Home » Pet Health » Feline Infections and Viruses » How Does FeLV Cause Cancer?


Feline Leukemia Virus (FeLV) is the most common virus leading to cancer in cats. It is a retrovirus, which means it uses your cat’s cells to help it multiply and spread throughout the body. Although the name suggests it is a virus and cancer, FeLV is not directly cancer-causing. However, there are a few ways the virus can lead to cancer development:

    • Uncontrolled cell growth. Because the virus enters the immune cells to replicate, there is a possibility some mechanism within the cells might be disturbed, leading to uncontrolled growth of the cell. This unchecked growth can result in tumor formation and cancer.
    • Weak immune system. FeLV is known for its prolonged ability to weaken the cat’s immune system. An immune system not functioning at full strength can significantly increase a cat’s susceptibility to developing cancers.
    • Overactive immune system. Viruses can also cause chronic inflammation and increased immune system activation, especially of the immune cells called T cells. Reports have shown that increased immune activity may lead to cancer development.

Blood-based cancers, such as leukemia and lymphoma, are the most common cancers linked to FeLV. Cats infected with FeLV are about 60 times more likely to develop one of these cancers than non-infected cats, which is lower than reported before the 1980s when up to 80% of cats with lymphoma or leukemia were FeLV-positive. Evidence suggests that this decrease in prevalence is primarily due to the development of effective FeLV prevention methods, such as vaccination.

Cancer treatment

While methods of preventing FeLV exist, there is currently no treatment that eliminates FeLV once your cat is infected. However, treatments are available if your pet develops a secondary disease from the virus, like cancer. If your cat develops cancer from a FeLV infection, your vet can treat them with conventional cancer therapy, including surgery, radiation, and chemotherapy. Vets will use radiation if the cancer is in one body area. Radiation plus chemotherapy is more beneficial for cancers that have spread. Thankfully, cancerous lymphomas are very responsive to chemotherapy. Some owners may be hesitant to give chemotherapy and radiation to their FeLV-positive cats because FeLV is known to suppress the immune system. However, many researchers believe FeLV does not disqualify a cat from chemotherapy in the right situations.

Despite the cancer risk associated with FeLV, your vet can provide options that give your cat a good quality of life for years after diagnosis. Whether those therapies handle secondary infections or target the cancer directly, know that FeLV is manageable. Therefore, talk to your vet about appropriate treatments if you have a FeLV-positive cat so you can plan those many tomorrows with your pet.

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Last Updated: October 14, 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 2012, ‘Clinical aspects of feline retroviruses: a review’, Viruses, vol. 4, no. 11, pp. 2684-2710.

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.

Henry, C & Higginbotham, ML 2009, Cancer Management in Small Animal Practice-E-Book, Elsevier Health Sciences.

Mauldin, GE, Mooney, SC & Meleo, RE 1995 ‘Chemotherapy in 132 cats with lymphoma: 1988-1994’, Proceedings of VCS, pp. 35–36.

Moore, AS, Cotter, SM, Frimberger, AE, Wood, CA, Rand, WM & L’Heureux, DA 1996, ‘A comparison of doxorubicin and cop for maintenance of remission in cats with lymphoma’, J Vet Intern Med, vol. 10, no. 6, pp. 372–375.

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

For reprint requests, please see our Content Usage Policy.