A retrospective case-control study of pregnancy failure in Thoroughbred horses in Australia

Pregnancy failure is a serious economic and welfare concern in the Thoroughbred horse industry, yet its incidence and risk factors in Australia remain unclear. This retrospective, nested, case-control study investigated pregnancy failure in resident mares on studs in the Hunter Valley, Australia, in 2021-2022, in early (46-150 days), mid (151-270 days), and late (>270 days) gestation. We found an annual incidence risk of 5.4 pregnancy failure cases/100 45-day pregnancies (0.05 cases/mare; 95 % CI 0.04-0.07), with full-term (perinatal) losses (≥320 days) accounting for 24 % of cases (95 % CI 14-37 %). There was no significant difference in loss according to mare age or reproductive history. Control mares had nearly six times higher odds of being vaccinated against Salmonellosis (Salmonella enterica subsp enterica serovar Typhimurium) compared to case mares (OR 5.92, 95 % CI 1.2-29.7). Environmental factors like paddocks with native trees were also associated with increased losses. The study provides evidence that Salmonella sp. might be an important cause of pregnancy failures in Australia. In addition, the findings contribute valuable baseline data for developing targeted surveillance strategies for pregnancy failure in Australian Thoroughbreds. Further investigation of factors such as Salmonella sp. is warranted to enhance breeding success and ensure welfare of Thoroughbred mares in Australia.