Bovine Leukosis: A Silent Threat

Abstract

Bovine leukaemia virus (BLV) establishes lifelong infection of predominantly B lymphocytes and causes enzootic bovine leukosis, yet most infected cattle remain outwardly healthy. This mismatch between biological persistence and visible disease has encouraged under-recognition in many cattle industries, even where herd-level prevalence is high. This critical narrative review evaluates BLV as a silent threat by integrating evidence on viral persistence, host and viral determinants of proviral burden, transmission ecology, production and welfare consequences, diagnosis, surveillance, control, and the unresolved question of human-health relevance. Literature was selected through live searches of accessible biomedical and veterinary sources, verification of bibliographic metadata and digital object identifiers, and critical comparison of mechanistic, observational and field-intervention evidence. The evidence is strongest that BLV is maintained by transfer of infected cells, that proviral load is a useful but incompletely standardised marker of infectiousness and disease risk, and that national or herd-level control is feasible when repeated testing is coupled with removal, segregation and rigorous prevention of blood transfer. Evidence for reduced longevity and production is persuasive at herd and lifetime scales, but simple seropositivity is an imprecise predictor; high proviral load, persistent lymphocytosis, parity, management and survivorship materially modify observed effects. Immune dysregulation is biologically credible and supported by vaccination and immunological studies, although causal links with individual secondary diseases remain less secure. Reported detection of BLV markers in human tissues is inconsistent across laboratories and does not establish zoonotic causation. The central policy failure is therefore not absence of control options, but failure to align diagnostic purpose, herd prevalence, economic incentives and sustained implementation. Future progress requires harmonised proviral-load standards, longitudinal multi-herd studies, transmission experiments reflecting modern production systems, economic evaluations that include welfare and replacement costs, and rigorously blinded human studies with sequence-level confirmation and contamination control.