The Feline Leukemia Virus (FeLV)

Cats are prone to catching a number of infections. Of particular interest is the number of slow viral infections that they are susceptible to.

FeLV is one of the most common.

How do cats get it? How is it spread?

The FeLV virus is a member of the Retroviridae family, and is somewhat related to FIV virus, with the FeLV being in the subfamily oncovirus and the FIV in the subfamily lentivirus. According to the Cornell Feline Health Cente , about 2 to 3% of American cats are infected. The Leukemia virus is shed in the saliva, nose discharges, and in lower concentrations in urine as well. The virus is relatively unstable and cannot live outside an infected cat for more than a few hours in a dry environment. Cat bites can be a source of infection. The virus is carried form the saliva of the infected cat and is introduced into the tissue of the recipient. Cats that are housed closely together and share food or water dishes are also at risk. Infected pregnant queens may also transmit the virus to their kittens ( >20 % of kittens infected). Abortion or stillborn kittens may result. There is a strong, natural resistance to FeLV infection that improves as cats mature. Kittens less than four months of age are at a much greater risk of infection than are adults.

What happens if cats get it?

There are a number of possible outcomes after infection. The presence of viremia (virus in the bloodstream) may progress to full infection, sequestered infection, or no disease. There are a proportion of cats exposed to the virus that develop immunity and clear the virus infection so they do not carry the virus in their blood and bone marrow. It turns out after careful study that many of these cats are really latent carriers of the infection, but some of these cats with deeply integrated sequestered virus can live a healthy life without ever developing disease symptoms.

Some cats, especially young kittens, when infected become progressive and will become ill within weeks of the exposure. Other cats will contract the virus and then sequester it (hold it inactively) in their bone marrow or other tissues for a variable length of time. Later, when stressed or treated with immune suppressing medications, the leukemia virus can erupt and cause active disease.

Cats infected with the virus can exhibit a wide array of diseases. System-wide infections, diarrhea, skin infections, eye disease, respiratory tract infections, bladder infections, abortion and infertility, anemia, nervous signs, and cancers can all be associated with FeLV infection. About 80% of cats with lymphoma cancer have FeLV infection. Other cats may develop leukemia, fibrosarcoma, or other tumors. The virus attacks the cat’s immune system and leaves them open to infection by any infectious agent. Any severe, chronic illness can be a sign of FeLV infection.

How can infection be prevented?

Although scientific research may eventually discover a cure for this disease, preventing infection is the best way to protect a cat. Control exposure to other cats by keeping her indoors, having the cat vaccinated if she contacts other cats (especially strays), testing all new cats adopted or bought before they are brought home, and keeping the cat’s environment clean will all help to prevent the disease. Although the FeLV vaccines available are effective, it is important to remember that they do not give 100% protection against infection in all cats. At-risk kittens will be vaccinated at 9 to 10 weeks of age, with a booster given in 3 weeks, and then given in an interval dictated by the lifestyle and risk of exposure of the cat. Cats should be tested negative for the virus before vaccination.

How can the veterinarian diagnose this disease?

Testing for FeLV is somewhat complex. It is important to remember that not all tests are 100% accurate under all conditions. If a cat has a positive FeLV test by the ELISA method, then a second test using a different method will be used to verify the results. Often this second assessment will be sent out to a laboratory for an IFA-type test. According to the Cornell Feline Health Center, “the majority of IFA positive testing cats remain infected for life”. Those cats, as well as many of those that are ELISA positive are shedding FeLV in the saliva and are infectious to other cats.

Sometimes, “discordant" test results occur. Discordant tests are seen where results are conflicting. According to the American Association of Feline Practitioners, discordant tests may be a consequence of the stage of infection, the variability of host response, or technical problems with the testing system.

If results of two ELISA tests are discordant, an IFA test will be performed. If results of an ELISA and an IFA test are both positive, the cat is very likely to be persistently infected. If the result of an ELISA test is positive and the IFA test is negative, both tests should be performed again in 60 days. If they remain this way, recent research suggests that these cats are infected, just not circulating significant virus (viremic). Kittens may be tested for FELV at any age but the virus may not be detected until weeks to months after birth, and therefore most kittens are not tested until weaning.

Is it treatable?

At this time there is no cure for FeLV infection. Although some therapies may help alleviate clinical signs or in some cases produce temporary remission of infections, they are not permanent cures. The disease is usually eventually fatal. Chemotherapy may allow the infected cat to continue life in a healthy state for weeks to months. Antiviral drugs or immune system modulators used in people may also be used off-label to treat infected cats and may provide some clinical improvement for a while.