Copper Pipe, Lack of Corrosion Control, and Very Low pH May Have Influenced the Trajectory of the Flint Legionnaires’ Disease Outbreak

Abstract

Uncertainties remain regarding factors influencing the Legionnaires’ disease (LD) outbreaks in Flint, MI, after the city switched source waters and interrupted corrosion control. Notably, there was lower incidence of LD associated with residential versus hospital exposure in summer 2015 versus 2014. To test possible causes for this phenomenon, we designed a bench-scale study with an influent pH of 7.3, which demonstrated prolific growth of Legionella pneumophila in the presence of cross-linked polyethylene (PEX) pipe (103 cfu/mL), but 2-log lower L. pneumophila levels and >100 μg/L Cu2+ with copper pipes. However, when phosphate was present with copper pipes, soluble copper and Cu2+ levels decreased and L. pneumophila increased to a range comparable to that of PEX pipe. Additionally, we reveal that monthly LD incidence in Flint residences during the outbreak months was positively correlated with distribution system pH (R2 = 0.67, p = 0.01). We hypothesized that copper service lines and pipes in residences provided biocidal levels of Cu2+ at the very low pHs (7.3–6.9) in late summer 2015. Copper is rarely used in large service lines, point of entry pipes, or storage in hospitals where pH was not correlated to LD incidence. Interactions of pH, corrosion control, and pipe material should be considered in diagnosing LD outbreaks.