Relationships between herd bovine leukemia virus infection status and reproduction, disease incidence, and productivity in Swedish dairy herds

A program has been initiated in Sweden to eradicate bovine leukemia virus (BLV) infections in dairy and beef cattle. As the first step, all dairy herds were screened in order to identify those with BLV-infected animals. The objective of this study was to ascertain possible relationships between herd BLV infection status and measures describing disease incidence, reproduction and production. BLV infection status was determined in 14 424 herds, using an enzyme-linked immunosorbent assay (ELISA) test applied to bulk-milk samples. Relationships were assessed with logistic and ordinary linear regression analyses. There were many statistically significant associations between herd BLV infection status and measures of disease incidence, reproduction and production, but most were of low magnitude. The risk for diseases with infectious etiology seemed to be greater among BLV-infected herds, while the risk for non-infectious diseases did not differ. The culling rate was higher and milk production was lower in BLV-infected vs. BLV-free herds. The effect of BLV infection on fertility was only minor.