The bacterium that causes Q fever in humans has been identified in some aborted equine foetal tissue samples in Australia, but researchers stress this does not prove it is a cause of abortion in horses.
Coxiella burnetii causes a disease known as coxiellosis in humans and a range of animals, although it is most commonly called Q fever in humans.
Acute forms in humans involve a flu-like illness, pneumonia, or hepatitis. Chronic forms can cause inflammation around the heart and chronic liver disease.
Animals that are infected may not show clinical signs of disease but may develop reproductive disorders, abortions and stillbirths or deliver weak offspring.
Depression, fever, gut inflammation, and pneumonia have been reported in experimentally infected horses. Some overseas studies have detected C. burnetii in equine aborted foetuses or placentas, but the association of the bacterium with reproductive losses are unclear.
C. burnetii has been detected in people who ride horses or visit horse facilities, but contact with other livestock or ticks at those facilities have often been considered the source of infection.
Cases of horses infecting humans have not been reported, but studies have suggested that equine veterinarians or breeders could potentially be at higher risk of infection.
The Australian study involved molecular-based analysis of 600 aborted equine foetal tissue samples submitted to the University of Melbourne’s diagnostic laboratories from 1994 to 2019.
Rumana Akter and her fellow researchers at the university focused on detecting levels of equine herpesvirus 1, C. burnetii, Leptospira species and Toxoplasma gondii.
The prevalence of C. burnetii was found to be 4%, with peaks found in the samples between 1997–2003 and 2016–2018. The prevalence of C. burnetii in Victoria was 3% and in New South Wales it was 6%.
However, the loads of C. burnetii DNA in the tissue samples were low — much lower than those found in foetal samples from sheep and goats.
All the samples tested negative for Leptospira species and Toxoplasma gondii DNA.
DNA from Equine herpesvirus 1, which has long been linked to abortion in some mares, was detected at a prevalence of 3%.
“This study has provided evidence for the presence of C. burnetii in equine aborted foetal tissues in Australia, but the role of C. burnetii as a potential cause of abortion in Australia requires further investigation,” they reported in the open-access journal, PLOS ONE.
However, they stressed: “Detection of C. burnetii DNA is not sufficient in itself to implicate the bacteria as the cause of abortion.
“Indeed, the low loads of C. burnetii DNA detected in equine foetal tissues could indicate that the bacteria may not be the cause of the abortion event.”
Future work to determine the prevalence and load of C. burnetii infection in animals with normal reproductive outcomes would be important to understand the significance of the infection in horses, they said.
Studies to determine if the presence of C. burnetii DNA is associated with pathological changes would also be helpful.
“Together, such studies will help to clarify the role of C. burnetii in equine abortion in Australia.”
The authors noted that the dose of C. burnetii required for human infection is very low. “Therefore, people handling material associated with equine abortions may be at risk of being infected with C. burnetii.
“This is in addition to risks posed by possible infection with Chlamydia psittaci.
“Thus, it is recommended that appropriate measures to minimise the risk of infection, such as the use of personal protection equipment, vaccination and biosecurity procedures, be considered when handling aborted equine materials.”
The researchers said the levels of C. burnetii found in the samples tested was consistent with rates of 4% found in a German study, and 3.4% and 1.5% in French studies.
A similar study reported that the prevalence of C. burnetii was 8% in abortion cases in the Netherlands in 2011.
Leptospira species were not detected in any cases of abortion in this current study.
While research by others has revealed a high Leptospira seroprevalence in clinically normal horses in Queensland, no studies until now had investigated Leptospira infection in equine abortion cases in Australia.
“The negative results of the current study suggest that Leptospira species may not be associated with equine abortion in Australia, although it would be beneficial for future studies to incorporate higher numbers of samples from other geographical regions, including Queensland, as the samples in this current study were mostly from Victoria and New South Wales.”
The absence of T. gondii DNA in the current study was consistent with results from a study in Hungary between 1998–2000, in which all 96 equine aborted foetal tissues were negative for T. gondii.
“This finding is despite previous studies which have shown that T. gondii is able to cross the equine placenta in an experimental model.”
“Serological surveys suggest that subclinical Toxoplasma infection is common among horse populations worldwide, although it should be noted that no such studies have been conducted in Australia.
“Taken together, these results suggest that T. gondii is not associated with abortion in horses in Australia.”
The study team comprised Akter, Alistair Legione, Fiona Sansom, Charles El-Hage, Carol Hartley, James Gilkerson and Joanne Devlin, all affiliated with the University of Melbourne.
Akter R, Legione A, Sansom FM, El-Hage CM, Hartley CA, Gilkerson JR, et al. (2020) Detection of Coxiella burnetii and equine herpesvirus 1, but not Leptospira spp. or Toxoplasma gondii, in cases of equine abortion in Australia – a 25-year retrospective study. PLoS ONE 15(5): e0233100. https://doi.org/10.1371/journal.pone.0233100