The implementation of chloramine disinfection halts Legionnaires disease outbreak in a community water system

Abstract

A Legionnaires’ disease outbreak occurred in a small city in northern Minnesota, USA from April 2023 to June 2024. The source of the Legionella bacteria was suggested to be the non-disinfected, groundwater supplied community water system. The goal of this investigation, therefore, was to assess the occurrence of Legionella pneumophila throughout the water distribution system and in selected public buildings (i.e., premise plumbing samples) and the effectiveness of chloramine disinfection at resolving the outbreak. L. pneumophila were quantified in two sets of water samples collected prior to and two sets collected after introducing chloramines in June 2024. Before disinfection, L. pneumophila was detected in only 1/16 (6.3%) of water distribution samples but frequently detected (5/10, 50%) in premise plumbing samples at concentrations ranging from 102.0 to 104.0 MPN per liter via the Legiolert assay and from 102.5 to 104.3 gene copies per liter via digital PCR (gene target = mip). Within 15 days of the initiation of disinfection, all water distribution system samples were negative for L. pneumophila via Legiolert and digital PCR while 3/6 (50%) of premise plumbing samples were positive for mip (concentrations =102.6, 103.9, and 108.2 gene copies per liter) by digital PCR. Most of the premise plumbing samples were negative via the Legiolert assay, except for the sample that had the highest concentration of mip genes (101.6 MPN per liter). After 168 days of chloramine disinfection, all water distribution system and premise plumbing samples were negative for L. pneumophila. These results show that the community water system supplied by groundwater supported the growth of substantial levels of L. pneumophila in premise plumbing but not within the water distribution system. Furthermore, the implementation of chloramine disinfection quickly (< 3 weeks) reduced the levels of L. pneumophila in tap water, which coincided with a cessation of new cases of Legionnaires’ disease.