Felis ISSN 2398-2950

Feline leukemia virus

Synonym(s): Feline leukaemia virus, FeLV

Contributor(s): Susan Dawson, Margaret J Hosie, Melissa Kennedy, Brian J Willett




  • Family: Retroviridae.
  • Group: Gammaretrovirus.
  • Subtypes A ,B, C, and T determined by host cell spectrum.


  • Retro because the viral RNA is reverse transcribed into DNA (proviral DNA).

Active Forms

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Clinical Effects



  • Lymphoid tissue in oropharynx   →   low titer viremia   →   systemic lymphoid tissue   →   bone marrow   →   high titer viremia   →   salivary gland and intestinal epithelia.
  • Following ingestion, the virus grows in the cells of the oropharynx and is transported via mononuclear cells in the blood to the bone marrow.
  • FeLV grows in the bone marrow, releasing p27 and virus into the plasma.
  • Virus reaches other organs and multiplies in the tissues where there are dividing cells, such as the epithelial cells of the oropharynx, upper respiratory tract and salivary glands.
  • The incubation period between infection and viremia is from 2-3 weeks onwards.
  • If cat doesn't clear the infection before virus reaches bone marrow, the cat is more likely to become persistently infected.


  • Large amounts of virus is shed in the saliva, virus may also be shed in the urine and milk.
  • The virus is fragile, therefore transmission occurs directly from cat to cat.
  • Requires long periods of close contact to infect adult cats as older cats  become increasingly able to mount an effective immune response after 4 months of age.
  • Transplacental transmission also occurs - kittens often diein uteroor are stillborn.
  • FeLV transmission to kittens has been recorded in a latently infected non-viremic cat via the milk (Pacitti et al, 1986).

Pathological effects

  • Cats that overcome viremia usually have high titers of virus neutralizing antibodies (VNA). Cell-mediated immunity is important; not all immune cats develop VNA and protected vaccinated cats do not have demonstrable VNAs.
  • Immunosuppression - main cause of death.
  • Thymic lymphosarcoma Lymphoma: mainly young cats (under 3 years old).
  • Multicentric lymphosarcoma Lymphoma.
  • Alimentary lymphosarcoma Lymphoma.
  • Lymphoid leukemia Chronic lymphoid leukemia.
  • Myeloid leukemia Myeloid leukemia.
  • Anemia: mainly non-regenerative Anemia: overview.
  • Reproductive disorders are observed rarely nowadays  Infectious infertility in the female.


Control via chemotherapies

  • Interferon-omega Interferon reduces the number of healthy FeLV-infected cats showing clinical signs and increases longevity.
  • The antiretroviral compound 3'azido-2',3'-dideoxythymidine (AZT) can reduce viral load and increase longevity. Side-effects (non-regenerative anemia) may develop at high doses.
  • If FeLV-infected cats are sick, prompt and accurate diagnosis is important for early treatment with appropriate medication.
  • Immunosuppressive drugs including corticosteroids should be avoided unless used to treat FeLV-associated malignancies or immune-mediated disease.
  • Blood transfusions Blood transfusion may be useful in anemic cats.
  • Granulocyte colony-stimulating factor can be considered in leukopenic cats.
  • Treatments for lymphosarcoma Lymphoma using vincristine Vincristine, prednisolone   Prednisolone  and cyclophosphamide Cyclophosphamidehave been described (Squires & Gorman, 1990).
  • Some cases of lymphoma respond well to chemotherapy, leading to remission in many cases and some cats showing no recurrence within 2 years.

Control via environment

  • FeLV is easily killed by any disinfectant.
  • FeLV-positive cat can remain healthy with good quality of life for months or years. It is important to isolate from other cats to protect both the FeLV-infected cat as well as the uninfected contacts.


  • Vaccination is recommended for all cats at risk of exposure from 8-9 weeks of age and again at 12 weeks, with a booster 1 year later to complete the primary vaccination course.
  • ABCD does not recommend reliance on vaccination to protect FeLV-negative cats living together with FeLV-infected cats.
  • Most vaccine manufacturers recommend annual boosters.
  • ABCD suggests booster vaccination at 2-3 year intervals is sufficient for cats over 3-4 years of age as adults are significantly less susceptible to FeLV infection.
  • There are several FeLV vaccines available: the first was an inactivated virus vaccine but newer vaccines contain either the viral envelope protein expressed inE.colior a canarypox virus carrying genes encoding viral proteins.

Other countermeasures

  • A highly effective strategy for the prevention of FeLV infection is by testing and prevention of uninfected cats contacting infected cats. This system has been successfully used to maintain the FeLV-free status of most serious cat breeders.
  • All the cats in the household are FeLV tested and FeLV-negative cats are separated from FeLV positive cats.
  • The cats are retested after 12 weeks, at which time any cats incubating FeLV should test positive and any which were only transiently positive should test negative.
  • Again, positive and negative cats are separated.
  • Any new cats entering the FeLV-free household should test FeLV negative before admission.
  • Annual retests are strongly advised.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Lutz H, Addie A, Belák S et al (2009) Feline leukaemia. ABCD guidelines on prevention and management. J Feline Med Surg 11 (7), 565-574 PubMed.
  • Muirden A (2002) Prevalence of feline leukaemia virus and antibodies to feline immunodeficiency virus and feline coronavirus in stray cats sent to an RSPCA hospital. Vet Rec 150 (20), 621-625 PubMed.
  • Dawson S, Gaskell R & Jarrett O S (1999) Vaccination in cats - an update. In Practice 21 (2), 71-74 VetMedResource.
  • Nowotny N, Uthman A, Haas O A et al (1995) Is it possible to catch leukaemia from a cat? The Lancet 346 (8969), 252-253 PubMed.
  • Squires R & Gorman N T (1990) Antineoplastic chemotherapy in cats. In Practice 12 (3), 101-111 VetMedResource.
  • Pacitti A M, Jarrett O, Hay D (1986) Transmission of feline leukaemia virus in the milk of a non-viraemic cat. Vet Rec 118 (14), 381-384 PubMed.

Other sources of information

  • Jarrett O (1994) Feline leukaemia virus. In: Feline Medicine and Therapeutics. Chandler E A, Gaskell C J, Gaskell R M (eds). Blackwell Science, Oxford, OX2 OEL. pp 473-487.
  • McCaw D L (1994) Advances in therapy for retroviral infections. In: Consultations in Feline Internal Medicine. August J R (ed). pp 21-25.


  • Companion Animal Diagnostics,Veterinary Diangostic Services, Faculty of Veterinary Medicine, University of Glasgow, Bearsden Road, Glasgow, G61 1QH, Scotland. Tel: + 44 141 330 5777; Fax: + 44 141 330 5748; E-mail:  companion@vet.gla.ac.uk .
  • Department of Veterinary Medicine, Bristol University, Langford House, Langford, Bristol, BS18 7DU.
  • European Advisory Board on Cat diseases (ABCD): www.abcd-vets.org .