The effect of bovine leukemia virus infection on health and growth of nonreplacement dairy calves.

The objective of this cohort study was to determine if bovine leukemia virus (BLV) positivity, identified using quantitative PCR (qPCR), was associated with health and growth outcomes in nonreplacement dairy calves. A commercial calf-raising facility located in Southwestern Ontario was used, where a total of 768 male dairy calves (726 Holstein, 42 crossbred), estimated to be 3 to 10 d of age, had 3 blood samples taken at 1 d, 30, and 84 after arrival to the facility. Whole blood EDTA samples were taken and sent to the University of Calgary Faculty of Veterinary Medicine for qPCR analysis to determine positivity for BLV. Calves were determined to be positive using a baseline threshold of >200 relative fluorescence unit by <40 cycles. All calves were health scored 2 times daily for fecal consistency and respiratory disease, using the UC Davis respiratory scoring chart. Additionally, calves were weighed at time of arrival and weekly thereafter until they left the facility at 84 d. Mixed linear regression models were built to assess ADG (kg/d) over 84 d and feed efficiency (ME/kg of gain), whereas a Poisson model was built to assess the number of observations with a respiratory score of ≥5. Further, a negative binomial model was built to assess the number of observations with diarrhea (score of ≥2), and a zero-truncated Poisson model was built to assess the number of observations with no clinical signs of respiratory disease (e.g., respiratory score of 0). Treatment for respiratory disease and diarrhea was also analyzed using a Cox proportional hazard model. Two calves had incomplete blood sampling and were therefore removed from the analysis, leaving a total of 766 calves. A total of 43 (5.9%) calves tested positive for BLV via qPCR testing over the 84-d period. Calves that were BLV-positive were associated with a greater number of observations with a respiratory score of ≥5 (incidence rate ratio [IRR] = 1.91, 95% CI: 1.69 to 2.17) and a lower number of observations with a score of 0 (IRR = 0.98, 95% CI: 0.95 to 1.00). There was no difference identified between BLV positivity and ADG (kg/d), feed efficiency (ME/kg of gain), or diarrhea. Overall, this study identifies an association between BLV status and respiratory disease occurrence in preweaning calves. Although the underlying mechanisms and long-term effects remain unclear, these findings highlight the need for further research to determine whether BLV plays a causal role in respiratory disease in calves.