"The chronic lymphoproliferative disorder, bovine leukosis, is caused by the deltaretrovirus bovine leukemia virus (BLV). Surveys indicate that 39% of the US beef cow-calf operations have at least one BLV-infected animal compared with 83% of dairy herds. Most BLV infected cattle remain asymptomatic and act as carriers of the virus while less than 5% progress to lymphosarcoma. There are limited studies on the impact of BLV in beef cattle. Understanding the impact as well as identifying important route of disease transmission in beef cattle enterprises is important to better design intervention strategies. The primary goals of the studies described in this this dissertation were to 1) further understand the risk of breeding bulls in the transmission of BLV and 2) to determine the effect of BLV on the survival of cattle in beef and dairy herds. We first determined BLV prevalence in breeding beef bulls and the presence of BLV provirus DNA in genital secretions (smegma and semen). In our study population, 44.6% of beef bulls were seropositive for BLV and 48.7% of herds had at least 1 BLV-infected bull. Bovine leukemia virus provirus DNA was detected in smegma samples of 4/54 (7.4%) BLV-seropositive bulls and represent a potential risk for the transmission of BLV from infected bulls to uninfected cows during natural service breeding programs. To evaluate this risk, we exposed BLV negative heifers to a BLV positive bull during a defined 38-day breeding period. Although BLV provirus was found in the smegma and blood of the BLV positive bull prior to and after the breeding period, we detected no evidence of seroconversion or presence of BLV provirus DNA in the blood of naive heifers. These results suggest that BLV infected bulls that are healthy and aleukemic may not be a significant risk of BLV transmission during a defined breeding season.We next evaluated the impact of BLV infection on beef and dairy cow' longevity within herds. The presence of BLV antibodies in blood was not associated with a change in beef cow longevity over 2 years monitoring period, but decreased survival was observed in cattle in which BLV infection had advanced clinically as indicated by a high BLV provirusload in blood. In dairy cows, we demonstrated that infected females lived significantly shorter than their negative herd mates and were at a 30% greater hazard of being culled compared with BLV negative cows. In summary, there is high prevalence of BLV in breeding beef bulls which could serve as a source of transmission both within and between herds. Based on our study results, the risk of transmission of BLV from healthy infected bulls to naive heifers is low when bulls are housed with heifers for a defined breeding period, but this scenario should not be considered without risk. BLV does not appear to have an impact on beef cow longevity, but in contrast, is associated with decreased longevity in dairy cows. These studies provide important information for supporting and designing risk based BLV control programs."--Pages ii-iii.
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