A 25-year retrospective study of Chlamydia psittaci in association with equine reproductive loss in Australia

is primarily a pathogen of birds but can also cause disease in other species. Equine reproductive loss caused by has recently been identified in Australia where cases of human disease were also reported in individuals exposed to foetal membranes from an ill neonatal foal in New South Wales. The prevalence of in association with equine reproductive over time and in different regions of Australia is not known. This study was conducted to detect in equine abortion cases in Australia using archived samples spanning 25 years. We tested for in 600 equine abortion cases reported in Australia between 1994 to 2019 using a real-time quantitative PCR assay targeting the 16S rRNA gene followed by high-resolution melt curve analysis. Genotyping and phylogenetic analysis was performed on positive samples. The overall prevalence of in material from equine abortion cases was 6.5 %. -positive cases were detected in most years that were represented in this study and occurred in Victoria (prevalence of 7.6 %), New South Wales (prevalence of 3.9 %) and South Australia (prevalence of 15.4 %). Genotyping and phylogenetic analysis showed that the detected in the equine abortion cases clustered with the parrot-associated 6BC clade (genotype A/ST24), indicating that infection of horses may be due to spillover from native Australian parrots. This work suggests that has been a significant agent of equine abortion in Australia for several decades and underscores the importance of taking appropriate protective measures to avoid infection when handling equine aborted material.