Feline leukemia (FeLV) is an infectious disease supported by a retrovirus of the  Oncovirinae subfamily, and causes immunosuppression, neoplastic diseases and anemia.

It is widespread and is one of the main causes of death in cats, related to risk factors such as access to the external environment or cohabitation (catteries, breedings, shelters). Transmission mainly occurs through contact with infected organic fluids (especially saliva) and bites wounds. Vertical transmission (in utero and through milk) is also possible. Susceptibility is greater in young cats. After initial infection, the virus replicates at the level of the oropharyngeal lymphoid tissue and then in the intestinal and respiratory mucosa and salivary glands; finally it spreads to the bone marrow. Viremia, usually appears after 2-4 weeks and is accompanied by fever, discomfort and lynfoadenopathy and developes according to different clinical pictures depending on different factors: the immune status and the age of the cat, the strain and viral concentration as well as the duration of exposure.

in this way 3 courses are possible:

1. Elimination of the virus: immunocompetent cats neutralize the virus and the infection disappears in 4-8 weeks.
2. Transitional viremia: the virus located in the bone marrow is neutralized by the immune system (after 3 weeks of viremia), causing a latent infection that can reactivate following a stimulus or stress reaction. At this stage cats are not infectious and reactivation becomes more uncommon as latency stage persist. However, the cellular alteration carried by the virus can lead to neoplastic diseases or myelosuppression.
3. Persistent viremia: when the immune system fails to contain the infection and this persists for more than 16 weeks the cat dies within 3 years from the diagnosis due to the development of diseases related to virus.

Diseases related to infection include malignant lymphoid neoplasms, myeloproliferative disorders, anemia (regenerative and non-regenerative), “panleukopenia-like” syndromes, various diseases related to induced immunosuppression and various immune-mediated diseases.

Atypical infections: persistent local viral replication with poor and intermittent production of antigens occurs very rarely.
Tests for the diagnosis of FeLV allow for the detection of the antigen.
The prevalence of FeLV infection has dropped significantly over the past 25 years, thanks to reliable diagnostic tests and vaccines.