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Leukemia, or cancer of the white blood cells, was the first disease associated with the feline leukemia virus (FeLV) and, thus, the source of its name. The term “leukemia” is often used rather loosely to include all of the diseases associated with the virus, even though most are not cancers of the blood. This virus causes many other fatal diseases in addition to leukemia.

EeLV is a retrovirus, which means it has the ability to integrate into the genetic material, or DNA, of the host. Retroviruses are sometimes called “the ultimate genetic parasites.” There are three subtypes of FeLV, and the diseases it causes depend on the particular subtype involved. The feline immunodeficiency virus, or feline AIDS virus, is another feline retrovirus.


  • Fever
  • Lethargy
  • Vomiting
  • Swollen lymph nodes
  • Weight loss
  • Chronic and recurrent infections
  • Diarrhea
  • Weakness
  • Pale gums
  • Difficulty breathing

Risk Factors

Cats at greatest risk for contracting FeLV are those that live in close, direct contact with an infected cat. Fighting is a known risk factor because the virus is shed in saliva. Spending time outdoors unsupervised with potential contact with cats whose FeLV status is unknown is also a risk factor. Kittens may contract the virus from the mother via the placenta.


The diagnosis of FeLV infection in cats is based on the presence of virus protein in the blood. The FeLV ELISA test can be performed on a blood sample by a veterinarian.  This test turns positive within a few days of infection and, in some cases, may later turn negative if the cat’s immune system eliminates the infection.  This test is frequently used as a screening test for detecting FeLV infection.

The FeLV IFA test detects the presence of virus protein in circulating white blood cells and must be performed in a specialized laboratory. This test is frequently used to confirm positive ELISA test results or when the ELISA test is negative and the veterinarian has a strong suspicion that the cat is infected with FeLV (a false-negative ELISA test result). A positive result on the IFA test usually means that the cat has a permanent infection and likely won’t be able to successfully eliminate the virus, a condition referred to as “persistently viremic” (virus in the blood).


A vaccine is available to protect cats from FeLV. Although it isn’t 100 percent effective, the vaccine is strongly recommended for all kittens and for adult cats that may be exposed to FeLV-infected cats. For adult cats that stay indoors at all times, are not in contact with another cat that goes outdoors, or are not in contact with a FeLV-infected cat living in the home, the vaccine is generally not recommended. There has been a decline in the incidence of FeLV infection and related diseases since FeLV vaccination use became widespread. Many owners are concerned that the vaccine will cause a cat to test positive for the virus, but this is not true.


Feline leukemia weakens the immune system so prevention of secondary infections is very important. Keeping your cat indoors can help prevent exposure to other infectious agents and feeding it a high-quality diet can help support the immune system.

Some veterinarians are now using antiviral medications, such as those used for people with HIV, for cats with feline leukemia, but studies have not yet proven those medications to be effective. You may need to find a specialist if you are interested in using these therapies.


The prognosis is dependent on many factors. In general, 80 percent of all persistently FeLV-positive cats die within three years, and most of these deaths occur within the first six months after detection. A cat that is transiently positive may expect a normal life span, or it may become ill if latent virus in the body is reactivated.

Some forms of leukemia (blood cancer) are unresponsive to all available forms of cancer treatment, while others may respond to chemotherapy. Most cats that develop leukemia have an average survival time of less than one year. Because the virus is not affected by treatment, the cat will always remain infected with FeLV.

Current Research

Click here to find out about Morris Animal Foundation–funded research into feline leukemia.  

Morris Animal Foundation (MAF) urges pet owners not to implement any suggestions on animal health treatments without prior consultation with a licensed veterinarian. If your pet is experiencing health issues, contact your veterinarian. MAF does not endorse any of the medical treatments described in these videos. The Foundation funds research to enhance medical options available to veterinary professionals and their patients.