What is Feline Leukemia Virus (FeLV)?


What is Feline Leukemia Virus (FeLV)?

This leukemia is not cancer, but rather a virus (that can lead to cancer in some situations) and it is one of the most potent and deadly virus strains to affect cats. There are three different types of FeLV infections. FeLV-positive cats can have one, two or all three types of the infection.


FeLV-A occurs in all cats infected with the virus. It causes the suppression of the immune system.

  • FeLV-B occurs in about half of infected cats, and it causes cancer, tumors and other abnormal tissue growth.

  • FeLV-C is the rarest type of infection, it and causes severe anemia.

Generally speaking,  FeLV is not a very resilient virus and does not survive outside a cat’s body for very long. It is however very easily spread from cat to cat.  It is spread from cat to cat through contact of bodily fluids such as blood, saliva and mucous. The most common forms of transmission are through fighting and mutual grooming. There is also risk that kittens can become infected in the womb or from the mother when they’re suckling as newborns.


Younger cats and those with suppressed immune systems are more susceptible to the virus. If your cat is exposed to the virus there are several outcomes:

  • It makes antibodies and fights off the virus and is not affected.

  • It becomes a carrier of the virus without showing any symptoms initially. These cats can spread the virus to other cats easily because they show no signs of illness. These cats may develop symptoms of the virus after a long period of time.

  • The virus quickly weakens the immune system and various chronic health problems can develop.

  • The cat eventually develops symptoms of the B and C types of infections; resulting in cancer, tumors, other abnormal tissue growth or severe anemia.

It is not completely understood why some cats can make antibodies to FeLV and are able to fight off the infection, while others become infected by the virus.

What are the symptoms of FeLV?

Unfortunately, there are no definitive tell tale symptoms of FeLV. Symptoms seen will be associated with the type of infection the cat is suffering from.

Those suffering from a suppressed immune system may exhibit signs of:

  • Bladder, skin, or upper respiratory infections

  • Stomatitis – Oral disease that includes ulceration of gums and interior of the mouth

  • Persistent infections

  • Loss of appetite

  • Poor coat condition

  • Progressive weakness and lethargy

  • Fever

Those suffering from a tumor or cancer system may exhibit signs of:

  • Enlarged lymph nodes

  • Weight loss

  • Progressive weakness and lethargy

Those suffering from anemia may exhibit signs of:

  • Pale gums

  • Breathing difficulty

  • Lethargy and weakness


How is FeLV diagnosed?

If feline leukemia is suspected in your cat, blood testing is available. The blood tests can detect all three types of the virus (but it can’t distinguish between the specific types). There are two blood tests available for FeLV testing.

The enzyme-linked immunosorbent assay (ELISA) detects the antigens in the blood serum. This test is quick and convenient, as most veterinary clinics can run them in clinic. This test can detect the antigens early in the course of infection, before the virus becomes wide spread. A cat that has tested positive with this test should be tested again several weeks later.  A negative results several weeks later indicate that the cat has developed immunity, and will likely never show any sign of infection. A second positive test several weeks later can indicate the infection is persistent.

The immunofluorescence assay (IFA) detects the antigens in the white blood cells once the virus has reached the bone marrow.  This is a send out test and may be recommended to follow up a positive ELISA test. Cats with a positive IFA result are generally positive for life. Negative results do not necessarily mean that the cat does not have FeLV. Negative IFA testing can occur in infected cats if their exposure to the virus is recent, as they may not be producing antibodies yet.

Besides the direct test for FeLV, a complete blood cell count (CBC) and chemistry screen might be recommended to if any other abnormalities are present. Cats with FeLV can also show:

  • anemia (abnormally low level of circulating red blood cells),

  • lymphopenia (abnormally low level of lymphocytes in the blood),

  • neutropenia (abnormal decrease in the number of circulating neutrophils, a type of white blood cell)

  • chemistry abnormalities could also suggest that the virus is effecting various organs in the body

How is FeLV treated and prevented?

There is no cure for FeLV, but feline leukemia positive cats can live long, full lives if they are otherwise healthy. It’s only once the cat becomes ill or starts to show symptoms associated with FeLV that concern rises about their well-being. There is no cure for cats with a persistent infection. Treatment will be done to control the symptoms of the illness the cat is showing and support the cat’s quality of life. Antibiotics for infections, blood transfusions for anemia, and vitamin and mineral supplements to support their immune system. Chemotherapy drugs may be helpful to treat FeLV-associated lymphoma, leukemia or other cancers, with varying degrees of success, as these cancers are not curable.

Preventative measures can be taken against FeLV. Vaccination is available and is recommended for all cats at risk for contracting FeLV (out-door cats, multi-cat houses, catteries, rescue societies/foster homes etc). The only sure way to prevent transmission of FeLV is to prevent exposure of your cat to infected cats by keeping them indoors or under direct supervision when outside.

If you have an otherwise healthy feline leukemia positive cat, keep them indoors and away from other cats so that they do not infect another cat. A FeLV-positive cat should be the sole cat in a household unless arrangements can be made to completely isolate that cat from any other cats in the household. If your FeLV-positive cat does pass away, all litter boxes, dishes and bedding should be replaced, carpet, floors, furniture should be disinfected where possible and it is recommended to wait a minimum of 30 days before introducing a new cat to the home.