INTRODUCTION
A mandatory Bovine Virus Diarrhea Virus (BVDV) control program started in Germany in January 2011. The program was based on ear-notch-testing of calves. A serological bulk-milk-surveillance of dairy cow herds is currently discussed. Regular prophylactical vaccination is not allowed anymore since May 2021. EU 2020/689 requires that surveillance has to detect a target seroprevalence of 50%. We assessed BVDV-seroprevalences in herds with recent BVDV-outbreaks.  
 
MATERIALS AND METHODS
In 11 dairy cow farms (mean 39 cows) with BVDV-positive calves the history of ear-notch-testing since 2011 and the BVDV-vaccination history were assessed. The duration of an outbreak was expressed as ‘PI-years’, i.e. number of years from the first BVDV-positive calf until the first serological testing in this study. Two outbreaks were assumed to be independent if they were separated by >2 years without BVDV-positive calves; the most recent was used for classification of the herd.
‘PI-year=1’ was recorded in 7 herds; in two of these farms one additional PI-year had been observed 4 and 5 years earlier. Four herds were classified as ‘PI-year>1’, one herd with 2, 3, 4, and 5 PI-years, respectively.
Individual milk samples were tested with commercially available ELISAs (Idexx, IDVet, Svanova – monophasic). Seroprevalences (SP) were calculated and median SP (MSP) of herds were compared for ‘PI-year=1’ and ‘PI-year>1’. Since 2018 three additional outbreak farms were serologically tested only with selected ELISAs and were vaccinated with an attenuated BVDV-vaccine based on genotype 1 and 2.
 
RESULTS
MSP in group ‘PI-year=1’ were 22.2%, 22.2% and 15.0% and in group ‘PI-year>1’ 82.0%, 56.6% and 59.8% for IDVet, Idexx and Svanova, respectively. In two herds only the mother of one BVDV-positive calf was antibody positive! One herd (PI-years=1) was epidemiologically linked to two other farms with 8 and 3 PI-years (both genotype 1f and 7 km distance to the study-farm).
Three additional herds (SP: IDVet 13.5% and 58.1%, Svanova 14.3%) were vaccinated. Notably, SP 13.5% was determined prior to vaccination after 3 BVDV-positive calves had been born 9 months apart. Only one positive calf was recorded 8 months after vaccination. In contrast, in an unvaccinated farm ‘PI-year=1’ two BVDV-positive calves had been detected two years later. One and even two years without BVDV-positive calves were observed in case of 3 and 2 outbreaks, respectively.
 
DISCUSSION AND CONCLUSIONS
MSP in herds ‘PI-year=1’ of about 20% indicated that the pre-scribed target seroprevalence of 50% is inappropriate for the surveillance of freedom; moreover, only 50% of herds exceeded 20%. Consequently, the sensitivity of antibody-ELISAs needs to be increased. Meanwhile, increased pool sizes (e.g. 100-200) of ear-notches are a compromise keeping costs and safety of surveillance in a balance. Transient infection spreads slowly within herds resulting in a prolonged duration of outbreaks, consequently, vaccination in case of BVDV-detection needs to be encountered as an emergency measure.
 
Acknowledgements
This study was financially supported by the Free State of Bavaria and the Bavarian Joint Funding Scheme for the Control and Eradication of contagious Livestock Diseases. PD Dr. K. Wernike (FLI) kindly performed the BVDV-genotyping.