Assessment of the Legionnaires’ disease outbreak in Flint, Michigan

Significance

Unresolved is the etiology of the 2014–2015 Legionnaires’ disease outbreak in Genesee County, MI. Flint is the most populous city in Genesee County, and the outbreak coincided with damaged water infrastructure and the subsequent Flint water crisis. The unprecedented disturbance in water quality within Flint’s drinking water distribution system allowed the evaluation of the statistical relationship between free chlorine residual and Legionnaires’ disease risk within a full-scale drinking water system. Through the integration of multiple datasets, results from numerous causal inference tests implicate changes in water quality, as reflected by changes in free chlorine residual, in the City of Flint as responsible for the outbreak. These findings provide public health professionals and engineers unparalleled scientific evidence to reduce waterborne disease.

Abstract

The 2014–2015 Legionnaires’ disease (LD) outbreak in Genesee County, MI, and the outbreak resolution in 2016 coincided with changes in the source of drinking water to Flint’s municipal water system. Following the switch in water supply from Detroit to Flint River water, the odds of a Flint resident presenting with LD increased 6.3-fold (95% CI: 2.5, 14.0). This risk subsided following boil water advisories, likely due to residents avoiding water, and returned to historically normal levels with the switch back in water supply. During the crisis, as the concentration of free chlorine in water delivered to Flint residents decreased, their risk of acquiring LD increased. When the average weekly chlorine level in a census tract was <0.5 mg/L or <0.2 mg/L, the odds of an LD case presenting from a Flint neighborhood increased by a factor of 2.9 (95% CI: 1.4, 6.3) or 3.9 (95% CI: 1.8, 8.7), respectively. During the switch, the risk of a Flint neighborhood having a case of LD increased by 80% per 1 mg/L decrease in free chlorine, as calculated from the extensive variation in chlorine observed. In communities adjacent to Flint, the probability of LD occurring increased with the flow of commuters into Flint. Together, the results support the hypothesis that a system-wide proliferation of legionellae was responsible for the LD outbreak in Genesee County, MI.