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Feline Leukemia and the Feline Leukemia Virus

Overview and Transmission

In humans, some viruses can lead to cancer. An example is human papilloma virus (HPV) which is the causative agent of most cases of cervical cancer. Viral infection can also lead to cancer in animals. Feline leukemia (FeLV; informally known as "Fee-Leuk") is an RNA virus (a retrovirus) that infects less than 2% of healthy, domestic cats in the U.S.(1) (2) Infection is more prevalent in high-risk populations (i.e. cats with outdoor access and/or frequent social interactions).(1)

The virus is spread from one cat to another via saliva, nasal secretions, feces, and milk.(3) (4) It is transmitted during various forms of contact, from friendly grooming to not-so-friendly biting. The virus can also be passed to a developing kitten during pregnancy. The age and time of infection affect the progression and clinical outcome of the virus. Kittens are more likely to be infected and more likely to develop more severe complications.(5) In most cases, FeLV initially infects lymphocytes in the back of the throat (oropharynx), which travel to the bone marrow, where virally infected cells are produced very rapidly.(6)

In most environments, the Feline leukemia virus cannot survive for long outside of the host. It can be "killed" with soap and disinfectants. At this time, studies show no evidence that FeLV can be transmitted from infected cats to humans.(7) However, because FeLV positive cats may carry other diseases, infants, elderly individuals, and immunosuppressed individuals may want to avoid contact.


Infected animals may develop anemia, lymphoma, and other conditions. The FeLV-C subtype binds to and impedes the function of a heme transport protein on the surface of developing red blood cells. The result is a decrease in red blood cell numbers (anemia).(8) (9) Signs of anemia in cats include paleness of the skin, tongue, gums, and mucous membranes surrounding the eye. FeLV- induced lymphomas are some of the most frequent tumors seen in cats. Symptoms depend on the location of the tumor, and may include weight loss, rough hair coat, loss of appetite, vomiting, diarrhea, respiratory distress, swelling of the lymph nodes and more. FeLV is also linked to diseases of the kidneys, joints, lymph nodes, small intestine, liver and nervous system. Depression of the immune system makes infected cats more susceptible to infections. As a result, they may be infected by organisms that healthy cats usually fend off. It is possible for FeLV positive cats to remain healthy. However, the prognosis is poor for cats with persistent active infection.(6)

Detection of FeLV

Veterinarians use several different laboratory tests to detect FeLV. These include 1) an antigen enzyme-linked immunosorbent assay (ELISA) and 2) an indirect immunofluorescent antibody assay (IFA). Both tests use blood samples to detect the presence of a protein that indicates FeLV infection. This protein is called p27, and is part of the structure of the virus.(6) Inconclusive results may require additional testing with other methods, such as a specific type of polymerase chain reaction (PCR), which can detect FeLV DNA in infected animals.

Treatment for FeLV Infection

Currently there is no cure for FeLV. However there are a number of things that pet owners can do to manage FeLV positive cats, as recommended by veterinarians and experts:

  • Visit the vet every 6 months for a physical examination, complete blood count, urine analysis, and other tests
  • Carefully monitor the cat's health (i.e. measure and record weight loss)
  • Provide cat(s) with a healthy and balanced diet. Avoid uncooked meat and eggs, as well as unpasteurized dairy products to prevent food-borne infections
  • Confine cats to indoor environments to reduce exposure to harmful organisms

Antiviral therapy has been used in FeLV positive cats, but the effectiveness of the drugs is limited and they can be toxic. Medications to control and restore the immune system (immunomodulators) are also used. However data to support the proposed health benefits of these agents are limited.(5) (7)

Prevention of FeLV Infection

Infection with FeLV can be prevented by vaccination. The vaccine is classified as non-core, which means it can be considered optional. However both the American Association of Feline Practitioners (AAFP) and the European Advisory Board on Cat Disease (ABCD) recommend that all cats with uncertain FeLV status and/or are at risk of exposure be vaccinated. Kittens are often vaccinated at 8-9 weeks of age and again at 12 weeks of age. Research shows that the vaccine will confer immunity for up to 1 year. Many vets recommend a booster vaccination 1 year after initial vaccination and annually thereafter. Because cats become less susceptible with age, some vets consider vaccination every 2-3 years sufficient for older animals.(6)

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References for this page:
  1. OConnor Jr TP, Tonelli QJ, Scarlett JM (1991) Report of the National FeLV/FIV Awareness Project. Journal of the American Veterinary Medical Association 199, 1348-1353. [PUBMED]
  2. Levy JK, Scott HM, Lachtara JL, Crawford PC (2006b) Seroprevalence of feline leukemia virus and feline immunodeficiency virus among cats in North America and risk factors for seropositivity. Journal of the American Veterinary Medical Association 228, 371-376. [PUBMED]
  3. Hardy WD Jr, Hess PW, MacEwen EG, et al. Biology of feline leukemia virus in the natural environment. Cancer Res 1976; 36: 582-588. [PUBMED]
  4. Pacitti AM, Jarret O, Hay D. Transmission of feline leukemia virus in the milk of a nonviraemic cat. Vet Rec 1986; 118: 381-384. [PUBMED]
  5. Levy J, Crawford C et al (2008), 2008 American Association of Feline Practitioners feline retrovirus management and guidelines. Journal of Feline Medicine and Surgery 10: 300-316. [PUBMED]
  6. Lutz, H et al (2009) FELINE LEUKEMIA: ABCD guidelines on prevention and management. Journal of Feline Medicine and Surgery 11, 565-574. [PUBMED]
  7. Cornell University College of Veterinary Medicine (Accessed July 2009) Feline Leukemia Virus [http://www.vet.cornell.edu/fhc/brochures/felv.html]
  8. Riedel N, Hoover EA et al (1988), Pathogenic and host range determinants of the feline aplastic anemia retrovirus. Proc. Natl. Acad. Sci. 85: 2758-2762. [PUBMED]
  9. Weiss RA, Tailor CS (1995), Retrovirus Receptors. Cell 62: 531-533. [http://www.cell.com/issue?pii=S0092-8674(00)X0394-7]
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