Feline Leukemia Virus – What is it?

By:  Dr. Hilary Slaven

This is such a complex and potentially devastating disease.  We get a lot of questions about it from pet owners, and frankly it can be tough to diagnose and manage Thebes the Catfrom a clinician’s standpoint.  Let’s see if we can clear up a few mental fuzzies about this subject today.

This disease affects cats ONLY.  This means that it can only be spread between cats and cannot be passed on to humans or dogs.

The virus is transmitted through close contact with an infected cat, i.e. cat bites, grooming, sharing a litter box, food or water dishes.  Saliva is the primary carrier, but blood and urine and feces and mother’s milk can all carry the virus too.  Many cats are infected as kittens, whether still in the womb or through nursing and close contact with an infected mother.

After a cat is infected, several things can happen.  

  1. Cat’s immune system responds and fights off the virus, clearing it from it’s system.
  2. Cat’s immune system responds but cannot fully fight off the virus, and it stays hidden in the body as a ‘latent’ infection for years.  These cats do not act sick, and can live a normal lifespan.  They can also live for a number of years and then eventually become actively infected and ill.
  3. Cat’s immune system cannot fight off the virus, and ends up with infection throughout the body, including the bone marrow.  This is life-long and cannot be undone.
    1. Bone marrow infection can lead to secondary infections because the immune system is not working properly.
    2. Bone marrow infection can also lead to anemia and platelet abnormalities, which can lead to weakness, blood loss and eventually death.
    3. FeLV virus also causes a variety of cancers in cats by ‘turning on’ cancer activity in normal cells.


How do we know if a cat is infected with FeLV?   Veterinarians have a quick blood test called an ELISA that tests for the virus in the bloodstream.  It is a good screening test for cats that have the virus active in their bloodstream.  Any positives should be followed up with another ELISA in 2-4 months, or a different blood test called an IFA.  An IFA test is different from an ELISA because it detects a positive infection in the bone marrow and is a good confirmatory test.  This helps to determine if your kitty fought off initial infection, or whether the infection has progressed into the bone marrow.  Cats with ‘latent infection’ and are not sickly may not show up positive on either test.

What do we do for sick, FeLV confirmed positive cats?  Because the disease is life-long, we try to keep cats from getting secondary infections.  This means keeping them indoors in a protected environment, where owners can also keep watch over daily activities (eating, drinking, using the litter box, playing) and observe any changes that could indicate a change in the health of the kitty.  Regular exams with your veterinarian (every 6 months or so) help to detect secondary issues early on.  Regular bloodwork and deworming can help prevent problems as well.

What is the typical lifespan for an FeLV-positive cat?  A cat with a latent infection that isn’t showing signs of illness can live a normal lifespan.  Kitties that are actively sick with positive ELISA and IFA tests can live for 1-3 years with good care.

Can we prevent infection?  Testing strays before adopting them into your household can help identify disease before exposing other naïve cats to FeLV.  Keeping known FeLV viremic cats away from other naïve cats is key as well.  There are several FeLV vaccines on the market that do have efficacy when given yearly as boosters.  They are not 100% effective but are a very good idea for cats that spend time outside or around known FeLV-positive cats.