Bovine leukemia virus and cow longevity in Michigan dairy herds.

To determine the association between infection with bovine leukemia virus (BLV) and cow longevity, a stratified random sample of 3,849 Holsteins in 112 Michigan dairy herds was followed for an average of 597 d following testing for BLV antibodies with an ELISA milk test. The hazard ratio of 1.23 indicates that BLV-positive cows were 23% more likely than their BLV-negative herd mates to die or be culled during the monitoring period. This result is adjusted for lactation number, which is also positively associated with an increased risk of leaving the herd. Because herd was included in models, the effect of BLV ELISA on cow longevity was a within-herd comparison in which BLV-infected cattle were compared with their uninfected herd mates. The analysis of 4 ELISA optical density (OD) groups demonstrated a dose response such that cows with higher OD values had decreased survival compared with cows with lower OD values. Cows with OD values above 0.5 were at 40% greater risk of dying or being culled than were their uninfected herd mates. These results support the contention that the association of BLV with cow longevity, when added to other economic impacts, may warrant the control of BLV in our US dairy cow population.