FeLV – Feline Leukemia Virus

Feline Leukemia or FeLV is caused by a retrovirus that can be spread through bodily secretions. Viremic FeLV-infected cats shed virus in many body fluids, including saliva, feces, nasal discharge, milk and urine. FeLV transmission occurs through sustained close contact among cats. Behaviors such as mutual grooming, sharing of food and water bowls and litter boxes, and fighting can contribute to transmission, primarily via saliva. It can also be spread through bite wounds and across the placenta from a mother cat to her kittens. FeLV affects the white blood cells/immune system, making the cat more susceptible to infections and disease.

Electron micrograph of feline leukemia virus

Clinical research suggests that many cats may remain infected with FeLV for life following exposure, but may revert to a regressive state with a low risk of clinical disease.1-3 Following exposure, cats may exhibit mild clinical signs, such as fever and malaise, or may remain asymptomatic. For cats that remain persistently infected, this acute phase is followed by a period of asymptomatic infection that may last months or years. Ultimately, progressive infection occurs, with development of one of several FeLV-associated disorders (eg, lymphoma, anemia) or a secondary disease associated with immune dysfunction (eg, opportunistic infections, oral inflammatory disease).

FeLV is very easily broken down outside of the host and remains infectious for mere minutes in the environment; in moist secretions it may survive until dried. It is readily inactivated by soap, disinfectants, heat and drying.

Early in the course of the disease, cats do not appear to be sick. But as immune function declines, cats become susceptible to life-threatening infections and certain types of cancer.

A cat with advanced FeLV which exhibit some or all of these symptoms:

  • lethargy
  • poor appetite
  • weight loss
  • vomiting
  • diarrhea
  • oral inflammation
  • pale mucous membranes
  • neurological disorders
  • chronic eye problems

When a cat has FeLV in her body, she has lots of FeLV in her body, so an antigen test can be used instead of an antibody test such as a standard FIV test. The big caveat with FeLV screening is that a cat’s immune system is sometimes able to fight off the infection, so a single positive test could indicate an early infection that may still be eradicated. Two positive antigen tests at least 90 days apart are needed to definitively diagnose FeLV.

FeLV vaccines are quite effective but unfortunately are associated with rare but potentially lethal injection site sarcomas, an aggressive type of cancer. Because young cats are at highest risk for infection and it’s difficult to determine what their lifestyles will be like until they are grown, the AAFP (American Association of Feline Practitioners) recommends that all kittens be vaccinated for FeLV and receive a booster when they come in for their first annual check-up. After that, only continue to vaccinate cats that are at high risk for disease (e.g., those that go outside or that live with a FeLV positive housemate). Discuss with your veterinarian if this vaccine should be included with the core vaccines (FVRCP and Rabies) for your kitten or young adult cat.

Onset of immunity is 2–3 weeks after primary vaccination depending on the product. Minimal information concerning maximum duration of immunity for FeLV vaccines is available. Results of several studies indicate that FeLV vaccine-induced immunity persists for at least 12 months following vaccination.4-6 In a 2 year challenge study, a greater proportion of glucocorticoid-treated control cats (11/11 cats; 100%) developed persistent FeLV viremia when compared with vaccinated cats (2/12 cats; 16.7%).7 This study suggests that duration of immunity induced by some FeLV vaccines may last for at least 2 years.

Because of ease of transmission, typically through saliva, FeLV+ cats should be kept separated from FeLV- cats or live only with other FeLV+ cats. Care should be taken by the by the owner to not spread the infection themselves. Wash hands with soap and hot water after handling an infected cat or their belongings including food and water dishes, toys, bedding, litter, etc.

Treatment protocols for FeLV deal with any complications that arise quickly and aggressively. Cats can remain healthy for years after being diagnosed unless they are already immune-compromised at the time they first test positive. Providing good nutrition and preventive health care and protecting them from exposure to potential pathogens can extend that time period. Unfortunately, once a cat’s quality of life declines to an unacceptable level because of FeLV infection, hospice care and/or euthanasia are the only effective ways to relieve suffering.

Cats with FeLV should have complete blood counts performed twice yearly due to their increased risk of hematological diseases.

Although a diagnosis of FeLV can be emotionally devastating, it is important to realize that cats with FeLV can live normal lives for prolonged periods of time. The median survival time for cats after FeLV is diagnosed is 2.5 years. Once a cat has been diagnosed with FeLV, careful monitoring of weight, appetite, activity level, elimination habits, appearance of the mouth and eyes, and behavior is an important part of managing this disease. Any signs of abnormality in any of these areas should prompt immediate consultation with a veterinarian.

  1. Gomes-Keller MA, Gonczi E, Tandon R, Riondato F, Hofmann-Lehmann R, Meli ML, et al Detection of feline leukemia virus RNA in saliva from naturally infected cats and correlation of PCR results with those of current diagnostic methods. J Clin Microbiol 2006; 44: 916–922
  2. Hofmann-Lehmann R, Huder JB, Gruber S, Boretti F, Sigrist B and Lutz H. Feline leukaemia provirus load during the course of experimental infection and in naturally infected cats. J Gen Virol 2001; 82: 1589 –1596.
  3. Pepin AC, Tandon R, Cattori V, Niederer E, Riond B, Willi B, et al. Cellular segregation of feline leukemia provirus and viral RNA in leukocyte subsets of long-term experimentally infected cats. Virus Res 2007; 127: 9–16.
  4. Hofmann-Lehmann R, Holznagel E, Aubert A, Ossent P, Reinacher M and Lutz H. Recombinant FeLV vaccine: longterm protection and effect on course and outcome of FIV infection. Vet Immunol Immunopathol 1995; 46: 127–137.
  5. Hoover EA, Mullins JI, Chu HJ and Wasmoen TL. Efficacy of an inactivated feline leukemia virus vaccine. AIDS Res Hum Retroviruses 1996; 12: 379–383.
  6. Harbour DA, Gunn-Moore DA, Gruffydd-Jones TJ, Caney SM, Bradshaw J, Jarrett O, et al. Protection against oronasal challenge with virulent feline leukaemia virus lasts for at least 12 months following a primary course of immunization with Leukocell 2 vaccine. Vaccine 2002; 20: 2866–2872.
  7. Jirjis F, Davis T, Lane J, Carritt K, Sweeney D, Williams J, et al. Protection against feline leukemia virus challenge for at least 2 years after vaccination with an inactivated feline leukemia virus vaccine. Vet Ther 2010; 11: E1–6.

Information on this site is for general information purposes only and is provided without warranty or guarantee of any kind. This site is not intended to replace professional advice from your own veterinarian and nothing on this site is intended as a medical diagnosis or treatment. Any questions about your animal’s health or diet should be directed to your veterinarian.

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