Feline leukemia virus (FeLV) is a retrovirus belonging to the oncornavirus subfamily, which means it is a cancer-causing virus.
FeLV can cause severe anemia and suppress the immune system, leaving the cat vulnerable to a variety of opportunistic diseases. FeLV is spread primarily through cats’ saliva. It can also spread through blood, tears, feces, and urine. Most cats acquire the virus from their infected mothers at birth or through prolonged direct contact, such as mutual grooming. It can also spread through bite wounds and rarely through the shared use of litter boxes or feeding dishes. FeLV cannot survive very long outside a cat’s body. It is easily destroyed with soap and water.
Studies show that FeLV is present in community cats at an equally low rate as owned cats. In several large-scale Trap-Neuter-Return programs, the incidence of FeLV-positive test results was found to be 1 to 2 percent. Urban cats are more likely to be infected than rural cats due to dense populations. Other variables may factor in, such as age and overall health, causing some colonies to have a higher or lower incidence.
A vaccination for FeLV exists, but it is not 100 percent effective. Owners should consult their veterinarians and determine their cat’s risk factors for FeLV when deciding whether to vaccinate. For community cats, the American Association of Feline Practitioners does not recommend FeLV vaccination if resources are needed for higher priorities, like spay or neuter surgeries.
How FeLV Exposure Can Affect a Cat
One of three things can happen when a cat is exposed to FeLV:
- Transient infection and immunity: The cat may experience a transient viral infection, fight off the virus, and develop future immunity. Kittens less than 16 weeks old are much less likely to fight off viral exposure.
- Persistent infection and disease: When the initial infection is not overcome, the kitten or cat becomes persistently infected. The feline leukemia virus eventually moves to the bone marrow and compromises the immune system. Although a cat in this situation may be asymptomatic for several years, FeLV-related diseases typically develop within two to three years. Cats with this type of infection shed the virus throughout their lives, meaning they can infect other cats with the virus. Shedding the virus means that the virus is present in cats’ bodily secretions, like saliva.
- 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 who has no disease but 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, meaning they are not always infectious to other cats. The latent phase of FeLV infection seems to be temporary for most cats, who become free of the virus within a few years after initial infection.
Signs a cat has FeLV include:
- Loss of appetite
- Weight loss
- Poor coat condition
- Persistent fever
- Inflammation of the gums and mouth
- Skin, urinary, and upper respiratory tract infections
- Persistent diarrhea
- Seizures, behavior changes, and other neurological disorders
- A variety of eye conditions
It is important to note that many of these symptoms can be signs of non-FeLV related disease.
Other conditions FeLV can cause include:
FeLV Testing Is Not Recommended for Community Cats
Alley Cat Allies does not support testing community cats for FeLV. In addition to low rates 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 used when invested in providing spay and 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 percent accurate and can yield false positive results.
Community cats with FeLV should remain in their outdoor homes since they can’t live indoors and aren’t socialized to people. Although there is no known cure, supportive care can improve the quality of life, health, and longevity for owned and community cats with FeLV. Community cat caregivers and owners alike can help cats with FeLV by providing care including:
- Shelter or a quiet place to rest
- Good nutrition
- Promptly taking cats to the veterinarian when they appear ill
 “Prevalence of feline leukemia virus infection and serum antibodies against feline immunodeficiency virus is unowned free-roaming cats,” JAVMA, Vol 220, No. 5, March 1, 2002