Lifetime effects of infection with bovine leukemia virus on longevity and milk production of dairy cows.

Enzootic bovine leukosis (EBL) is an economically important disease of dairy cattle caused by bovine leukemia virus (BLV). The economic impacts of the infection have been debated in the literature. The present study was conducted to determine the lifetime effects of BLV infection on longevity and milk production of dairy cows in Canada. The data were aggregated from a combination of two data sets: 1) BLV serum-ELISA test results from Canada-wide surveys of production limiting diseases, which took place between 1998 and 2003 in 8 provinces, and 2) longitudinal production data for all cows in the former study, extracted from the Canadian dairy herd improvement database. All participant cows had been culled or died by the onset of this study. A historical cohort study was designed, including cows which tested positive to BLV-antibodies in their first lactation (positive cohort, n=1858) and cows which tested negative in their second or later lactations (negative cohort, n=2194). To assess the impacts of infection with BLV on longevity (the number of lifetime lactations), a discrete-time survival analysis was carried out. The effect of BLV on the lifetime milk production (the sum of all life 305-day milk production) was evaluated using a multilevel linear regression model. Overall, 4052 cows from 348 herds met the eligibility criteria and were enrolled in the study. In the longevity model, the interaction term between time (lactation number) and BLV-status was highly significant. Cows which were positive to BLV had consistently greater probabilities of being culled (or dying) than the test-negative cows. In the milk production model, the interaction term between BLV-status and longevity of the cows was highly significant; indicating that lifetime BLV effects on the total milk production was dependent on the lactation in which the study cows were culled/died. Infected cows with 2 and 3 lactations showed significantly lower life milk productions [-2554kg (-3609 to -1500) and -1171kg (-2051 to -292), respectively] compared with their negative counterparts with 2 and 3 lactations. As the cows lived longer (>3 lactations), the differences in life milk production between the two cohorts were no longer significant. Overall, it was predicted that the test-positive cows produced substantially lower milk compared to the test-negative cows throughout their study lifespans. With the high prevalence of BLV in Canadian dairy cows and its detrimental economic impacts, pursuing broad-based control programs in Canada should be evaluated.