats are at the greatest risk
and their lifestyle is most likely to change in the future, the AAFP panel
felt that it may be appropriate to suggest initial FeLV vaccination for
all kittens, with subsequent annual vaccinations only for those that
continue to be at-risk. In any case, owners should discuss issues of FeLV
testing and vaccination with their veterinarian so the best decision can
be reached for each individual cat.
References:
- Colloquium on FeLV/FIV: Tests and Vaccination, Journal of the American
Veterinary Medical Assoc, Vol 199, No 10, 1991
- Recommendations for Feline Leukemia Virus Testing, American Association
of Feline Practitioners Newsletter, Vol 14, No 1, April 1996
- Feline Vaccination Guidelines, Advisory Panel on Feline Vaccines of the
American Association of Feline Practitioners and the Academy of Feline
Medicine, 1997
- Weiss RC, Cummins JM, Richards, AB. Low-dose orally administered alpha
interferon treatment for feline leukemia virus infection. J Amer Anim Hosp
Assoc 199 (10): 1477-1481, 1991
- Rojko JL, Hardy WD. Feline Leukemia Virus and other retroviruses, in
Sherding RG (ed), The cat: diseases and clinical management, second
edition, WB Saunders, Philadelphia, pp 263-432, 1994.
Please Note: The Winn Feline Foundation provides the feline health
information on this site as a service to the public. Diagnosis and
treatment of specific conditions should always be in consultation with
one's own veterinarian. The Winn Feline Foundation disclaims
all warranties and liability related to the veterinary advice and
information provided on this site.
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