Prevalence of antibodies to bovine viral diarrhoea virus in bulk tank milk and associated risk factors in Scottish dairy herds.

Bulk tank milk samples were collected from 374 dairy farms in Scotland in 2007/2008 along with questionnaire data relating to the management of the farm. Milk samples were tested for antibodies to bovine viral diarrhoea virus (BVDV) using a commercially available (Svanova) kit and percentage positivity scores calculated according to the manufacturer's guidelines. There were 220 farms that did not routinely vaccinate for bovine viral diarrhoea (BVD), and these were distributed according to the Swedish BVD eradication classes as 12.7 per cent, 22.3 per cent, 44.5 per cent and 20.5 per cent for Classes 0, 1, 2 and 3, respectively. A more sophisticated statistical method (finite mixture modelling) which does not depend on arbitrary thresholds and categories suggested a 73 per cent prevalence of herds with high mean levels of antibodies. Risk factor analysis suggested that routine vaccination for BVD, suspicion of BVD, housing of pregnant cows with calves, total number of cows and the proportion of cows that were dry were all associated with increased BVDV antibodies in bulk milk. The inclusion of BVD within the farm's health plan was associated with decreased BVDV antibodies in the bulk milk.