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Feline Leukemia Virus

Dr. Lila Miller, D.V.M., ASPCA

Feline Leukemia Virus (FeLV) is one of the most feared causes of disease in cats, being responsible for almost one-third of their cancer deaths. This complex and generally misunderstood disease was first reported in 1964, and it remains one of the most researched feline disease problems, despite the fact that although estimates vary according to the geographic region, the incidence of the disease in healthy free roaming cats has been reported to be as low as 1-3%. There have been advances in the development of therapeutic regimens, but no treatment has resulted in a cure or removal of the virus from the cat once it is established. Epidemics of feline leukemia in shelters do not occur, but leukemia is a problem largely because it can be undetected in healthy carriers of the virus and because it is still a fatal disease. One of the biggest controversies surrounding the disease for shelters is whether or not feral cat trap neuter release (TNR) programs and shelters should perform routine screening tests for the virus, and how to handle the apparently healthy cats that test positive.


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Feline leukemia is caused by a retrovirus that only infects members of the feline family. It is not believed to be contagious to humans. It is readily destroyed in the environment, being susceptible to inactivation with heat, soap, most disinfectants and drying. Even in the proper environment, it will only last outside the cat’s body for 24-48 hours. For these reasons, cats may be safely held in shelters with minimal risk of disease spread, but only if they are housed in individual cages, and rules about proper sanitation and washing hands between handling cats are strictly enforced. In addition, long waiting periods do not have to be observed before adopting a new cat into a household that previously housed a leukemia positive cat.


Feline leukemia is most commonly spread by direct oronasal contact between cats and through mutual grooming and shared contaminated water bowls and litter boxes. The virus is shed in many body fluids, including urine, saliva, milk, tears and blood, and in feces. It is found in higher concentrations in saliva than blood. It can also be transmitted across the placenta, through contaminated needles, fomites (hands or other inanimate objects), surgical instruments or blood transfusion. Healthy cats that are infected and considered to be viremic will shed virus constantly in levels as high as those encountered in sick cats.

When cats are infected with feline leukemia virus, they will test positive within a few days to a week, but if they are healthy and immunocompetent, they will rid themselves of the virus within about 2 weeks and then test negative. In the case of latent carriers, the virus establishes itself in the bone marrow and may escape detection via routine testing. Latent carriers do not shed virus and are thus not contagious, and may rid themselves of the infection eventually. The third outcome is that the immune system is incapable of mounting a good response and the cat persistently tests positive (is viremic) and eventually becomes sick. According to Greene’s infectious disease text, the death rate for healthy, persistently viremic cats in multicat households is 50% in 2 years and 80% in 3 years, although there are also indications that many cats given appropriate care may survive for several years.

The symptoms of feline leukemia infection can be very vague and non-specific, including vomiting, loss of appetite and weight, lethargy, anemia, etc. As the infection progresses, symptoms may be seen that correlate with the organ system that is affected by the virus. For example, if the intestines are affected, diarrhea may be seen. The most common cancers include leukemia and lymphoma. Abortions and stillbirths may occur. These cats are also susceptible to infections by other organisms because of the immunosuppressive nature of the infection, and commonly succumb to other diseases. If routine FeLV screening is not performed, it is always indicated in cats with diseases that are non- responsive to therapy or in cats with recurrent fevers and other signs of clinical disease.

Diagnostic Testing

The American Association of Feline Practitioners (AAFP) recommends that the feline leukemia status of every cat should be known. It remains the decision of the shelter whether or not the status of the cat should be determined prior to adoption and whether this is the best use of the shelter’s resources if they are limited. Screening tests should be a priority in shelters that group house cats or neuter before adoption. Whatever decision is made, no healthy cat should be euthanized based upon the results of one single test. A positive test only indicates that the cat is infected with the virus, and the tests are not 100% accurate all of the time. The AAFP website ( has a detailed description of feline leukemia testing. The following is a summary of some of the most important testing principles.

  • The ELISA test is commonly used in shelters and private practices to screen for the presence of the leukemia virus (or antigen) in blood, tears or saliva, although it is recommended to use serum or plasma only.
  • Vaccination or maternal antibodies do not interfere with the test so it can be performed on cats at any age (although newborns may not test positive for several weeks).
  • Queens and each member of a litter should be individually tested.
  • It is best to perform the test at least one week after a leukemia exposure has occurred. If the cat tests negative, retests should be performed, with the final test 90 days after the exposure to be comfortable that a cat is truly negative.
  • Any healthy cat that tests positive should be retested with the IFA test, which is considered confirmatory.
  • Discrepancies between ELISA and IFA test results should be resolved before a final decision is made about a healthy cat.

Treatment and Management

Treatment options are beyond the scope of this article. Treatment is not advised in the shelter environment because the shelter is stressful and presents the opportunity for other diseases to infect these vulnerable cats. Healthy FeLV positive cats should be vaccinated (FVRCP and rabies), dewormed, neutered, kept indoors and monitored closely for development of other disease symptoms. They should be housed separately, and not with FIV positive cats. If adopted they should go to a household with no other cats, or one with cats that have been vaccinated against the disease. Kittens are at much higher risk of contracting the disease than adult cats, so households with kittens should be avoided. Owners should be advised about the disease, its risks and the cat’s prognosis, to take the cat to their veterinarian twice a year for a checkup, to feed a complete and balanced feline diet, and to examine the cat’s mouth periodically for dental disease. Stress should be avoided, as is the feeding of raw, undercooked or unpasteurized food. Antiviral and immunotherapy treatment protocols may be initiated if they become ill, but it should be remembered that while they may prolong and improve the quality of life, they do not offer a cure.

A FeLV vaccine is available. Cats should be tested before the vaccine is given. Avoid vaccinating FeLV positive cats. The vaccine is generally believed to be effective, but it is not recommended for routine use in the shelter. The vaccine has been implicated in the development of vaccine- associated sarcomas (tumors) and should be given SQ over the lateral aspect of the left thigh in case a sarcoma develops that necessitates limb amputation.

© 2004 ASPCA

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