Bovine leukosis (BL) is a lifelong disease of dairy cattle caused by the bovine leukosis virus. Although most infections appear to be subclinical, a proportion of cows over 3 years old (30 – 50%) develop persistent lymphocytosis, and a smaller percentage (5%) develop malignant tumors (lymphosarcomas) in various internal organs. It has been reported this disease can reduce milk production (up to 1000 kg/lactation), cow fertility, and longevity due to premature death of cows. Moreover, it produces economic losses associated with condemnation of carcasses after slaughter.
As a result of decades of systematic control and eradication programs, BL has been successfully controlled and eradicated from many countries (some countries in the European Union, Australia, New Zealand…); however, BL is common in North American dairy cattle. Recent surveys indicate that 89% of US dairy farms had cattle seropositive for BL. Similarly, in a 2015 study from Canada, 78% of 315 surveyed dairy herds from 7 provinces had antibodies to BL.