Equine psittacosis: an emerging cause of equine abortion and neonatal illness in Australia

Chlamydia psittaci is an obligate, intracellular, bacterial pathogen generally associated with clinical and subclinical infection of birds. It is a zoonotic pathogen in humans causing psittacosis a serious respiratory disease and reported to cause infection in animals including cattle, sheep and horses. Although there have been sporadic reports of disease due to C. psittaci in horses since the last century, reports from Australia over the last decade have highlighted the potential of disease in horses and zoonotic transfer. Epizootics of abortions and stillbirths in mares and serious neonatal disease, termed equine psittacosis (EP), have highlighted the potential of C. psittaci to both cause disease in horses and recognise them as major mammalian vectors for zoonotic transmission. Molecular characterisation techniques for these Australian isolates have demonstrated that the majority of equine associated C. psittaci strains have identified the globally disseminated pathogenic 6BC/ST24 type. Diagnosis is primarily through molecular techniques to identify C. psittaci genomically as seroconversion has not been reliably observed in horses. In tissues from abortion samples histopathological changes typically include lymphohistiocytic placentitis though this is neither a sensitive or specific finding in cases of EP. Neonatal foal illness is characterised by severe interstitial pneumonia and disease is generally fatal. Recognition of EP has heightened both awareness of C. psittaci as an equine abortigenic pathogen and the zoonotic threat that infected horses pose. Personal protective equipment should be donned by exposed personnel and strict biosecurity and control measures should be enforced following equine abortion or foetal loss cases and neonatal illness pending diagnostic evaluation.