Reproductive performance of high producing lactating cows in Coxiella-infected herds following vaccination with phase-I Coxiella burnetii vaccine during advanced pregnancy

• We assess the effect of phase I vaccination against Coxiella burnetii in pregnant cows.
• First study to address the effects of vaccination on reproduction of dairy cows.
• Vaccinated animals were more likely to conceive than control cows.
• Seronegative animals seemed to be responsible for the improved reproductive performance.

This study was designed to assess the safety of phase I vaccination against Coxiella burnetii in advanced pregnancy and the effect of vaccination on subsequent reproductive performance of high producing dairy cows. C. burnetii serostatus was determined in 719 dairy cows by individual serological testing. According to their serostatus, cows were randomly assigned to a control (n = 359) or vaccine (n = 360) group (inactivated phase I on Days 171–177 and 192–198 of gestation, Coxevac-Ceva Sante Animale). Using a χ2-test, vaccination had no effect on abortion before parturition, retention of placenta and stillbirth, either in seropositive as in seronegative cows. Cox's proportional hazards model revealed that cows in the vaccine group were 1.22 times more likely to conceive during the first 150 days in milk than cows in the control group. Moreover, the likelihood of pregnancy was lower in multiparous cows, cows with a retained placenta and cows undergoing first AI during the warm season compared to the remaining animals (by factors of 0.75, 0.69 and 0.69, respectively). In animals testing seronegative for C. burnetii, the likelihood of pregnancy was 1.25 times higher in vaccinated cows compared to non-vaccinated seronegative animals. No effect of vaccination on subsequent fertility was detected in seropositive animals. In conclusion, the results of this study indicate that phase I vaccination against C. burnetii during advanced pregnancy in dairy cows is safe and improves subsequent fertility of C. burnetii seronegative animals.