FeLV: feline leukaemia virus
A preventable infection with several possible outcomes, not all of them bad.
Key points. FeLV spreads between cats more readily than FIV, through close contact. A positive screening test is not a final answer: follow-up testing separates cats with progressive infection from those whose immune system is controlling the virus. A vaccine exists and is recommended for kittens.
What it is
Feline leukaemia virus is a retrovirus that can suppress the immune system, cause severe anaemia, and increase the risk of cancers such as lymphoma. Prevalence has fallen markedly over recent decades thanks to testing and vaccination: about 2 to 3 percent of cats in the US and Canada test positive, with much higher rates among sick or high-risk cats (Cornell).
How it spreads and how contagious it is
- The virus is passed mainly in saliva during prolonged close contact such as mutual grooming and sharing bowls, and by bites. Urine, faeces and milk are less common sources.
- Mother to kittens before birth, during birth and while nursing is an important route.
- The virus is fragile and survives only hours outside a cat (up to about 48 hours in moist conditions, per the ABCD). Environmental spread is unlikely, and the ABCD says no waiting period is needed before a new cat uses a space once an infected cat lived there, though thorough cleaning is advised.
- Cats at highest risk: those living with infected or unknown-status cats, free-roaming cats, kittens of infected mothers, and groups of several cats of unknown status.
- Spread by contaminated needles, instruments or blood transfusion is possible, which is why blood donors are screened.
Possible outcomes after exposure
Exposure does not always lead to lifelong disease. Depending on the cat's age and immune response, infection can follow one of several paths:
- Abortive: the immune system clears the virus. The cat tests negative and is not infectious.
- Regressive: the virus becomes integrated into the cat's cells but is not actively replicating in the blood. These cats are generally not infectious and often stay well, but the virus can reactivate, for example under stress or immunosuppression.
- Progressive: the virus replicates persistently, the cat sheds virus to others and is at risk of FeLV-related disease. Kittens are far more likely to follow this course than adults.
- Focal infection, confined to certain tissues, is rare.
Sources disagree on how often each outcome occurs, and estimates differ between studies, so treat any percentages with caution.
Testing and diagnosis
- The standard screening test is a blood test for the viral protein p27 (an antigen test, ELISA or point-of-care). It mainly detects progressive infection.
- A positive screening result can be a false positive, particularly in low-risk cats, and needs confirmation. A PCR test detects the integrated virus and so also detects regressive infection.
- Because status can change over time, positive cats are retested several weeks later (Cornell suggests 6 to 12 weeks) to tell progressive from regressive infection. After a possible recent exposure, test again about 3 to 6 weeks later (Cornell) or about 6 weeks later (ABCD).
- Unlike FIV, FeLV antigen tests are not confused by maternal antibodies or vaccination, so kittens can be tested at any age.
- Every new cat, and every cat before it joins a household, should have its FeLV (and FIV) status known. If one cat is positive, test the others.
Prognosis
This depends strongly on the type of infection.
- Progressive infection: a shortened life expectancy. Cornell gives a median survival of roughly 2.5 years after diagnosis; International Cat Care states that an estimated 80 to 90 percent of cats with a persistent infection die within 3 to 4 years of diagnosis. Many cats nevertheless stay well for a long time, and individual outcomes vary.
- Regressive infection: generally a good prognosis (ABCD), provided stress and immunosuppression are avoided.
The AAFP guidance is that a decision for euthanasia should never be made solely on a cat's retrovirus status.
Day-to-day care
- There is no cure. Care focuses on preventing and promptly treating secondary problems such as infections and anaemia (which may need transfusion).
- Keep the cat strictly indoors if it is progressively infected, to protect other cats and to limit exposure to infections. A secure outdoor enclosure can give fresh air.
- Vet check-ups about every 6 months are a common recommendation, with weight and blood tests. Monitor appetite, weight, activity, litter habits, mouth, eyes and behaviour at home and report changes promptly.
- Neuter, feed a complete commercial diet (avoid raw food and unpasteurised dairy), and keep parasite control up to date.
- Some antiviral drugs have been used, but they can have significant side effects and may be unavailable (Cornell). Ask your vet.
- Reduce stress, which may help regressive cats avoid reactivation.
- Vaccination in infected cats is discussed on the vaccination page.
When to call a vet urgently: breathing difficulty, collapse, a cat that is straining to pass urine or producing none, repeated vomiting, not eating for 24 hours or more, or sudden weakness in the back legs. See signs that need a vet urgently.
Can an FeLV-positive cat live with FeLV-negative cats?
This is more difficult than with FIV, because FeLV spreads through ordinary close contact. International Cat Care states that ideally FeLV-positive cats are kept separately in a home, and Cornell likewise advises separating infected and uninfected cats and not sharing bowls or litter trays. Where complete separation is not possible, International Cat Care suggests keeping vaccinated, uninfected cats together and reducing stress in multi-cat homes. Cats that are regressively infected are generally not infectious.
If you already share a home with an FeLV-positive cat, speak to your vet about testing and vaccinating the others. If one kitten in a litter tests positive, the ABCD advises treating the whole litter as infected and keeping it apart from uninfected cats until each has been tested.
Vaccination
FeLV vaccines reduce the risk of infection but do not protect every cat. The AAHA/AAFP treat FeLV vaccination as core for kittens (non-core for adults, depending on risk), the WSAVA 2024 guidelines advise it for young cats and for adult cats with outdoor access, and the ABCD recommends it for all cats at risk. Cats should be tested negative before being vaccinated, because vaccination does not benefit already-infected cats. Details are on the vaccination page.
People and dogs
FeLV is a cat virus and is not known to infect people or dogs. Normal hygiene (handwashing) after handling any sick animal is sensible.