Abstract
Quantitatively estimating an individual's risk of infection by an airborne pathogen requires knowledge of the expected dose and the pathogen's infectious dose. Based on our review of the published literature on tularemia, we conclude that the infectious dose of Francisella tularensis varies among individuals, but that a substantial proportion of the population can be infected by a single bacillus. We also conclude that infection can be initiated by inhaling bacilli carried on respirable particles (diameters less than 10 µm) or nonrespirable particles (diameters between 10 µm and 100 µm). Regression analyses based on two-parameter Weibull and log-normal models of human inhalation dose-infection data aggregated across three studies indicate that approximately 30% of individuals who inhale a single F. tularensis bacillus will develop tularemia. Further, when the organism is carried on particles with diameters on the order of 1 µm, it is estimated that the deposition of a single bacillus produces infection in 40% to 50% of individuals; thus, when F. tularensis is carried on respirable particles, the estimated ID50 via inhalation is close to one deposited bacillus. These results are consistent with separate analyses using nonparametric methods and with experimental animal models in which infection is observed after injection of a single bacillus. The risk of person-to-person transmission of tularemia is generally considered negligible, perhaps due to a low concentration of F. tularensis in respiratory fluids. However, viable F. tularensis bacilli are present in human respiratory fluids, and can be carried in inspirable particles (diameters less than 100 µm) which are emitted during coughs and sneezes.