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Feline Leukemia Virus (FeLV)

Feline leukemia virus is a retrovirus belonging to the oncornavirus subfamily, which means it is a cancer-causing virus. A confirmed positive test result should only be considered an indication of retrovirus infection, not clinical disease. Cats infected with FeLV or FIV may develop diseases that are not necessarily caused by retrovirus infection.

FeLV can cause severe anemia and/or suppress the immune system, leaving the cat vulnerable to a variety of opportunistic diseases. Infected cats shed FeLV primarily in their saliva, although the virus is also present in the blood, tears, feces, and urine. Most cats acquire the virus from their infected mothers or through prolonged intimate contact, such as mutual grooming. Fighting is not considered a major mode of transmission. FeLV cannot survive very long outside a cat's body. The virus loses its infectivity within minutes and is easily destroyed with soap and water.

In several large-scale Trap-Neuter-Return programs, the incidence of FeLV positive test results was found to be 1-2%. Urban cats are more likely to be infected than rural cats, because populations are denser. Other variables may cause some colonies to have a higher or lower incidence.

How FeLV Exposure Affects a Cat
One of three things can happen when a cat is exposed to FeLV:

  1. Transient infection and immunity: The cat may experience a transient viral infection, fight off the virus, and develop future immunity. Evidence reveals up to 70 to 80% of adult cats exposed to FeLV survive the initial stage of infection and acquire immunity. Kittens under 16 weeks are much less likely to fight off viral exposure.
  2. Persistent infection and disease: When initial infection is not overcome, the kitten or cat becomes persistently infected or viremic. The feline leukemia virus eventually moves to the bone marrow and compromises the immune system. Although a viremic cat may be asymptomatic for several years, FeLV-related diseases typically develop within two to three years. Persistently infected cats can shed the virus throughout their lives.
  3. Latent infection and immunity: Cats that become persistently infected do not always develop disease. Some individuals produce an effective immune response to the virus while continuing to harbor the virus in the body. This results in a latent or carrier state—that is, an infected cat with no disease that may transmit FeLV to other cats. Latently infected cats appear to resist FeLV-related diseases. Unlike cats with persistent infections, latently infected cats shed the virus intermittently. The latent phase of FeLV infection seems to be temporary for most cats, which become free of the virus within a few years after initial infection.

Against FeLV Testing
Alley Cat Allies does not support testing feral cats for FeLV. Besides the reasons previously stated— low rate of disease, low likelihood of transmission between adult cats, and poor viability of the virus— the cost of testing is substantial. We believe that funds are more effectively invested in providing neuter services.

Moreover, FeLV tests can provide inconclusive results:

  • A cat in the initial stage of FeLV infection may test negative.
  • A cat exposed to FeLV may test positive during the transient phase of the infection and then test negative if the virus is overcome.
  • Tests are not 100% accurate and can yield false positive results.

Treatment
Although there is no known cure, supportive care can improve the quality of life, health, and longevity for feral cats with FeLV. Supportive care can include:

  • Shelter
  • Good nutrition
  • Reduced stress
  • Prompt treatment of illness

New treatments, known as immunotherapies, are said to boost a weakened immune system. These therapies are popular, but their effectiveness is largely unproven. For more information about immunotherapy, visit “Feline Leukemia (FeLV) Treatments Page” at www.angelfire.com/il/felv.

Read more complete information about selection and interpretation of FeLV and FIV tests and management suggestions for infected cats from the American Association of Feline Practitioners.