Feline leukemia virus (FeLV) is not cancer itself, but it is a virus that can cause cancer in cats. The name is misleading because “leukemia” sounds like a diagnosis of blood cancer, when in reality FeLV is a retrovirus, more comparable to HIV in humans. About 10 to 20% of infected cats will eventually develop lymphoma or a related cancer. The rest may face other serious health problems, including anemia and immune suppression, without ever developing a malignancy.
Why the Name Is Confusing
FeLV gets its name from the type of cancer researchers first linked it to: leukemia, a cancer of the blood and bone marrow. But the virus itself is just that, a virus. It belongs to a family of retroviruses that insert their genetic material into a cat’s DNA. Once embedded, the virus can hijack normal cell growth, but it doesn’t always do so. Many cats live with FeLV for years without developing cancer, though the infection does shorten life expectancy and leaves them vulnerable to a range of other illnesses.
How FeLV Actually Causes Cancer
When cancer does develop, it happens through a multi-step process. The virus inserts itself near genes that control cell growth. FeLV carries powerful genetic switches that can crank up the activity of these neighboring genes. If those genes have the potential to drive uncontrolled cell division (oncogenes), the virus essentially flips them into overdrive. This is the core mechanism behind FeLV-related tumors.
The virus also affects which cell types it infects. Certain strains of FeLV bind more tightly to cell receptors, allowing them to spread faster through the body or reach cell types they wouldn’t normally access. This means different viral strains can lead to different diseases. One subtype (FeLV-A) is found in nearly all infected cats and is considered less aggressive on its own. When it recombines with genetic material inside the cat to produce a second subtype (FeLV-B), the risk of lymphoma and leukemia increases significantly. A third subtype (FeLV-C) is associated with severe anemia rather than cancer.
Types of Cancer Linked to FeLV
Lymphoma is the most common cancer caused by feline leukemia virus. FeLV-associated lymphomas tend to arise from T-cells and most often appear in two locations: the mediastinum (the space in the chest between the lungs) and throughout multiple lymph nodes at once, known as multicentric lymphoma. In one study of FeLV-positive cats with lymphoma, 62.5% had mediastinal tumors and 31.2% had multicentric disease. These forms are strongly correlated with FeLV infection and are more frequent in cats carrying both the A and B subtypes of the virus.
Actual leukemia, the blood cancer, also occurs but is less common than lymphoma. Among FeLV-positive cats diagnosed with leukemia, 40% had acute myeloid leukemia, 30% had chronic myeloproliferative disease, and 30% had acute lymphoid leukemia. These cancers affect the bone marrow and blood-forming cells, leading to abnormal white blood cell counts and progressive organ damage.
FeLV-positive cats also develop cancers in less typical locations, including the nasal passages and spinal cord, though these are rarer.
Not Every Infected Cat Gets Cancer
The outcome depends heavily on whether the infection progresses or whether the cat’s immune system manages to suppress it. Cats with progressive infection, meaning the virus is actively replicating and circulating in the bloodstream, face the highest cancer risk and the worst prognosis. In one Brazilian study, cats with progressive FeLV had a median survival time of just 30 days after diagnosis of clinical disease, largely because they were already severely ill by the time they were tested. Overall, cats with progressive infection survive roughly three years after initial infection.
Cats with regressive infection tell a different story. Their immune systems push the virus into a dormant state. The virus is still present in the DNA of some cells, but it’s not actively producing new copies. These cats showed no significant reduction in life expectancy compared to uninfected cats in the same study. This distinction matters enormously: a positive FeLV test does not automatically mean a cat will develop cancer or die young.
How FeLV Testing Works
Standard screening uses a blood test that detects a viral protein circulating in the bloodstream. These point-of-care tests are widely available at veterinary clinics and can return results in minutes. A positive result means the virus is present, but it doesn’t tell you whether the infection will progress or regress.
A second type of test detects the virus inside bone marrow cells. Cats only test positive on this test once the virus has established itself deep in the bone marrow, which represents a more advanced stage of infection. Because these two tests measure different stages, it’s possible for results to conflict. A cat might test positive on the initial screening but negative on the bone marrow test, indicating the infection hasn’t fully taken hold yet. Retesting a few weeks later helps clarify whether the cat is heading toward progressive or regressive infection.
How Vaccination Changed the Landscape
Before widespread vaccination, roughly 70 to 80% of cats with lymphoma tested positive for FeLV. That number has dropped dramatically. Current studies from the U.S., Germany, and the U.K. report that fewer than 14.5% of feline lymphomas are now associated with FeLV or the related feline immunodeficiency virus. Vaccination programs deserve most of the credit for this shift.
Four FeLV vaccines are available in the United States, though they vary in how well they work. In a head-to-head comparison, one inactivated whole-virus vaccine completely prevented persistent infection in all vaccinated cats after direct viral challenge, while a live vectored vaccine allowed half of the vaccinated cats to become persistently infected. No vaccine is 100% effective in the real world, but vaccination combined with keeping cats indoors and away from infected animals remains the most reliable way to prevent FeLV-related cancers.
What This Means for a Diagnosed Cat
If your cat has tested positive for FeLV, the virus is present but cancer is not guaranteed. The 10 to 20% lifetime cancer risk is significant, but it also means the majority of FeLV-positive cats will face other complications first, particularly infections their weakened immune systems struggle to fight off. Young cats are especially vulnerable to FeLV-associated lymphoma, particularly mediastinal forms, and the presence of the FeLV-B subtype appears to correlate with more aggressive disease and higher mortality.
Monitoring is key. Regular veterinary checkups can catch early signs of lymphoma or leukemia, such as swollen lymph nodes, difficulty breathing (from a chest mass), unexplained weight loss, or persistent fever. Cats with progressive infection benefit from being kept indoors, both to protect their compromised immune systems and to prevent transmission to other cats.