Transmission of Bovine Leukosis Virus

Bovine leukosis virus is an exogenous retrovirus (Retroviridae, Oncovirinae) that infects lymphocytes of cattle. Infection with bovine leukosis virus and the concomitant antibody response are lifelong. Infection can result in several outcomes, including production of antibodies against bovine leukosis virus without other evidence of infection, inversion of the T:B lymphocyte ratio, persistent lymphocytosis, and clinical lymphosarcoma. The prevalence of an infection in a population of animals is the proportion of the group infected at any given time. Surveys have shown the prevalence of bovine leukosis virus infection in cattle populations ranging from 0 to nearly 100%. This wide range of prevalence levels is likely due to variations in risk factors such as husbandry practices, insect vectors, and genetic resistance. For example, prevalence tends to be higher in dairy than beef cattle and in cattle in Southern vs Northern states. The relative importance of the known modes of transmission of bovine leukosis virus has not been established in beef cattle. Also, the economic impact of bovine leukosis virus infection in beef cattle has not been examined. However, the presence of cattle infected with bovine leukosis virus in a herd drastically reduces opportunities to export cattle and/or semen to many countries. Excluding an early transient viremia, the virus locates in lymphocytes as a DNA provirus. Because of its ceil-associated nature, transmission is believed to occur by movement of infected lymphocytes to susceptible animals. Intradermal, subcutaneous, intramuscular, or intravenous inoculation of as little as one microliter of blood or intracutaneous inoculation of 2,500 lymphocytes from an infected animal (equivalent to .5 microliter of blood) results in transmission of bovine leukosis virus. Transmission of an infectious agent in this manner is a form of horizontal transmission. Other means of horizontal transmission have been investigated, including casual contact in common housing; animal husbandry procedures such as dehorning without sanitizing the dehorner between cattle, tattooing with common pliers, rectal palpations with common sleeves, and injections with common needles; and blood feeding arthropods. In addition, transmission from the dam to calf, termed vertical transmission, has also been shown to occur with bovine leukosis virus. The purpose of these studies was threefold: 1) to characterize the bovine leukosis virus status of the U.S. Meat Animal Research Center cattle population, 2) to investigate the extent and significance of vertical transmission of bovine leukosis virus in the U.S. Meat Animal Research Center cow herd, and 3) to investigate the role of specific management practices in horizontal transmission of bovine leukosis virus in the U.S. Meat Animal Research Center cattle herd