Breeding beef bulls as a source of BLV transmission

Abstract

Bovine leukosis is a chronic lymphoproliferative disorder caused by bovine leukemia virus (BLV) that leads to economic losses in the beef and dairy industries. The USDA NAHMS Beef 1997 study estimated that 38% of cow-calf beef herds and 10.3% of individual adult cows in the US are BLV seropositive. Most BLV infected animals are asymptomatic carriers of the virus while a proportion of infected cattle develop lymphocytosis (30-40%) and lymphosarcoma (5-10%). The major route of virus transmission is iatrogenic through the transfer of blood from infected cattle. BLV proviral DNA has also been identified in nasal secretions, saliva, milk, colostrum, and semen, which could serve as potential sources for virus transmission. In the US, natural service accounts for 90% of the breeding in beef cow-calf operations. During natural breeding, minor genital tract trauma resulting from copulation may lead to blood transfer and transmission of BLV. Alternatively, BLV transmission may occur via transfer of infected secretions including semen and smegma during copulation. Little is known about the prevalence of BLV in breeding bulls and few studies have evaluated semen or smegma as a potential route of BLV transmission. The objectives of this study were to 1) determine the prevalence of BLV in breeding bulls presented for breeding soundness exams (BSE) at Michigan State University (MSU), 2) compare lymphocyte counts in BLV positive and negative bulls, and 3) evaluate the presence of BLV proviral DNA in bull smegma and semen.