Reproductive and productive performance in vaccinated and non-vaccinated dairy cows with antibodies against bluetongue virus serotype 3.

In August 2024, Bluetongue virus serotype 3 (BTV-3) emerged among ruminants in Denmark, with many dairy farmers reporting decreased milk yield and reproductive disturbances. We compared reproductive performance and milk yield in BTV-3 antibody ELISA-positive and -negative vaccinated and non-vaccinated dairy cows. Fifteen randomly selected dairy farms confirmed with BTV-3 infection were included. Milk samples from all lactating cows were tested for BTV-3 antibodies using an indirect ELISA. Cow-level data on milk yield and reproductive variables were retrieved, and associations between ELISA-status and reproductive outcomes were assessed separately in vaccinated and non-vaccinated cows. Milk yield was compared to the farm- and parity-specific values. Among vaccinated cows, those testing ELISA-positive had a 2.86 (95% CI: 0.98-8.3) times higher chance of abortion compared to ELISA-negative cows. Non-vaccinated, ELISA-positive cows had a 1.75 times higher chance of late abortion (>120th day of gestation; 95% CI: 1.0-3.07) and 1.29 (95% CI: 1.08-1.53) times higher chance of return-to-service following the first insemination, compared to ELISA-negative. In October 2024, non-vaccinated ELISA-negative cows produced 3 kg less energy-corrected milk (ECM) per day compared to expected September yields, with the deficit increasing to 5 kg ECM by January. ELISA-positive non-vaccinated cows showed even lower yields. Vaccinated cows were also affected but exhibited smaller yield reductions and faster apparent recovery. These findings indicate that BTV-3 infection was associated with adverse reproductive and productive performance in dairy cows, with differences observed according to vaccination status.