Bovine leukemia virus (BLV), the causative agent of Bovine leukosis, is an exogenous C-type oncogenic retrovirus of the BLV-human T-lymphotropic virus group.
The prevalence of BLV infection varies from country to country. Many European countries, Australia, and New Zealand have eradication programs in place that have led to negligible rates of BLV infection. Although voluntary control programs are in place in the USA, prevalence is high compared to much of the rest of the world. The most recent surveys in the USA estimate that 44% of dairy and 10% of beef cattle are infected with the virus. Prevalence tends to increase on dairies with increasing herd size, while the converse is true in beef cattle. In general, the prevalence of viral infection increases with age.
Cattle are infected with BLV through the transfer of blood and blood products that contain infected lymphocytes. Once infected, cattle develop a lifelong antibody response, primarily to the gp51 envelope protein and the p24 capsid protein. B lymphocytes harbor the integrated provirus but rarely express viral proteins on their cell surface. The exact site of viral replication and expression that drives the immune response remains elusive.
Under experimental conditions most routes of viral exposure can successfully transmit infection. However, many of these settings are unlikely to be encountered naturally. Many bodily fluids, including urine, feces, saliva, respiratory secretions, semen, uterine fluids, and embryos, have been examined for their ability to transmit BLV and are considered to be noninfectious. Only on rare occasion has virus been found in these fluids. Colostrum from BLV-positive cows contains virus and has been found to be infectious experimentally. However, colostrum also contains large amounts of antibody and it is believed that the protective effects of colostral antibody outweigh the infectious potential when colostrum is administered in a normal fashion.
The majority of BLV transmission is horizontal. Close contact between BLV-negative and BLV-positive cattle is thought to be a risk factor. Many common farm practices have been implicated in viral transmission, including tattooing, dehorning, rectal palpation, injections, and blood collection. Vectors such as tabanids and other large biting flies also may transmit the virus. Vertical transmission may occur transplacentally from an infected dam to her fetus, intrapartum by contact with infected blood, or postpartum from the dam to the calf through the ingestion of infected colostrum. Any material that is blood contaminated or lymphocyte rich has the potential to infect animals with BLV.