Distribution System Operational Deficiencies Coincide with Reported Legionnaires' Disease Clusters in Flint, Michigan.

We hypothesize that the increase in reported Legionnaires' disease from June 2014 to November 2015 in Genesee County, MI (where Flint is located) was directly linked to the switch to corrosive Flint River water from noncorrosive Detroit water from April 2014 to October 2015. To address the lack of epidemiological data linking the drinking water supplies to disease incidence, we gathered physiochemical and biological water quality data from 2010 to 2016 to evaluate characteristics of the Flint River water that were potentially conducive to Legionella growth. The treated Flint River water was 8.6 times more corrosive than Detroit water in short-term testing, releasing more iron, which is a key Legionella nutrient, while also directly causing disinfectant to decay more rapidly. The Flint River water source was also 0.8-6.7 °C warmer in summer months than Detroit water and exceeded the minimum Legionella growth temperature of 20 °C more frequently (average number of days per year for Detroit was 63 versus that for the Flint River, which was 157). The corrosive water also led to 1.3-2.2 times more water main breaks in 2014-2015 compared to 2010-2013; such disruptions have been associated with outbreaks in other locales. Importantly, Legionella spp. and Legionella pneumophila decreased after switching back to Detroit water, in terms of both gene markers and culturability, when August and October 2015 were compared to November 2016.