Bluetongue BTV-3 cuts milk yield and disrupts fertility in dairy cows

Bluetongue virus serotype 3 (BTV-3) can cause substantial production and reproductive losses in dairy cattle, according to a Danish study conducted after the 2024 outbreak. Infected cows showed more abortions, more returns to service after insemination and milk-yield losses reaching around 5 kg of energy-corrected milk per cow per day. Vaccinated herds appeared less affected, although the study could not prove a causal protective effect of vaccination.

The study involved 15 dairy farms in southern and western Jutland with confirmed BTV-3 infection. Researchers tested milk from lactating cows for BTV antibodies and linked the results with reproductive and milk-production records. The final dataset included 883 vaccinated and 2,930 non-vaccinated cows.

Abortions and poorer conception

Across the study population, 3.7% of ELISA-positive cows aborted, compared with 1.9% of ELISA-negative cows. Among vaccinated cows, antibody-positive animals had 2.86 times higher adjusted odds of abortion, although the confidence interval was wide. In non-vaccinated cows, late abortion between days 120 and 260 of gestation was significantly associated with BTV antibodies. Non-vaccinated ELISA-positive cows were also more likely to require another insemination after the first service, with crude odds 1.29 times higher than in ELISA-negative cows.

Milk yield fell sharply

Milk production also declined after the outbreak. By winter, losses exceeded 5 kg of energy-corrected milk per cow per day in some non-vaccinated groups. Vaccinated cows generally showed smaller reductions and appeared to recover faster. The authors describe the study as the first cow-level analysis of BTV-3 effects on both milk production and reproduction in vaccinated and non-vaccinated dairy cattle.

Vaccination signal, but important limitations

The results suggest that vaccination may reduce production losses, but only three farms were predominantly vaccinated and vaccination status was strongly linked to farm management. The observational design therefore does not allow a firm conclusion that vaccination itself caused the better outcomes.