Feline Leukemia Virus (FeLV) in Cats

What is feline leukemia?

Discovered in 1964, the feline leukemia virus (FeLV) is a retrovirus belonging to the subfamily orthoretrovirinae which is in the same family as the feline immunodeficiency virus (FIV).

More deaths occur from the feline leukemia virus than any other pathogenic disease in cats. Currently, 2%-3% of cats who go outdoors in the United States are FeLV positive.

Four subgroups of FeLV exist:

  • A
  • B
  • C
  • T

Only subgroup A is transmissible between cats. The other subgroups arise de novo or as results of recombination with an endogenous feline DNA sequence. Hence, there is very good evidence this virus is quite ancient and may well have evolved more than one time over the last 10,000,000 years.

The virus is neoplastic, which means it causes cancer, predominantly lymphoma, it is also immunosuppressive and bone marrow suppressive.


Large quantities of the virus are shed via saliva and nasal secretions. Smaller amounts of virus are shed in the urine, feces, and milk. Kittens under twelve months of age are most susceptible to FeLV infection due to their immature immune systems.

Modes of transmission:

  • Direct contact:  Mutual grooming, nose-to-nose contact, bites and mating.
  • Vertical: Unborn kittens can become infected in-utero.
  • Lactogenic: Kittens can become infected via their mother’s milk.
  • Fomites: Inanimate objects such as litter trays shared food bowls (possible, but low-risk).

The virus is extremely fragile and is quickly destroyed in the environment and is highly susceptible to heat and disinfection.

What does FeLV do?

Once the virus enters the body, it replicates in the lymphoid tissue surrounding the site of initial penetration. Having been infected, there are three possible outcomes, each with an equal (33%) likelihood of occurrence:

  1. Transient viremia. The virus is present in the blood and saliva for less than twelve weeks. The cat develops neutralising antibodies, which destroy the disease. The cat is no longer infected and cannot transmit the disease to other cats. It does not become sick and has a normal life expectancy.
  2. Persistent viremia. The virus is present in the blood and saliva for over twelve weeks and has made its way to the bone marrow; once it reaches the bone marrow, the virus remains with the cat for life. The cat doesn’t mount an effective immune response and is susceptible to many potentially fatal diseases. Mortality ranges from six months to three years.
  3. Latent infection. It produces neutralising antibodies to destroy the virus, but the virus isn’t extinguished completely and persists in the bone marrow and T-cell lymphocytes. The cat usually doesn’t develop diseases or cancers related to FeLV. The virus can reactivate during times of stress or concurrent illness.


The incubation period of FeLV can range from weeks or years after exposure. Clinical signs vary widely, depending on the disease type and on which organs are involved. Some symptoms caused (directly or indirectly) by FeLV include:

Around 80% of cats die within a year of infection, and almost all will die within two to three years.


ELISA (enzyme-linked immunosorbent assay): This test can detect the antigen p27 in whole blood (blood that hasn’t been separated into components), blood serum (the straw-coloured liquid portion of blood), blood plasma, saliva, and tears of the cat. It is possible to get a false positive or a false negative. In the case of a positive from whole blood, a second test performed on blood plasma or blood serum is recommended.

IFA (immunofluorescence assay): Testing detects an antigen in leukocytes (white blood cells) or platelets.


There is no cure for FeLV, and the goal of treatment is to manage FeLV-related diseases and keep the cat as healthy as possible. The veterinarian will tailor the treatment and care of the FeLV-positive cat according to its circumstances.

Keeping your cat in a stress-free environment, feeding a nutritious diet, and avoiding exposure to disease are all important keys to helping a cat with FeLV. Treatments/management may include:

  • Regular check-ups: It will be necessary for the veterinarian to see the cat regularly to monitor its health.
  • Vaccinations: Different vaccination schedules for the FeLV positive cat such as only using a killed vaccine instead of a modified live vaccine.
  • Interferon: Interferon is a natural protein released by cells that have been invaded by viruses and assist the immune response by inhibiting viral replication.
  • AZT (azidothymidine): An antiviral drug used in humans with HIV. It can produce quite severe side effects in cats. A veterinarian will need to closely monitor your cat.
  • Antibiotics: To treat or prevent secondary infection.
  • Chemotherapy: For cats with lymphoma.
  • Keep the cat indoors, or with access to a safe cat enclosure.


The virus is very fragile outside the body and is unable to survive more than a few years in the environment. The following disinfectants are effective against FeLV.

  • Bleach (sodium hypochlorite, calcium hypochlorite or sodium dichloroisocyanurate)
  • Trifectant or Virkon-Potassium peroxomonosulfate
  • Virox, Accel – Accelerated hydrogen peroxide


Do not let cats free roam, either keep your cat inside or provide a cat enclosure.

There is a vaccine for FeLV. This is recommended for cats for high-risk cats. No vaccine is 100% effective; therefore it cannot be assumed that a vaccinated cat is completely safe.

Avoid having FeLV-positive and negative cats together.

Test all new cats for FeLV.


  • Julia Wilson, 'Cat World' Founder

    Julia Wilson is the founder of Cat-World, and has researched and written over 1,000 articles about cats. She is a cat expert with over 20 years of experience writing about a wide range of cat topics, with a special interest in cat health, welfare and preventative care. Julia lives in Sydney with her family, four cats and two dogs. Full author bio