BLv

Bovine leukemia virus (BLV) is a retrovirus of the genus Deltaretrovirus within the Retroviridae family. It infects cattle and causes enzootic bovine leukosis, a disease characterized by persistent infection and, in a minority of animals, development of malignant lymphoid tumors. BLV is found worldwide, with higher seroprevalence in dairy herds and regions with less aggressive control programs.

Genome and biology: BLV is an enveloped, single-stranded RNA virus with a proviral DNA stage integrated into the host genome. The genome encodes the structural proteins gag, pol, and env, along with regulatory genes such as tax and rex. Some isolates express additional regulatory elements, and recent work has described viral microRNAs.

Transmission and epidemiology: Most BLV infections are asymptomatic. About 30% show persistent lymphocytosis, and around 5–10% of infected cattle develop B-cell lymphosarcoma after several years. Transmission occurs primarily through contact with infected blood and secretions, including iatrogenic transmission via shared needles, dehorning equipment, and other instruments, and through colostrum and milk from infected dams.

Clinical disease: Lymphosarcoma may involve the thymus, heart base, or abomasum; signs include enlarged lymph nodes, weight loss, and decreased milk yield. Many infected animals remain clinically normal for life.

Diagnosis: Serology (ELISA or other antibody tests) detects BLV exposure; PCR or proviral load assays detect integrated virus. A complete blood count may reveal persistent lymphocytosis, and tumor biopsy is used when masses are present.

Prevention and control: No widely available vaccine exists. Herd-management measures reduce transmission, including test-and-segregate or cull strategies, strict biosecurity, single-use equipment, and pasteurization of colostrum.

Zoonosis and treatment: There is no evidence that BLV infects humans; there is no specific antiviral therapy, and control efforts focus on herd-level management.