Chlamydia psittaci and Australian Thoroughbreds: a two-year study on C. psittaci infection in pregnant mares and their foals

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Foal loss due to Chlamydia psittaci (C. psittaci) is considered a threat to the Thoroughbred industry in Australia. We aimed to better understand C. psittaci infection in late pregnant mares and their foals and assess potential risks involved with this infection. A two-year study involved seven stud farms from the Hunter Valley region in Australia. Mucosal swabs from 155 mares and their foals and placental samples were tested for C. psittaci and Equine herpesvirus-1 (EHV-1) using species-specific qPCR assays. A cross sectional analysis of the 2019 (n=90) and 2020 (n=65) mares and foals utilised multilevel logistic regression (MLR) to assess potential risk factors. Multilocus Sequence Typing (MLST) was used to determine genotypes of the infecting strains detected in chlamydial foal loss. The study detected C. psittaci at an estimated prevalence of 18.9% (17/90; CI 95% 12.14, 26.18) in 2019 and 10.8% (7/65; CI 95% 0.05, 0.22) in 2020 in healthy mares and foals at low loads of infection compared to high infection loads reported in chlamydial foal loss cases(P<0.05). MLR determined that foals being born in winter (Odds ratio = 5.1; CI 95% 0.21, 1.69) was a significant predictor of C. psittaci detection. There was no foal loss to C. psittaci in the study group, but chlamydial foal loss was reported in both years in the region (2019 [n=1]; 2020 [n=3]). MLST resolved a clonal endemic C. psittaci sequence type (ST) 24 strain in all foal loss cases.The main limitations of this study concerned equine selection and sampling was performed by each farms veterinary team which may have resulted in bias. Potential contamination of swabs at foal birth collection time point. Prevention strategies in Australia should focus on temporal management and new research into correlations between yearling respiratory health and C. psittaci is warranted.