The effect of bovine viral diarrhoea virus on fertility in dairy cows: two case-control studies in the province of Styria, Austria.

Bovine viral diarrhoea (BVD) leads to substantial economic losses in beef and dairy herds worldwide. Two case-control studies were carried out using production data from 1996 to 2012 to analyse the impact of BVD virus (BVDV) on fertility in dairy herds in the province of Styria during an eradication programme. In study 1, herds in which at least one persistently BVDV-infected (PI) animal was detected (case herds) were compared to a group of control herds proven free from BVDV infection (contro herds). In study 2, within BVD infected herds the period during which P animals were present (exposed period) was compared to the period after successful BVD eradication (unexposed period). Calving interval (CAl) and the probability of a first service conception (FSC) were used as indicators in a mixed regression model to investigate the impact of BVD on reproductive performance. The model results indicated that BVD had a significant influence on CAl and FSC. Cows from control herds were 1.1 times more likely to conceive at first service compared to cows from case herds and cows served during the BVDV unexposed period were 1.3 times more likely to conceive at first service than those inseminated during the exposed period. In BVD-infected herds the CAI averaged seven days shorter in unexposed periods than in exposed periods. Besides BVD the animal breed and the parity substantially impact the analysed fertility indicators.