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Feline Leukemia Virus

December 1, 2017

Feline Leukemia Virus

The feline leukemia virus (FeLV) is a slowly progressive virus that usually persists in cats’ bodies and interferes with normal immunity, with identified stages of infection. FeLV infection results in an increased risk of infections and cancers. 


Cats tend to show signs of disease as young adults, usually from maturity to about five years of age, although the virus is often acquired when they are kittens.

Signs of the virus vary:

  • Cats may develop primary leukemia/lymphoma.
  • Digestive tract, oral cavity, skin or nervous system infections may lead to diarrhea, periodontal disease, abscesses, or unsteady balance, respectively due to immune system deficiency. 
  • Blood disorders such as low red blood cells (anemia) or low white blood cells;
  • Lymph nodes may be enlarged, and upper respiratory infections may become recurrent and difficult to manage.
  • Other general signs of poor health may occur including weight loss, dull or sparse hair coat, and reproductive abnormalities.

Virus Transfer and Risks

Transfer of the virus can occur from mother to babies, between cats housed together, or in feral outdoor cat colonies. Sharing food and water bowls, mutual grooming, fighting, and shared litter boxes can all provide opportunity for virus transfer.

Once infected, the virus travels through the body’s circulatory system and in some cats the virus fuses with the cat’s own genes and becomes dormant in the bone marrow (as a provirus). This is referred to as a regressive infection. These cats do not test positive on routine clinic tests and may appear normal for a while.

In other cats the infection becomes persistent and these cats can excrete the virus, which may infect other cats. These cats usually die from the virus or associated complications within two to five years of the initial infection.


Tests are available and are used to screen both sick and healthy cats. The infection can be silent for months to years so testing plays an important role. Young kittens and cats exposed to infected cats should be retested in four months, even if initial tests results were negative.

Some veterinary clinics use ELISA tests to check for circulating virus protein. Newer laboratory polymerase chain reaction tests (PCR) may be more sensitive and can identify even small amounts of the provirus in the system. An immunofluorescent assay (IFA) on blood smear or bone marrow samples is also used for testing cells for leukemia virus. Your veterinarian can discuss the pros and cons of each type of test with you.

Prevention Strategies and Management of Infected Cats:

  • Keep sick/actively infected cats indoors and isolated.
  • Vaccinate cats living in the same environment as FeLV-positive cats, and vaccinate cats that go outside. The vaccination of adult cats is based on risk. Indoor cats are at lower risk than roaming outdoor cats. Although vaccination is not 100 per cent effective, it is the best means of prevention.
  • Quarantine any newcomers, and test them to ensure they are FeLV-negative before bringing them into a home or facility.
  • Reduce stress and crowding, and provide quality nutrition and appropriate preventive health care.
  • Avoid breeding cats that test positive for the virus. Young kittens are at highest risk of contracting the disease since their immune system is not as well developed as that of an adult cat.
  • Perform twice yearly or more frequent health assessments of sick cats as directed by your veterinarian.
  • Use appropriate disinfection to clean an environment that has had an infected cat resident before introducing new cats or kittens; the virus does not live long on surfaces.


There is no cure for the disease currently. Newer trials with antiviral medication, chemotherapy for associated cancers, T-cell immunomodulator and omega interferon therapy show some promise for chronic management. Your veterinarian can explain these modalities further. Supportive care is provided for complications associated with disease such as secondary infections and anemia.

Talk to your veterinarian about precautions needed to keep your cat safe from FeLV virus, or to help manage an infected cat.

Dr. Kathleen Cavanagh, Consulting Online Editor CVMA

Dr. Beth Hanselman, Specialist Consultant
DVM DVSC Dip DACVIM                                                                     

November 27, 2017