Veterinarian-written / veterinarian-approved articles for your cat.

Feline Leukemia Virus

cat_vet_visitCats are prone to catching a number of infections. Of particular interest is the number of chronic viral infections to which they are susceptible. Feline Leukemia Virus or FeLV is one of the most notorious.

How do cats get the Feline Leukemia Virus? How is it spread?

According to the Cornell Feline Health Center, about 2 to 3 percent of American cats are infected. The Leukemia virus is shed in the saliva and nose discharges of an infected cat. The virus is relatively unstable and cannot live outside an infected cat for more than a few hours in a dry environment. It is spread from cat to cat via shared food and water bowls and close contact, i.e., grooming. FeLV can be transmitted from a mother cat to her kittens in utero, through mother-kitten contact, or through her milk. Cat bites are another source of infection. Kittens are especially susceptible to contracting FeLV. After four months of age, natural resistance develops in many cats. Adults are relatively resistant to infection with FeLV.

What happens if cats get it?

When a cat first contracts FeLV, they are termed “viremic,” meaning there is virus in the bloodstream. The viremic stage may progress to:

  1. Persistent infection. Some cats, once infected, become persistently infected within weeks of exposure. Virus is found in their blood and bone marrow. This is especially the case in young kittens exposed to FeLV.
  2. Sequestered infection. 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, if the cat is stressed or treated with immune suppressing medications, the leukemia virus can erupt and cause active disease.
  3. No disease. There are a proportion of cats exposed to the virus that develop immunity and clear the virus from their system, leaving no trace of FeLV in their blood, bone marrow, or other tissues. 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.

Cats with persistent FeLV infection develop any of three major disease syndromes: immune suppression, anemia, or lymphoma. Immune suppression occurs in most cats. The virus attacks the cat’s immune system and leaves them open to infection by any infectious agent. System-wide infections, diarrhea, skin infections, eye disease, respiratory tract infections, bladder infections, and oral infections are common. Any severe, chronic illness can be a sign of underlying FeLV infection.

How can infection be prevented?

Definitive information on the diagnosis and management of FeLV can be found on the American Association of Feline Practitioners (AAFP) website and a detailed summary brochure can be found here.

Although scientific research may eventually uncover a cure for this disease, preventing infection is the best way to protect your cat. The following measures are recommended:

  • Vaccinate indoor-outdoor cats for FeLV if they contact other cats (especially strays).
  • Test all new cats before they are brought home.
  • In multi-cat households, periodically retest cats who may be at risk. This includes outdoor cats, escapees, cats with bite wounds, and recently adopted kittens.
  • In multi-cat households, keep food, water, and litter pans as sanitary as possible.

Although FeLV vaccines are effective, it is important to remember that they do not give 100% protection against infection in all cats. At-risk kittens can 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. Vaccination will not protect a cat that has already been infected with the virus.

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. Here are some important facts about FeLV tests as outlined by the American Association of Feline Practitioners:1

  • Most initial screening for FeLV is conducted using a simple blood test called the ELISA. Kittens may be ELISA-tested at any age but the virus may not be detected until weeks to months after birth. A positive ELISA test indicates that there is FeLV virus in the cat’s body.
  • It can take up to 3 months after initial exposure for a cat to become ELISA positive. For this reason any at-risk cat or kitten, i.e., a new adoptee, should be retested for FeLV after 3 months have passed, regardless of an initial negative test.
  • FeLV vaccination does not cross-react (i.e., interfere) with subsequent FeLV testing. If a previously FeLV-vaccinated cat tests positive, true infection should be suspected.
  • If a cat has a positive FeLV test by the ELISA method, then a second blood test called the IFA will be used to verify the results. This test indicates that there is FeLV replicating in the cat’s bone marrow and circulating in the bloodstream.
  • If the IFA test is positive, this means the cat is persistently infected. IFA-positive cats, as well as many ELISA-positive cats, shed FeLV in the saliva and are therefore infectious to other cats.
  • Sometimes, “discordant" test results occur. For example, two sequential ELISA tests may disagree, or a cat may test ELISA-positive but IFA-negative. 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.
  • Cats with discordant test results should be considered contagious to other cats.
  • Your veterinarian is best able to walk you through the fine points of FeLV diagnostics and the implications for your individual cat.

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. Sadly, 85% of cats with FeLV infection die within 3 years of the diagnosis.

Medications may allow the infected cat to continue life in a healthy state for weeks to months. Some veterinarians may prescribe certain antiviral drugs or immune system modulators normally intended for humans. These may provide some clinical improvement for a while. Never give any medication to your cat unless specifically prescribed by a veterinarian.

Advances are continually being made in the diagnosis and management of FeLV. For the latest information, it’s always best to consult your veterinarian.


  • 1. Levy, J. et al. “2008 American Association of Feline Practitioners’ feline retrovirus management guidelines.” Journal of Feline Medicine and Surgery 10 (2008): 300-316.
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