A quantitative microbial risk assessment model for human inhalation exposure to Legionella

Since the first reports that the bacteria Legionella pneumophila causes Legionnaire's disease (LD) there has been unresolved conjecture about its infectious dose for humans. Legionella are not uncommon in water from building hot water systems, whirlpool spas, hot springs, and cooling towers. It has arguably been held that only high level contamination of aerosols from such sources represents a significant risk for Legionnaires' disease for immune competent individuals, and in part due to that view, control of Legionella contamination is neither broadly regulated nor uniformly practiced. Yet, LD remains a significant contributor to human morbidity and mortality. Establishing the dose-response for humans is an important component in better understanding the extent of the risks from exposures both high and low.; The hypothesis that Quantitative Microbial Risk Assessment (QMRA) estimates for LD agree with reported rates was tested by comparing predictions with rates reported for three outbreaks. The reported rates of seroconversion as a measure of subclinical infection (only for outbreak A and analyzed for two subgroups) fell at the 70th to 96th (95% CI 60 to 98th) percentiles of the calculated risk distributions. The clinical severity infection rates reported for outbreaks A, B and C fell at the 86th to > 99th (95% CI 70 to >99 th) percentiles of the estimates. Mortality rates for outbreaks B and C (not reported for A) were below the calculated rates but fell within an order of magnitude. The hypothesis of agreement is accepted since the predictions fell within an order of magnitude or less of the reported rates.; Additional research is appropriate to further validate the QMRA model and also to evaluate its capability of encompassing a broader range of human susceptibility and risk factors. Nevertheless, the projected risks (e.g., 1% clinical and subclinical severity infection rates at doses of 92 and 0.17 CFU respectively) suggest that there are significant public health consequences from low level exposure to Legionella. The new quantitative risk information for LD may motivate increased initiatives for improved Legionella contamination control, with a resulting reduction of the extent of human exposure, and a reduction in the prevalence of LD.