Home » Pet Health » Feline Infections and Viruses » What is Feline Leukemia Virus?


The feline leukemia virus (FeLV) is one of the most common retroviruses infecting domestic cats. Retroviruses are viruses that enter the body of an organism, attach themselves to the host cell surface, and insert their genetic material into the cells. The viral genetic material then hijacks the host cells’ replication machinery to produce multiple viral copies of itself.

The feline leukemia virus infects the cells in the cat’s throat and multiplies. The virus then enters the bloodstream and travels to the bone marrow, home to blood cell production. Here, the virus infects bone marrow cells and causes further viral multiplication.

There are four types of FeLV:

    1. FeLV-A is the most common type of FeLV and is associated with the characteristic weakened immune system. FeLV-A often results in lymphoma and leukemia.
    1. FeLV-B is a derivative of FeLV-A and is associated with a high risk of tumor formation, leukemia, and lymphoma.
    1. FeLV-C is a rare mutation of FeLV-A that causes severe anemia and bone marrow disorders.
    1. FeLV-T is another rare mutation of FeLV-A that attacks specific immune cells called T-cells, leading to an impaired immune system and lymphomas.

As described above, FeLV is a virus that uses the cat’s cells to replicate. A mutation (or change) to the cell’s genetic code can occur when the virus FeLV invades the cells. FeLV also causes a severe weakening of the cat’s immune system. The weakened immune system, along with the changes in the genetic code of the cells, can eventually lead to the development of cancer. Sometimes this does not happen until years after the infection.

While cancer development can result in FeLV-infected cats, there are also other diseases the cats can develop. For example, while the infected cats may not develop cancer, they can instead develop diseases like anemia, which results in a dangerously low number of red blood cells.

Despite the risks associated with FeLV, there are ways to manage the disease so that your cat can lead a good quality of life. Be sure to speak to your vet about FeLV, and if your cat becomes infected with the virus, discuss the treatment options available.

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Last Updated: September 21, 2022

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The following sources were referenced to write the content on this page: 

Fujino, Y, Ohno, K & Tsujimoto, H 2008, ‘Molecular pathogenesis of feline leukemia virus-induced malignancies: insertional mutagenesis’, Vet Immunol Immunopathol, vol. 123, no. 1-2, pp. 138-143.

Hartmann, K & Hofmann-Lehmann, R 2020, ‘What’s new in feline leukemia virus infection’, Vet Clin North Am Small Anim Pract, vol. 50, no. 5, pp. 1013-1036.

Sakaguchi, S, Shojima, T, Fukui, D & Miyazawa, T 2015, ‘A soluble envelope protein of endogenous retrovirus (FeLIX) present in serum of domestic cats mediates infection of a pathogenic variant of feline leukemia virus’, J Gen Virol, vol. 96, no. 3, pp. 681-687.

Willett, BJ & Hosie, MJ 2013, ‘Feline leukaemia virus: half a century since its discovery’, Vet J, vol. 195, no. 1, pp. 16-23.

The Pet Cancer Foundation’s medical resource for pet owners is protected by copyright.

For reprint requests, please see our Content Usage Policy.