The Infectious Dose of Coxiella Burnetii (Q Fever)

Abstract

Quantitative estimation of an individual's risk of infection due to airborne pathogens requires knowledge of the pathogen's infectious dose, in addition to estimates of the pathogen's airborne concentration and the person's exposure duration. Based on our review of the published literature on Q fever, we conclude that the infectious dose of Coxiella burnetii is likely one rickettsia, and that the probability of a single organism initiating infection is approximately 0.9. Findings in experiments exposing guinea pigs to C. burnetii via intraperitoneal injection and inhalation of respirable aerosols firmly support a “one-hit” Poisson model of infection. Findings in experiments exposing human subjects to C. burnetii via inhalation of respirable aerosols fail to provide convincing evidence that the one-hit Poisson model applies to human infection; however, inference from the human studies is limited by the small numbers of subjects and lack of quantification of the exposure concentrations. Given the presence of C. burnetii in sputum, the prevalence of cough in Q fever patients, and the ability of the pathogen to initiate infection via the respiratory tract, we believe that person-to-person transmission of C. burnetii via inhalation of respiratory aerosol is possible.