Feline Leukaemia Virus Infection: A Critical Appraisal of Outcome Classification, Diagnostic Inference, and Control Strategy

Abstract

Feline leukaemia virus (FeLV) has been studied for six decades, yet the conceptual framework used to describe infection has changed more rapidly than the diagnostic and epidemiological evidence base that supports it. The binary distinction between persistently viraemic and non-viraemic animals has been replaced by a four-category scheme comprising progressive, regressive, abortive, and focal infection, and this scheme now underpins clinical guidelines, shelter policy, and prevalence reporting. The present critical narrative review examines whether the evidence supporting that framework is strong enough to bear the interpretive weight now placed upon it. Literature was identified through open scholarly databases and citation searching, appraised for design adequacy and reference-standard independence, and synthesised thematically rather than catalogued. Several conclusions are well supported. Endogenous FeLV sequences are confined to the genus Felis, recombine readily with exogenous virus to generate multiple subgroup B variants within individual hosts, and vary in copy number in ways that correlate with disease outcome. Proviral DNA persists in tissues of regressively infected cats for years, and quantitative antigen and provirus measurements at diagnosis are associated with survival. Other conclusions are considerably weaker. The four outcome categories are defined by combinations of test results rather than by independently verified biological states, which introduces circularity when those same tests are used to estimate category prevalence. Reported positive predictive values for point-of-care antigen testing in apparently healthy populations are low, and several widely cited sensitivity and specificity estimates were derived against reference standards that measure the same analyte. Published survival estimates for progressive infection differ by more than an order of magnitude between cohorts, a discrepancy that is more plausibly explained by case ascertainment and health status at enrolment than by biological difference. Vaccine efficacy studies almost uniformly use prevention of antigenaemia as the endpoint and therefore provide little information about prevention of regressive infection. Priority research needs include reference standards for outcome assignment that are independent of the screening assays, prospective cohorts with standardised enrolment, challenge studies reporting proviral as well as antigenaemic outcomes, and prevalence surveys designed for comparability rather than local description.