Preventing Pregnancy Losses in Mares with Salmonella Vaccine

Just because your mare made it through the first 40 days of pregnancy successfully doesn’t mean you can let your guard down. Pregnancy failures after 45 days of gestation are actually more common than early pregnancy losses, and the reasons for those losses are not clear. In fact, the causes of pregnancy failure vary geographically. Potential causes of pregnancy loss in Australian Thoroughbreds were recently explored, and one surprising finding was that vaccinating mares against Salmonella enterica serovar Typhimurium appears to have a protective effect against abortion.^*

In the study, data were collected from broodmares residing on stud farms located in Hunter Valley, New South Wales, Australia. Stud data (e.g., number of broodmares, control of pests such as birds, insects), feeding practices, biosecurity, and mare data (e.g., age, number of 45-day pregnancies, number of pregnancy failures, vaccinations) were examined.

Pregnancy loss was divided into early (46-150 days pregnancy), mid (151-270 days), and late (more than 270 days) stages. Of the 810 pregnant broodmares on the six stud farms, 59 cases of abortion were included in the study. The control group included 120 pregnant mares residing on the same stud farms that did not suffer pregnancy loss.

Some of the key findings of this study included:

• Age at first breeding, previous pregnancy failures, and number of times bred to achieve pregnancy at 45 days gestation were similar between case (i.e., mares with pregnancy loss greater than 45 days) and control mares;
• Mare age did not influence the likelihood of pregnancy failure;
• A history of pregnancy failure did not predict future pregnancy loss in this study; and
• Most pregnancy losses were late term, occurring on or after 270 days gestation, and loss of full-term foals accounted for one-quarter of all pregnancy failures.

“This study also found that vaccinating mares against Salmonella Typhimurium was potentially protective against pregnancy failure. Specifically, control mares had almost six times higher odds of being vaccinated against this pathogen than case mares that suffered pregnancy loss,” said Kathleen Crandell, Ph.D., a nutritionist for Kentucky Equine Research.

Thus, Salmonella Typhimurium may be an important cause of pregnancy failure in Australia. Further studies are needed to evaluate the potentially protective effect of this vaccine.

“Supporting pregnant mares throughout the entire pregnancy must include appropriate biosecurity as well as routine veterinary examinations to identify high-risk late pregnancies. In addition, providing a balanced diet through regular feeding and maintaining a healthy weight can go a long way to supporting broodmares nutritionally through the delicate times of gestation. Finally, supplementing pregnant mares with long-chain fatty acids, like those found in EO-3, support the immune system and have beneficial effects on various reproductive parameters,” added Crandell.

Consider these five tips when feeding broodmares in late gestation.

^*Wilson, C.S., J. Carrick, P. Shearer, J. Heller, and V.J. Brookes. 2025. A retrospective case-control study of pregnancy failure in Thoroughbred horses in Australia. Preventive Veterinary Medicine:106424.