Epidemiology of Chlamydia psittaci infections in pregnant Thoroughbred mares and foals.
Authors: Anstey S, Lizárraga D, Nyari S, Chalmers G, Carrick J, Chicken C, Jenkins C, Perkins N, Timms P, Jelocnik M
Journal: Veterinary journal (London, England : 1997)
Summary
# Editorial Summary: Chlamydia psittaci in Thoroughbred Mares and Foals Between 2016 and 2019, *Chlamydia psittaci*—traditionally recognised as an avian pathogen—emerged as a cause of late-term abortion in Australian Thoroughbred mares, prompting investigation into the epidemiology of this infection across 14 stud farms. Researchers collected mucosal swabs from 228 pregnant mares and their foals, alongside environmental faecal samples from birds on the same properties, using qPCR to detect *C. psittaci* and equine herpesvirus-1 (EHV-1). Whilst no mares tested positive to either pathogen, 13.2% of newborn foals carried *C. psittaci* at low levels and 14.5% carried EHV-1, with 1.3% co-infected; notably, foals born during winter months were significantly more likely to be infected (adjusted odds ratio 15.83), and a dominant strain (ST24) correlated strongly with frost events during the preceding gestation period. The findings suggest vertical or periparturient transmission remains possible despite negative maternal serology, that environmental conditions and avian reservoirs (detected in 5.3% of collected bird samples) warrant investigation, and that practitioners should maintain heightened vigilance during winter foaling seasons in at-risk regions, particularly given the potential for subclinical infection to progress to clinical disease in immunologically immature foals.
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Practical Takeaways
- •Monitor newborn foals born during winter months more closely for C. psittaci infection, as seasonal risk is substantially elevated; consider environmental and management factors specific to winter foaling
- •Avian presence on stud farms may serve as a reservoir for C. psittaci; implement biosecurity measures to minimize contact between birds and pregnant mares, particularly during winter
- •Traditional risk factors such as maiden mare status, vaginal sutures, and prior farm history did not predict infection risk in this study, suggesting focus should be on environmental and seasonal factors rather than individual mare management practices
Key Findings
- •C. psittaci detected in 13.2% of healthy newborn foals despite 0% prevalence in pregnant mares, suggesting perinatal or environmental transmission
- •Foals born in winter showed 15.83-fold increased odds of C. psittaci infection compared to other seasons
- •Co-infection with both pathogens occurred in only 1.3% of foals, indicating independent transmission pathways
- •A dominant C. psittaci sequence type (ST24) correlated significantly with frost events during reported abortion cases in Hunter Valley (2016-2019)