Factors associated with being a bovine-virus diarrhoea (BVD) seropositive dairy herd in the Møre and Romsdal County of Norway.

Factors associated with being a bovine-virus diarrhoea (BVD) seropositive dairy herd were studied in a case-control study of 314 dairy herds in the Møre and Romsdal County of Norway. Information was collected through a mailed questionnaire, and associations were modeled using conditional logistic regression after selecting variables by a best-subset procedure. Purchasing of animals, use of common pasture, herd-to-herd contact over pasture fences, purchasing cattle with insufficient health (about BVD) documentation, and not using dairy advisors were associated with higher risk. In addition, younger farmers were more likely to have a bovine-virus diarrhoea seropositive herd than older farmers. 'Other animal traffic', including use of common animal housing in the summer months and exchange of calves between farmers were also risk factors. Collectively, these factors could explain 51% of the seropositive herds.