Bulls as a source of bovine leukemia virus during natural breeding

Bovine leukosis is a chronic lymphoproliferative disorder in cattle caused by the deltaretrovirus, bovine leukemia virus (BLV). Most BLV infected animals remain asymptomatic and act as carriers of the virus; 30 to 40% of infected cattle develop  a persistent lymphocytosis, while less than 5% progress to lymphosarcoma. The major route of virus transmission is believed to be iatrogenic through procedures that permit the transfer of blood between cattle. Proviral DNA has been identified in nasal secretions, saliva, semen, and smegma. Natural transmission through these secretions has not been clearly demonstrated. Our research group has identified the use of bulls in dairy herds as a risk factor for BLV infection at the herd level, and has also shown a high prevalence ( 45%) of infected beef bulls in Michigan. Natural service is still used in half of dairy operations across the US, and is the most commonly used breeding method (approximately 90%) in beef cattle herds in the US. Given these factors, there may be risk of BLV transmission during natural breeding via smegma or blood transfer following trauma to the penis, vulva, and vagina. The overall goal of our research team is to develop cost effective strategies to reduce the transmission of BLV in cattle herds. The objective of this study is to assess the risk of BLV transmission by breeding bulls during natural breeding.