Application of survival analysis to identify management factors related to the rate of BHV1 seroconversions in a retrospective study of Dutch dairy farms

The prevalence of Bovine herpes virus type 1 (BHV1) at dairy farms is dependent on several factors. First, the prevalence is influenced by introduction of BHV1 at the farms, which is dependent on the risk factors for introduction. Second, the BHV1 prevalence might also be influenced by reactivation of BHV1 within the farm, which might be affected by the management of the farm. In this study the relations between risk factors, management factors and the estimated time since latest BHV1 outbreak were investigated by means of Cox regression analysis. The results showed that direct animal contacts (i.e. purchase of cattle and returning export cattle) and occasional visitors increased the rate of BHV1 outbreaks on dairy farms. Management factors related to reactivation of BHV1 at dairy farms were all related to a loose housing system which incurred an increased risk of reactivation of BHV1 at the farm. The reactivation was facilitated when the barn was overcrowded (i.e. more cows than cubicles in the barn). To minimise the risk of introduction of BHV1 at a farm the farmer should banish direct animal contacts and limit the number of contacts with visitors. Stress and an overcrowded barn should be avoided.