Coxiella burnetii abortion in farmed plains bison (Bison bison bison) in western Canada.

Coxiella burnetii is a zoonotic bacterium that affects human and animal health globally and has detrimental socioeconomic effects on the livestock industry. This organism is known to cause abortion in sheep, goats, and cattle that is characterized by placentitis with minimal to no fetal lesions. In April 2025, the Diagnostic Services Unit at the University of Calgary Faculty of Veterinary Medicine (Alberta) received 11 aborted fetuses from a single herd of farmed plains bison (Bison bison bison). Initial submissions lacked placenta, delaying diagnosis. The 5th submission, which included placenta, revealed a necrotizing placentitis with intracellular bacteria within trophoblasts, highly suggestive of coxiellosis. In 4 of 11 cases, a definitive diagnosis of C. burnetii abortion was made, based on compatible histopathologic lesions in the placenta and detection of high numbers of organisms as evidenced by qPCR values. Coxiella burnetii was suspected in the remaining cases but could not be confirmed due to the absence of placenta. This report documents the first association of C. burnetii with abortion in plains bison and describes the histopathology and qPCR results observed during this outbreak. These cases underscored diagnostic challenges posed by incomplete submissions in abortion investigations. Key clinical message: This report highlights diagnostic challenges associated with abortion cases and the need for submission of both fresh and formalin-fixed placenta. Clinicians and diagnosticians should now include C. burnetii as a differential diagnosis for fetal loss in bison and ensure that appropriate biosafety measures are in place when handling specimens from ruminant abortions.