What is Feline Leukemia Virus?
Feline leukemia virus (FeLV) is a retrovirus that spreads between cats through close contact, mostly through saliva. It weakens the immune system and can lead to anemia and cancers such as lymphoma, but the infection itself is not a cancer, and it does not infect people.
FeLV is an infectious disease found in cats worldwide. Like other retroviruses, it can insert its genetic material into the cat's own DNA. The virus usually enters through the mouth or nose, multiplies in the lymphoid tissue of the throat, then spreads through the blood. What happens next depends on the cat's immune response. The American Association of Feline Practitioners (AAFP) guidelines describe three outcomes:
- Abortive infection. The immune system clears the virus before it settles into the cat's DNA. These cats test negative and are not infectious.
- Regressive infection. The virus sits in the cat's DNA as a provirus but is not actively circulating, so the cat does not shed it. This replaces the older term "latent infection." The virus can reactivate later, for example if the cat becomes immunosuppressed, and it can still pass in donated blood.
- Progressive infection. The virus keeps multiplying, including in the bone marrow, and the cat sheds it. These cats carry the highest risk of FeLV-related disease.
Kittens are the most likely to develop progressive infection, so early testing and protection matter most for them.
How is FeLV transmitted?
FeLV is shed mainly in saliva, and less often in nasal secretions, milk, urine and feces. The routes that matter in practice:
- Mutual grooming and other prolonged, close contact with an infected cat
- Sharing food and water bowls or litter boxes with an infected cat over time
- Bite wounds, usually from fights (less common than grooming and sharing)
- From an infected queen to her kittens before birth or through nursing
The virus does not survive long outside the body, and ordinary detergents and disinfectants kill it. Living alongside an infected cat is the real risk, not brief contact with a surface.
FeLV infects domestic cats and wild cats such as lions. It does not spread to people, and there are no known human cases.
Here is how the virus moves from cat to cat.
How common is FeLV?
In a large 2017 study of cats tested in the United States and Canada, about 3.1% were positive, according to the Merck Veterinary Manual. Cornell's Feline Health Center puts it at 2-3% of cats in the US and Canada, rising to as much as 30% in cats that are ill or at high risk.
Risk is higher for outdoor cats, unneutered males, cats that already have other diseases, and cats living with infected cats or cats of unknown FeLV status. In one European study, living with five or more cats was also a risk factor.
How is FeLV diagnosed?
FeLV is diagnosed with blood tests, and one result does not always settle it. The 2020 AAFP retrovirus guidelines recommend testing:
- Every cat as soon as possible after it joins a home or shelter
- After any possible exposure to an infected or unknown-status cat
- Before a first FeLV vaccination
- When a cat is sick, whatever its earlier results
The usual first step is an in-clinic screening test for an FeLV protein (the p27 antigen). A positive screen can be wrong, so your veterinarian will usually confirm it with a different test, such as PCR, which looks for the virus's genetic material, or a lab test like IFA.
Timing matters too. The screening test turns positive about 30 days after exposure at the earliest, so guidelines advise retesting at least 30 days after the last possible exposure, including for a new cat. A confirmed positive is often retested weeks to months later to help tell progressive from regressive infection. When results disagree, the cat is treated as possibly infected until the picture is clear.
FeLV vaccination does not cause a positive result on these tests.
What are the clinical signs?
Many infected cats show no signs early on, sometimes for a long time. Later illness often comes from the weakened immune system, which leaves progressively infected cats open to bacterial, fungal, viral and other secondary infections. Signs include:
- Loss of appetite and weight loss over time
- Poor coat condition
- Enlarged lymph nodes
- Persistent fever
- Pale gums, or inflamed gums and mouth (gingivitis and stomatitis)
- Repeated infections of the skin, bladder or upper airways
- Ongoing diarrhea
- Seizures, behavior changes or other neurologic problems
- Eye problems
FeLV is also linked to these conditions:
- Lymphoma (lymphosarcoma) and leukemia. These cancers develop in as many as 30% of cats with progressive infection, according to Merck. More on lymphoma in cats.
- Anemia. A drop in red blood cells, usually because the virus suppresses the bone marrow. More on anemia in cats.
- Reproductive problems. Fetal death, abortion and weak "fading" kittens.
Living with an FeLV-positive cat
A positive test is not an immediate crisis, and many infected cats live well for years. There is no proven antiviral cure, so care focuses on keeping the cat healthy and the virus away from other cats. Your veterinarian will tailor the plan, which generally includes:
- Keeping the cat strictly indoors
- Housing it apart from FeLV-negative cats, with its own bowls and litter box, and testing the other cats in the home
- Spaying or neutering
- Preventive checkups at least twice a year, plus the other vaccines your veterinarian recommends
- Prompt care for any illness, since these cats fight off infections less well
The FeLV vaccine does not help a cat that is already infected, and treatments aimed at the virus itself have not shown clear benefit in controlled studies.
Outlook. In large studies, cats lived roughly two and a half years on average after an FeLV diagnosis, compared with about six years for cats that tested negative. The range is wide: cats with regressive infection and many adults live much longer, while kittens tend to decline faster.
Call your veterinarian if your cat has a positive or unclear FeLV test, has a bite wound from an unknown cat, or is about to share a home with an FeLV-positive or untested cat. If your cat is FeLV-positive, call about any change in weight, appetite, energy, gums, eyes or behavior.
How to prevent FeLV
The most reliable protection is keeping uninfected cats away from infected ones.
- Test every new cat before it meets the other cats in your home.
- Keep cats indoors or supervised outside. Our indoor vs outdoor cats guide covers the tradeoffs.
- Separate FeLV-positive cats from negative ones, with no shared bowls or litter boxes.
- Vaccinate as your veterinarian advises. The AAFP recommends FeLV vaccination for all kittens and for at-risk adults, such as outdoor cats or cats living with positive or unknown-status cats. Test first. The vaccine does not protect every cat, so it does not replace testing and separation.
- Clean routinely. Household cleaners kill the virus, but cleaning only helps alongside keeping cats apart.
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Sources
- AAFP / Journal of Feline Medicine and Surgery: 2020 AAFP Feline Retrovirus Testing and Management Guidelines (Little et al.)
- Cornell Feline Health Center: Feline Leukemia Virus
- Merck Veterinary Manual: Feline Leukemia Virus Disease
- Merck Veterinary Manual for Cat Owners: Feline Leukemia Virus (FeLV)
This article is for general information and is not a substitute for advice from your veterinarian.
