Feline leukaemia and immunodeficiency viruses (FeLV and FIV)

What it is

Feline leukaemia virus (FeLV) and feline immunodeficiency virus (FIV) are retroviruses: they copy their RNA genome into DNA and integrate it into the DNA of the cat's cells as a provirus. Neither infects people; they are not zoonoses.

FeLV (gammaretrovirus). It is shed mainly in saliva and spreads by close, prolonged contact (mutual grooming, shared bowls and litter trays) and by bites, as well as from queen to kittens. After exposure, infection follows one of three courses:

• Abortive: the immune system eliminates the virus; the cat has no detectable antigen or provirus, only antibodies.
• Regressive: after an initial viraemia the cat controls replication and no longer has antigen in the blood, but the provirus persists in the bone marrow. It rarely sheds virus, can reactivate with immunosuppression and its blood can transmit infection in a transfusion.
• Progressive: viraemia is persistent, the cat sheds virus and often develops FeLV-associated diseases that shorten its life.

The typical diseases of progressive infection are lymphoma (especially mediastinal and multicentric in young cats) and leukaemia, cytopenias (particularly non-regenerative anaemia), myelodysplastic syndromes and immunosuppression with secondary infections.

FIV (lentivirus). It is transmitted mainly by bites during fights; transmission in stable households where cats do not fight is uncommon. It infects CD4+ T lymphocytes and macrophages. A brief acute phase (fever, lymphadenomegaly, neutropenia) is followed by an asymptomatic phase that usually lasts years, and in some cats an immunodeficiency then develops, with chronic infections, oral disease, neoplasia (mainly lymphoma) and sometimes neurological signs.

Who it affects

Cats with outdoor access. FIV predominates in adult intact males that fight. FeLV is acquired mainly when young; kittens are much more susceptible to progressive infection than adults. Cats living with many other cats of unknown status, stray cats and sick cats are more likely to be infected.

Anatomy involved

• Spleen: a target lymphoid organ of both viruses. In acute FIV infection there is lymphoid hyperplasia and generalised lymphadenomegaly; in advanced stages, lymphoid depletion. The spleen enlarges in FeLV-associated lymphoma and leukaemia, and in haemolytic anaemia due to haemoplasmas, which is more common in immunosuppressed cats. The bone marrow, the other haematopoietic organ, is the reservoir of provirus in regressive FeLV infection and the origin of the cytopenias, myelodysplasia and leukaemia.
• Small intestine: alimentary lymphoma is the most common form of lymphoma in cats, but today most cats with alimentary lymphoma are FeLV-negative (see the atlas entry "Feline alimentary lymphoma"). FIV slightly increases the risk of lymphoma. Both viruses can also cause enteritis, FeLV through crypt damage.
• Mandible: gingivitis, periodontitis and stomatitis are among the most common signs in cats with FIV and are also seen with FeLV. The inflammation affects the gingiva and mucosa covering the mandible and maxilla; "Feline chronic gingivostomatitis" has its own entry in the atlas.

Clinical signs

• FeLV: many cats with progressive infection are healthy for a time. Later come pallor and weakness from anaemia, fever, weight loss, recurrent infections, stomatitis, lymphadenomegaly, dyspnoea and pleural effusion from mediastinal lymphoma, reproductive problems and neurological signs.
• FIV: most infected cats have no signs for years. When signs appear they are non-specific: gingivostomatitis, fever, weight loss, lymphadenomegaly, chronic infections of the skin, airways or intestine, uveitis, kidney disease or behavioural changes.

Diagnosis

According to the AAFP guidelines, the status for both viruses of every cat at risk of infection should be known: when it joins a household, after exposure to an infected cat or a cat of unknown status, before vaccination against FeLV or FIV and whenever it is ill.

• FeLV: point-of-care tests detect p27 antigen in blood. A positive result in a healthy cat is confirmed, and repeated a few weeks later to distinguish progressive infection from transient viraemia. Provirus PCR on blood detects regressive infections, which are negative on antigen tests. FeLV vaccination does not interfere with these tests.
• FIV: point-of-care tests detect antibodies. In kittens under 6 months a positive may be due to maternal antibodies and is repeated after that age. Cats vaccinated against FIV test positive on most antibody tests; in them PCR or tests that distinguish vaccine antibodies are used, with limitations.
• A single result is not always enough to know the cat's status; sometimes the test must be repeated or methods combined.

Treatment and prevention

There is no antiviral treatment of proven efficacy; data on antiretroviral and immunomodulatory drugs are scarce. Management relies on regular check-ups (with complete blood count, biochemistry and urinalysis), keeping the cat indoors and neutered, good nutrition, parasite control, core vaccinations and prompt treatment of each intercurrent disease, including chemotherapy for lymphoma. Cats with progressive FeLV should not live with healthy unprotected cats. Cats with FIV can live with other cats if they do not fight. Infected cats must not be blood donors.

The FeLV vaccine is recommended for all kittens and for adults at risk of exposure, always after a negative test; it does not protect 100% and does not replace testing new cats. The FIV vaccine is only available in some countries, its protection under field conditions is doubtful and it complicates diagnosis. The basis of prevention is testing every cat, separating FeLV-infected cats and preventing fights (neutering, indoor life).

Key studies

• AAFP feline retrovirus guidelines (Little et al., 2020): the reference consensus for diagnosis and management of FeLV and FIV. It recommends knowing the status of every cat at risk of infection and warns that a single test may not be enough. It notes that some infected cats, especially those with FIV, live for many years with good quality of life and that there are hardly any data supporting antiretroviral or immunomodulatory treatments.
• Burling et al. (2017), cross-sectional study of 62,301 cats tested at 1,396 clinics and 127 shelters in the United States and Canada. Seroprevalence was 3.1% for FeLV and 3.6% for FIV. Adult age, outdoor access, illness and being an intact male were risk factors for both viruses. Among cats with oral disease, 4.7% were FeLV-positive and 9.7% FIV-positive; among those with abscesses or bite wounds, 5.5% and 12.5%. 13.9% of sick cats were infected, compared with 3.6% of healthy cats. A single point-of-care test was used, without confirmation.
• Spada et al. (2018), retrospective study of 103 seropositive cats (53 FIV, 40 FeLV and 10 with both) and 103 age- and sex-matched negative cats at a referral centre in northern Italy. Median survival was 3,960 days in negative cats, 2,040 with FIV, 714 with FeLV and 77 in coinfected cats. The hazard of death was 3.4 times higher with FeLV and 7.4 times higher with both viruses than in negative cats, whereas FIV did not significantly affect survival. A low red blood cell count at diagnosis was associated with shorter survival. It is a single-centre study.
• Liem et al. (2013), retrospective case-control study of 520 client-owned cats tested for FIV at a university centre in Sydney (Australia). Prevalence was 14.6%; being mixed-breed, male and older were risk factors. Laboratory abnormalities were equally common in infected and uninfected cats, lymphoid neoplasms were slightly more frequent with FIV and survival did not differ significantly.
• Castillo-Aliaga et al. (2026), retrospective cohort of 132 cats at a UK sanctuary followed over 20 years (22 uninfected, 82 with FIV, 24 with FeLV and 4 with both). Median life expectancy was 160.3 months in uninfected cats, 149.1 with FIV, 41.5 with FeLV and 74.2 with both. Both infections shortened life, although the effect of FIV was smaller; neoplasia, especially lymphoma, was associated with FeLV, and oral disease with FIV. The groups were small, particularly the uninfected group. It qualifies the earlier studies: FIV has a small effect on survival, but perhaps not none.
• Westman et al. (2016), case-control study in Australia of the FIV vaccine in cats with outdoor access; after applying the inclusion criteria, 89 vaccinated and 212 matched unvaccinated cats were analysed. 5 of the 89 vaccinated cats (6%) and 25 of the 212 controls (12%) were infected; the estimated protective rate was 56%, with a 95% confidence interval of −20 to 84, and the difference was not significant. It casts doubt on the vaccine's efficacy under field conditions.