What Is Feline Leukaemia Virus?
Feline leukaemia virus, or FeLV, is a retrovirus that affects cats worldwide and represents one of the leading infectious causes of illness and death in the domestic cat population. Despite its name, FeLV does not exclusively cause leukaemia — the virus suppresses the immune system broadly, making infected cats susceptible to a wide range of secondary infections and diseases, including various forms of cancer, anaemia, and neurological conditions.
FeLV belongs to the same family of viruses as feline immunodeficiency virus (FIV), though the two are quite different in their behaviour and prognosis. Understanding FeLV specifically is important for any cat owner, particularly those with cats that venture outdoors or live alongside other cats.
How FeLV Is Transmitted
Unlike some feline viruses, FeLV is relatively fragile outside the body and does not survive long in the environment. Transmission requires fairly prolonged or close contact with infected cats, which is why the virus spreads most readily in multi-cat households, catteries, and colonies of outdoor cats.
The primary route of transmission is through saliva. Cats that groom one another, share food and water bowls, or engage in social contact such as rubbing and nuzzling can pass the virus between them. Biting is also an efficient route of transmission, which means cats that fight with infected individuals are at elevated risk. Transmission can also occur from an infected queen to her kittens, either in the womb, through colostrum, or during nursing.
Casual contact — brief encounters between cats, shared litter trays used separately, or humans handling an infected cat and then a healthy one — is not considered a significant route of transmission. FeLV is not contagious to humans or other species.
Stages of Infection
Not every cat exposed to FeLV goes on to develop persistent infection. The outcome depends on the cat's age, immune status, and the dose of virus encountered. Kittens under sixteen weeks of age are particularly vulnerable and are much more likely to develop progressive infection.
Following exposure, cats may fall into one of several categories. Some mount a successful immune response and eliminate the virus entirely — these cats become immune and are not carriers. Others develop what is called regressive infection, where the virus integrates into cells but is suppressed by the immune system and does not progress to active disease; these cats may not test positive on standard screening and are generally not infectious to others. In progressive infection, the virus replicates persistently, the cat remains viremic (the virus is detectable in the blood), immune suppression develops, and health deterioration follows over months to years.
Testing for FeLV
Standard FeLV testing uses an ELISA (enzyme-linked immunosorbent assay) test to detect the p27 antigen — a protein produced by the virus — in the blood. This test can be performed in-clinic using a simple blood sample and delivers results within minutes. It is included in many combination tests that check for both FeLV and FIV simultaneously.
A positive result on the initial ELISA test should always be confirmed with a second test, ideally using a different method such as an IFA (immunofluorescent antibody assay) or PCR testing, which are typically sent to an external laboratory. False positives on initial screening do occur, and the implications of a positive result are significant enough to warrant confirmation before major decisions are made.
Testing is recommended for all cats before they are introduced into a multi-cat household, for cats with unknown history, for cats that have had potential exposure to an infected cat, and for any cat presenting with unexplained illness. Many vets also include FeLV testing as part of routine health screening for cats that spend time outdoors.
Health Implications and Life Expectancy
For cats with confirmed progressive FeLV infection, the prognosis is unfortunately guarded. Studies suggest that around 80 percent of cats with persistent viraemia die within three years of diagnosis, though individual outcomes vary considerably. Some cats remain relatively stable for longer periods, particularly those with attentive veterinary care and a low-stress environment.
FeLV-positive cats are susceptible to a wide range of secondary conditions, including:
- Lymphoma and other cancers, which are significantly more common in FeLV-positive cats
- Anaemia, often non-regenerative and progressive
- Chronic and recurrent infections that would not trouble a healthy cat
- Neurological disease, including seizures and behavioural changes
- Reproductive failure in breeding queens
Management focuses on maximising quality of life, preventing secondary infections through good husbandry and prompt treatment, and monitoring closely for early signs of complications. Regular veterinary check-ups — typically every six months for stable FeLV-positive cats — allow problems to be caught and addressed early.
FeLV-positive cats should ideally be kept indoors both to protect their own immune-compromised health and to prevent transmission to other cats. Vaccination against other preventable diseases remains important, as FeLV-positive cats have reduced capacity to fight infection.
Vaccination Against FeLV
A vaccine against FeLV is available and is classified as a non-core vaccine in the UK — meaning it is recommended based on lifestyle risk rather than for all cats universally. It is strongly advised for cats that go outdoors, live with other cats of unknown FeLV status, or have any potential exposure to infected animals.
Crucially, the vaccine is only effective in cats that are not already infected. All cats should be tested before vaccination to confirm negative status, as vaccinating an FeLV-positive cat provides no benefit and delays other management decisions.
The vaccine does not provide complete protection in all cases, so testing remains important even in vaccinated cats following potential exposure. However, it substantially reduces the risk of progressive infection and is a sensible precaution for any at-risk cat.