Enzootic bovine leukosis
Enzootic bovine leukosis is a disease of cattle caused by bovine leukemia virus (BLV). The virus causes a persistent, life-long infection in a subset of B cells. Malignant tumours (lymphomas) ultimately develop in 2–5% of infected animals, predominantly in adult cattle older than 3–5 years. Lymphomas invariably lead to death of the animal within months. Before 1960, BLV was endemic in dairy herds in Northern/Eastern Europe and North America. Since then it has spread to all continents. The disease has been successfully controlled and eliminated from many countries in Europe. There is no evidence to suggest that any significant reservoir of BLV exists among other species, nor for any role of BLV in human disease or cancers. BLV exhibits a slow, progressive spread within a herd and it is likely to persist if control measures are not applied. The main modes of transmission are perinatal from cow to calf, via colostrum and milk, and close contact that allows transfer of infected lymphocytes, such as dehorning and injections using non-sterile utensils. Transfer between herds is almost entirely by movement of infected animals. Suitable methods have been developed for diagnosis of BLV infection in specimens of blood, milk and lymphomas. EBL has a negative impact on milk yield and leads to increased premature culling. The welfare consequences of lymphomas vary according to the location and magnitude of organ involvement. Criteria for maintaining country freedom from EBL differ substantially between OIE and EU. Surveillance for freedom should be based on a combination of serological testing of adult animals and identification of lymphomas at slaughter.