An epizootic of <i>Chlamydia psittaci</i> equine reproductive loss associated with suspected spillover from native Australian parrots

<i>Chlamydia psittaci</i> is an avian pathogen capable of spill-over infections to humans. A parrot <i>C. psittaci</i> strain was recently detected in an equine reproductive loss case associated with a subsequent cluster of human <i>C. psittaci</i> infections. In this study, we screened for <i>C. psittaci</i> in cases of equine reproductive loss reported in regional New South Wales, Australia during the 2016 foaling season. <i>C. psittaci</i> specific-PCR screening of foetal and placental tissue samples from cases of equine abortion (<i>n</i> = 161) and foals with compromised health status (<i>n</i> = 38) revealed <i>C. psittaci</i> positivity of 21.1% and 23.7%, respectively. There was a statistically significant geographical clustering of cases ~170 km inland from the mid-coast of NSW (<i>P</i> &lt; 0.001). Genomic analysis and molecular typing of <i>C. psittaci</i> positive samples from this study and the previous Australian equine index case revealed that the equine strains from different studs in regional NSW were clonal, while the phylogenetic analysis revealed that the <i>C. psittaci</i> strains from both Australian equine disease clusters belong to the parrot-associated 6BC clade, again indicative of spill-over of <i>C. psittaci</i> infections from native Australian parrots. The results of this work suggest that <i>C. psittaci</i> may be a more significant agent of equine reproductive loss than thought. A range of studies are now required to evaluate (a) the exact role that <i>C. psittaci</i> plays in equine reproductive loss; (b) the range of potential avian reservoirs and factors influencing infection spill-over; and (c) the risk that these equine infections pose to human health.