Feline Retroviral Infections and Tumour Development: The Oncogenic Roles of Feline Leukaemia Virus and Feline Immunodeficiency Virus

Abstract

Feline leukaemia virus (FeLV) and feline immunodeficiency virus (FIV) are the two most clinically significant retroviruses of domestic cats, both carrying well-documented oncogenic potential. FeLV, a gammaretrovirus, is the most important feline tumour virus known, increasing the risk of lymphoma development up to 60-fold in progressively infected cats through mechanisms including insertional mutagenesis, proto-oncogene activation, and recombination with host cellular oncogenes. FIV, a lentivirus analogous to human immunodeficiency virus (HIV), increases lymphoma risk approximately 5-6-fold, primarily through chronic immune dysregulation, impaired tumour immunosurveillance, and sustained antigenic stimulation of lymphocytes, although direct insertional oncogenesis has also been demonstrated in rare cases. Co-infection with both viruses synergistically amplifies the risk of lymphoid malignancies up to 77-fold. Despite the marked decline in FeLV prevalence following widespread vaccination programmes, the overall incidence of feline lymphoma has not decreased proportionally, suggesting that FeLV-independent aetiological factors — including FIV, environmental carcinogens, and chronic inflammation — play increasingly important roles. This review provides a comprehensive overview of the oncogenic mechanisms, epidemiological associations, tumour types, and clinical implications of FeLV and FIV infections in domestic cats, with emphasis on recent advances in the understanding of retroviral oncogenesis.