How is feline leukaemia virus transmitted?

FeLV is shed in high concentrations in saliva, and to a lesser extent in nasal secretions, urine, faeces, and milk. The primary transmission routes are mutual grooming, shared food and water bowls, bite wounds, and from a queen to her kittens in utero or via milk. The virus is fragile in the environment — it is inactivated by drying, heat, and most household disinfectants within minutes. Close, prolonged social contact is required for transmission; casual exposure is much less likely to result in infection than, for example, feline panleukopaenia (see panleukopaenia glossary entry ).

Is there a vaccine for FeLV?

Yes — an effective vaccine exists and is recommended for at-risk cats. The WSAVA classifies FeLV vaccination as non-core (recommended based on lifestyle risk rather than universal). Cats with outdoor access, contact with unknown cats, or living in multi-cat households are candidates for vaccination. The FeLV vaccine is part of our cat vaccinations guide — discuss risk assessment with your vet at the annual booster appointment.

Can FeLV-positive cats live with FeLV-negative cats?

Generally not recommended. While transmission requires close social contact, cohabitation creates ongoing risk for the FeLV-negative cat. If a mixed household is unavoidable, strict management (separate feeding stations, separate litter boxes, avoiding mutual grooming) can reduce — but not eliminate — risk. FeLV-negative resident cats in the household should be vaccinated. The International Cat Care FeLV guide covers cohabitation management in detail.

What is the prognosis for a FeLV-positive cat?

It depends on infection outcome. Cats that achieve regressive infection (the virus becomes latent or is eliminated) may live normal lifespans. Cats with progressive infection — where the virus replicates persistently — have a median survival of 2–3 years from diagnosis, though many live longer with attentive care. FeLV-positive cats should be kept indoors, have twice-yearly vet check-ups, and any illness investigated promptly given their immunocompromised status. See our when to call the vet guide for signs that warrant an urgent appointment.

Feline Leukaemia Virus (FeLV) — Cat Glossary

What it is

Feline leukaemia virus (FeLV) is an oncornavirus — a retrovirus that can integrate into the host genome and, in some cats, cause cancerous transformation of blood cells (leukaemia and lymphoma). It is distinct from feline immunodeficiency virus (FIV), though both suppress immune function. FeLV is not transmissible to humans or other species.

FeLV is estimated to affect around 1–2% of cats in the UK, with higher prevalence in outdoor, multi-cat, and rescue centre populations. The International Cat Care FeLV resource is the most comprehensive freely available reference on the virus.

How the virus works

FeLV is a retrovirus — it reverse-transcribes its RNA genome into DNA and integrates into the host cell's chromosomes. In cats that develop progressive infection, this integration disrupts normal cell cycle control and immune regulation, leading to:

Persistent immunosuppression (vulnerability to secondary infections)
Bone marrow suppression and anaemia
Lymphoma (most commonly mediastinal or alimentary)
Leukaemia (less common than lymphoma)

Infection outcomes

Not all FeLV-exposed cats develop progressive infection. There are four recognised outcomes:

Abortive infection Best outcome

The immune system clears the virus completely before bone marrow integration. The cat tests negative and has no residual virus. Requires strong immune response — more common in vaccinated or previously exposed adult cats.

Regressive infection Latent carrier

The virus integrates into bone marrow cells but is suppressed by the immune system. The cat tests negative on standard antigen tests but harbours proviral DNA. Can reactivate if the cat becomes immunocompromised. Does not typically cause disease.

Progressive infection Persistent viraemia

The virus replicates continuously in bone marrow and circulating cells. The cat tests persistently positive on antigen tests and sheds virus. This is the outcome associated with immunosuppression, anaemia, and lymphoma. Median survival 2–3 years from diagnosis.

Focal/atypical infection Localised

The virus replicates only in specific tissues (e.g. mammary glands, bladder) without systemic viraemia. Can give inconsistent test results. Clinical significance varies.

Testing

The standard test is an ELISA antigen test (p27 protein) — available as an in-clinic snap test. A positive result should be confirmed by a second test 12 weeks later to distinguish progressive from regressive infection, as a newly infected cat may test positive initially and then clear the virus. The WSAVA vaccination guidelines recommend testing all new cats before introducing them to a resident FeLV-negative cat.

Prevention

Vaccination is effective and recommended for at-risk cats — those with outdoor access or contact with cats of unknown FeLV status. Indoor-only cats with no contact risk may not need the vaccine; discuss with your vet. Testing before vaccination is recommended (no benefit to vaccinating a cat already positive). See our cat vaccinations guide for the full schedule and how FeLV fits into the annual booster.

What it is

How the virus works

Infection outcomes

Testing

Prevention

Frequently asked questions