Prevalence of bluetongue serotype 3 following the 2023 epidemic and associated risk factors in Dutch dairy cattle

In September 2023, bluetongue virus serotype 3 (BTV-3) emerged in the Netherlands for the first time. To be prepared for a potential resurgence in 2024, a study was performed during early spring 2024, considered as a low-risk period, to estimate the prevalence of BTV-3 and identify associated risk factors in Dutch dairy cattle. Between December 2023 and April 2024, bulk tank milk (BTM) samples from 12,756 dairy herds were tested to estimate the herd-level and within-herd antibody prevalence. Additionally, an online survey was distributed to participating farmers to collect farm management information from 2023. Animal movement data were obtained from the national identification and registration database. Multivariable logistic regression was performed to quantify associations between potential risk factors and the probability of BTV-3 infection. Herd-level prevalence was 64% (95% CI: 63%-65%). Assuming that BTM test results provide a reliable indication of within-herd prevalence, the expected within-herd prevalence was 36% (95% CI: 35%-37%) in antibody-positive herds. Using BTM as a proxy for within-herd prevalence, the nationwide cow-level prevalence was estimated at 19% (95% CI: 19%-20%). For 5,253 dairy herds, both prevalence data and survey data were available. Factors associated with a lower odds of the presence of BTV-3 antibodies in bulk milk included the presence of large ventilation openings and use of mechanical ventilation while cattle were housed indoors. Factors associated with higher odds included grazing (both limited and longer periods), introduction of cattle from BTV-3 high-risk areas, presence of sheep on the farm, and the month of first BTV notification in the herd's postal code area. No association was found between the use of insecticides and the presence of antibodies. This study shows that although the herd level prevalence after the first BTV-3 epidemic season was substantial, cow-level prevalence at the national level was still relatively low, leaving a large population of cattle susceptible for BTV-3 in 2024. In addition, several management factors were identified, which inform preventive strategies to reduce the risk and the impact of future BTV-3 infections in dairy herds.