The impact of vaccination on bluetongue serotype 3–associated mortality and reduced milk production in Dutch dairy cattle

In 2023, bluetongue serotype 3 (BTV-3) emerged in the Netherlands, leading to a 19% prevalence of BTV antibodies in animals by spring 2024. This highlighted the urgent need for vaccination, which became available in May 2024. Despite this, BTV-3 spread extensively throughout 2024, causing farmers to perceive the vaccination as ineffective. High-quality census data on key health parameters from the cattle monitoring system were used to evaluate the impact of BTV-3 vaccination during the 2024 epidemic. Data were available for ~13,500 dairy herds from January 2020 to September 2024. Data included information on mortality, milk production, BTV-3 antibody status in the spring of 2024, and BTV-3 vaccination status. Herds were classified by BTV-3 status (no antibodies, low or high antibody levels) combined with the herds' vaccination status, for example, unvaccinated, fully but not timely vaccinated, or fully and timely vaccinated, to determine BTV-3 susceptibility at the start of the BTV-3 epidemic in 2024. Timely vaccination was defined as the full completion of herd immunization before BTV-3 began to spread in the area where the herd was located. Generalized estimated equations were used for analysis, with 4-wk moving average mortality per age category of cattle or average weekly milk production (kg/cow per d) as dependent variables. Mortality and milk production in vaccinated herds during the BTV-3 epidemic in 2024 were compared with (i) the period in which BTV-3 was not yet present and (ii) mortality and milk production in unvaccinated herds during the BTV-3 epidemic. Other explanatory variables included herd size, location (province), season, herd health status for several endemic diseases, and time trends. Herd and week were included as time series variables. Mortality in cows (≥2 yr) was significantly increased in all herds compared with mortality in BTV-3–free years. However, during the BTV-3 epidemic in 2024, mortality in fully and timely vaccinated herds was significantly lower compared with mortality in unvaccinated herds, with an incidence rate ratio (IRR) of 0.77 (95% CI: 0.67–0.89). Youngstock and calf mortality in fully and timely vaccinated herds did not significantly differ from the period herds were classified as BTV-3 free. During the BTV-3 epidemic in 2024 in unvaccinated herds, postnatal calf mortality (≤14 d old) increased with an IRR of 1.58 (95% CI: 1.49–1.67), preweaning calf mortality (15–55 d old) with an IRR of 1.28 (95% CI: 1.17–1.40), weaned calf mortality (56 d–1 yr old) with an IRR of 1.22 (95% CI: 1.08–1.38), and youngstock mortality increased with an IRR of 1.53 (95% CI: 1.32–1.76) compared with the period in which herds were classified as BTV-3 free. In addition, fully and timely vaccinated herds without naturally acquired antibodies had a 1.00 kg higher milk production per cow per day (95% CI: 0.40–1.61), and fully and timely vaccinated herds with high naturally acquired antibody levels had a 1.23 kg higher milk production per cow per day (95% CI: 0.63–1.91), compared with unvaccinated herds with a similar antibody status. In conclusion, vaccination significantly mitigated the impact of BTV-3 in 2024 in the Netherlands when cattle were fully and timely vaccinated.