The Importance of Ticks in Q Fever Transmission: What Has (and Has Not) Been Demonstrated?

Q fever is a widespread zoonotic disease caused by Coxiella burnetii, a ubiquitous intracellular bacterium infecting humans and a variety of animals. Transmission is primarily but not exclusively airborne, and ticks are usually thought to act as vectors. We argue that, although ticks may readily transmit C. burnetii in experimental systems, they only occasionally transmit the pathogen in the field. Furthermore, we underscore that many Coxiella-like bacteria are widespread in ticks and may have been misidentified as C. burnetii. Our recommendation is to improve the methods currently used to detect and characterize C. burnetii, and we propose that further knowledge of Coxiella-like bacteria will yield new insights into Q fever evolutionary ecology and C. burnetii virulence factors.

TrendsQ fever is a widespread zoonotic disease caused by Coxiella burnetii, an intracellular bacterium that infects humans and a wide range of vertebrates. Domestic ruminants are the main reservoir.C. burnetii is frequently detected in ticks, and laboratory experiments have revealed that at least some tick species are competent vectors. However, under natural conditions, Q fever is far more frequently airborne than vector-borne.Many Coxiella-like bacteria, closely related to but genetically distinct from C. burnetii, have been described in ticks and, very occasionally, in vertebrates. They likely behave as non-virulent tick symbionts. Their pathogenicity for vertebrates is largely unknown.Coxiella-like bacteria may have been misidentified as C. burnetii in past field studies. New means of detecting and characterizing tick-borne C. burnetii and Coxiella-like bacteria are needed.