Risk factors for introduction of BVDV in the context of a mandatory control program in Dutch dairy herds.

ABSTRACT

Bovine Viral Diarrhea Virus (BVDV) is a common viral disease in cattle, causing economic losses in naive herds where the virus is introduced. In the Netherlands, a BVDV control program has been in place since 1998, evolving from voluntary to mandatory participation for dairy herds since April 2018. Participation in the BVDV control program is not mandatory for non-dairy farms. The aim of this study was to determine risk factors for new introduction of BVDV into certified BVDV-free herds in the context of a national BVDV control program in dairy herds. In this retrospective case-control study, survey data were collected from 149 dairy farms that lost their BVDV-free status (case herds) and 148 matched dairy farms that maintained their BVDV-free status (control herds) between 2018 and 2021. The survey captured information about potential risk factors and herd characteristics in the 2 years leading up to the loss of BVDV-free status (case herds, virus detection in at least one animal or when seroconversion was detected) or remaining BVDV-free (control herds). Descriptive statistics and logistic regression with a backward selection and elimination procedure were used to identify potential risk factors associated with losing BVDV-free status. Risk factors were quantified as an Odds Ratio (OR) with the associated 95% confidence interval (CI). The risk factor with the highest OR for losing BVDV-free status was purchasing cattle from herds without BVDV-free status (OR 1.25, CI 1.10-1.41), followed by the farmer having another profession that resulted in contact with other cattle (OR 1.25, CI 1.06-1.47), housing young calves and adult cows in the same barn (OR 1.22, CI 1.08-1.38), having a permanent employee on the farm (OR 1.17, CI 1.04-1.31), having a group calving pen (OR 1.16, CI 1.03-1.32), escaped cattle from other farms that mingled with own cattle (OR 1.16, CI 1.01-1.33), and nearest distance to a non-dairy farm (OR 1.15, CI 1.03-1.28). Although the BVDV status of most dairy herds can be checked in an open register, approximately half of the farmers indicated that they purchased cattle from BVDV-free herds while they were actually purchasing from non-BVDV-free farms. Farmers should be stimulated to actively check the true BVDV status of the herd from which cattle are purchased to further reduce the risk of introduction. In addition, indirect contact with cattle from other farms through either the farmer or other on-farm staff should be avoided. It is strongly advised to work in these situations with proper biosecurity measures such as changing boots and coveralls. The results can be used to improve BVDV control programs to further decrease the prevalence.