Investigating an airborne tularemia outbreak, Germany

In November 2005, an outbreak of tularemia occurred among 39 participants in a hare hunt in Hesse, Germany. Previously reported tularemia outbreaks in Germany dated back to the 1950s. We conducted a retrospective cohort study among participants and investigated the environment to identify risk factors for infection. Ten participants had serologic evidence of acute Francisella tularensis infection; 1 other participant died before laboratory confirmation was obtained. Presence within 5 meters of the place where disemboweled hares were rinsed with a water hose was the risk factor most strongly associated with infection (risk ratio 22.1; 95% confidence interval 13.2-154.3). Swabs taken at the game chamber and water samples were PCR negative for F. tularensis. Eleven of 14 hare parts showed low-level concentrations of F. tularensis, compatible with cross-contamination. More than half of case-patients may have acquired infection through inhalation of aerosolized droplets containing F. tularensis generated during rinsing of infected hares.