There are now multiple reports implicating a global pathogen traditionally associated with birds to abortion in horses, researchers warn.
Chlamydia psittaci is an emerging pathogen that must be included in routine microbiological diagnostic testing of all equine abortions, according to Susan Anstey and her fellow researchers in Australia.
C. psittaci is traditionally regarded as a globally distributed bird pathogen that can spill over into other species. Infections are common in a variety of birds, including poultry, wild birds, and pet birds.
These infections are also found in humans, and to a lesser degree, domesticated livestock, such as sheep, cattle, horses, and pigs.
The disease, known as psittacosis or parrot fever, can cause fever and chills in humans, a headache and muscle aches, a dry cough, and occasionally pneumonia.
Spill-over transmission from the bird host to other animals, via direct or indirect contact, is thought to be the route of infection.
Compared to global studies, Australia has limited reporting in humans and domesticated livestock, such as sheep and cattle.
While molecular research has identified an extended global host range and significant genetic diversity, Australia has reported a reduced host range — in birds, horses and humans, with a dominance of clonal strains, denoted ST24.
In Australia, C. psittaci has found a preferred host niche in the nation’s parrots and Thoroughbred horses.
To better understand how widespread this strain type is in Australia, Anstey and her colleagues conducted molecular-based tests on samples from birds, horses, marsupials and cattle from Australia.
Their work, reported in the journal Pathogens, confirmed that clonal ST24 strains dominate infections of Australian parrots and equine hosts. However, they also identified novel hosts — the Australian white ibis, King parrots, racing pigeons, cattle, and a wallaby.
They also showed that strain diversity does exist in Australia. Their discovery of a novel strain, dubbed ST306, in a novel host, the Western brush wallaby, is the first detection in a marsupial, they said.
The authors noted that whilst C. psittaci infections in Australian parrots are not new, the first descriptions of the pathogen in Thoroughbred horses in 2014 were considered novel and raised biosecurity concerns regarding a potential emerging infection.
C. psittaci infection in a pregnant horse resulted in late-term pregnancy loss and a novel infectious aerosol transmission of C. psittaci to humans handling infected equine foetal membranes, as reported in 2017 papers.
Since then, C. psittaci has remained an annually reported cause of foal loss in the Hunter Valley region of Australia — a major Thoroughbred breeding center — as observed in a study from 2016 to 2020.
“A recent retrospective study has also provided evidence that this pathogen is not new in Australian horses and has been a cause of sporadic equine foal loss in Australia for over 30 years.”
These studies, they said, highlight potential under-reporting as C. psittaci was not previously considered in equine diagnostic panels.
“Addressing C. psittaci knowledge gaps is important considering the high individual value of the Thoroughbred foals and the zoonotic risk to farmworkers,” the study team said.
Discussing their findings, the researchers said previous Australian studies have focused on molecular descriptions of equine strains, which, on their own, cannot provide an informed understanding of the disease-causing ability of each strain and the mechanism of equine foal loss.
“We are still facing knowledge gaps on how C. psittaci is transmitted to the pregnant mare, how it colonises the placenta, and how pathological processes in the foetal membranes/organs are induced.
“If the infectious agent … is orally ingested from pastures contaminated with faecal droppings containing C. psittaci, information about the density of the bird population shedding C. psittaci is of importance.”
They said while parrots might play a major role in Australia, pigeons might be considered as a potential infection source in European countries.
Case numbers of abortions in different geographical regions are closely related to the type and density of horse farming and breeding, they noted.
They noted that mixed infection of C. psittaci and other abortion-causing agents, such as equine herpesvirus-1 have been reported, but it remains unclear whether one of the agents (or a combination of both) induced the abortion.
Further studies on the lesions arising from infection and inflammatory processes would help to understand the disease process, they said.
The study team acknowledged that a diagnosis of C. psittaci in horses can be challenging. “Whilst high loads are noted in clinical cases of both avian psittacosis and equine foal loss, the significance of detection of low levels in a healthy host remains unknown.
“Importantly,” they continued, “early detection for intervention in the pregnant mare is still lacking.
“Considering that Australia’s major Thoroughbred breeding center in the Upper Hunter Valley is in a potentially endemic ST24 region, what risk does the emerging strain diversity pose for an outbreak of foal loss and human psittacosis?”
They said there is a serious potential for cross-species transmission associated with C. psittaci. “Equine veterinarians and staff on equine breeding farms must take more precautions when dealing with the aborted equine conceptus and the affected mare.”
Birthing in horses can now be considered as a time of higher risk for transmission of C. psittaci to farmworkers, they said.
“Compared to global reporting, there is potential under-reporting in Australia, with less than 50 notified cases of psittacosis per annum in the past decade reported in the Australian National Notifiable Diseases Surveillances.
“To date, some regions are considered endemic, and this study has identified the upper Hunter Valley as another potential region.”
They said the recent development of a rapid diagnostic test for the pathogen in horses shows promise in helping to manage the risk for front-line workers.
They said their work has shown the predominance of a dominant ST24 strain in Australia, capable of causing serious disease in people and animals, and mortality in equines, as well as identifying a broader host range and found a new strain.
They said the descriptive framework around C. psittaci disease should be updated. There should be ongoing surveillance and strain identification for potential outbreak intervention, including cell biology work to learn more about the disease-causing potential.
The full study team comprised Anstey, Vasilli Kasimov, Cheryl Jenkins, Alistair Legione, Joanne Devlin, Jemima Amery-Gale, James Gilkerson, Sam Hair, Nigel Perkins, Alison Peel, Nicole Borel, Yvonne Pannekoek, Anne-Lise Chaber, Lucy Woolford, Peter Timms and Martina Jelocnik. They are variously affiliated with a range of institutions, including the University of the Sunshine Coast, the New South Wales Department of Primary of Industries, the University of Melbourne, the Western Australia Department of Primary Industries, the University of Queensland, Griffith University, the University of Adelaide, the University of Zurich in Switzerland, and the University of Amsterdam in The Netherlands.
Anstey, S.I.; Kasimov, V.; Jenkins, C.; Legione, A.; Devlin, J.; Amery-Gale, J.; Gilkerson, J.; Hair, S.; Perkins, N.; Peel, A.J.; Borel, N.; Pannekoek, Y.; Chaber, A.-L.; Woolford, L.; Timms, P.; Jelocnik, M. Chlamydia Psittaci ST24: Clonal Strains of One Health Importance Dominate in Australian Horse, Bird and Human Infections. Pathogens 2021, 10, 1015. https://doi.org/10.3390/pathogens10081015