The effect of bovine leukemia virus on dairy cow longevity

Abstract

Bovine leukemia virus (BLV) is a retrovirus of cattle that infects approximately 45% of all US dairy cattle, with about 90% of US dairy herds having at least one infected animal. Studies have found BLV infection to be associated with multiple measures of decreased immune function, which may explain the observed economic losses from milk production, decreased cow longevity, and predisposition to lymphoma and other diseases. Our objective was to measure the association between BLV infection and cow longevity in dairy cow operations. Ninety-one dairy herds from 9 US states volunteered to participate in this study. Milking dairy cows (n = 3,611) were tested for BLV antibodies using an ELISA milk test, and their presence in the herd was monitored for an average of 29 mo. The survival analysis controlled for herd and lactation number. Cows sold for dairy purposes were excluded, and individual cow results were not shared with producers so as not to influence culling decisions. Overall, 47.1% (1,701/3,611) of cows were BLV-positive by ELISA. The significant hazard ratio of 1.30 indicated that positive cows were 30% more likely than their negative herdmates to die or be culled during the monitoring period. These results are consistent with other studies in finding a negative effect of BLV infection on cow lifespan.

Summary: Dairy cows (n = 3,611) from 91 herds in 9 US states were tested for antibodies to bovine leukemia virus (BLV) with an ELISA milk test and subsequently followed for survival in their herds for an average of 29 mo. Controlling for age and herd effects, ELISA-positive cows were 30% more likely (P < 0.00001) than their negative herdmates to die or be culled during the monitoring period. Measures of the total economic impact of this virus on cattle production must include an assessment of any shortened lifespan.

Figure 1

Cumulative survival of cattle following bovine leukemia virus (BLV) ELISA testing, showing the proportion of cattle surviving (y-axis) and days since BLV testing (x-axis). Curve 1 = BLV-negative cows and curve 2 = BLV-positive cows; shaded bands indicate 95% CI. Survival data were collected for an average of 29 mo after ELISA testing. The significant hazard ratio of 1.30 indicates that BLV-positive cows were 30% more likely than their BLV-negative herdmates to die or be culled during the monitoring period.