Hospitals are often required to perform a supplemental disinfection of their water systems to protect individuals from hospital‐acquired Legionnaires' disease. The authors of this article recently studied one hospital where three cases of hospital‐acquired Legionnaires' disease were detected in less than two years. These cases were linked to Legionella colonization of the hospital's water system. Chlorine dioxide (ClO2) was considered a cost‐effective approach to disinfection given that ClO2 generators could treat the 23 buildings comprising the hospital complex from one central location. The authors evaluated the efficacy of maintaining a residual of 0.5 to 0.8 mg/L of ClO2 for Legionella control in the secondary distribution system of this 437‐bed hospital over a two‐year period. Monthly monitoring showed mean Legionella positivity at hot water outlets and cold building source water areas decreased from 23 to 12% and 9 to 0%, respectively (p< 0.05). ClO2 residuals decreased with increasing distance from the application point and temperature. Mean ClO2 concentrations were lowest in hot water outlets (0.08 mg/L) followed by cold water outlets (0.33 mg/L) and reservoirs (0.68 mg/L). Complete eradication (0% positivity) of Legionella was achieved after 1.75 years, and no cases of Legionnaires' disease were reported during this time.

Testing drinking water systems for the presence of Legionella colonization is a proactive approach to assess and reduce the risk of Legionnaires’ disease. Previous studies suggest that there may be a link between Legionella positivity in the hot water return line or certain water quality parameters (temperature, free chlorine residual, etc.) with distal site Legionella positivity. It has been suggested that these measurements could be used as a surrogate for testing for Legionella in building water systems. We evaluated the relationship between hot water return line Legionella positivity and other water quality parameters and Legionella colonization in premise plumbing systems by testing 269 samples from domestic cold and hot water samples in 28 buildings. The hot water return line Legionella positivity and distal site positivity only demonstrated a 77.8% concordance rate. Hot water return line Legionella positivity compared to distal site positivity had a sensitivity of 55% and a specificity of 96%. There was poor correlation and a low positive predictive value between the hot water return line and distal outlet positivity. There was no correlation between Legionella distal site positivity and total bacteria (heterotrophic plate count), pH, free chlorine, calcium, magnesium, zinc, manganese, copper, temperature, total organic carbon, or incoming cold-water chlorine concentration. These findings suggest that hot water return line Legionella positivity and other water quality parameters are not predictive of distal site positivity and should not be used alone to determine the building’s Legionella colonization rate and effectiveness of water management programs.

This article reviews how Legionella and other waterborne pathogens can present a risk to consumers of potable water, secondary disinfection options, and a case study on chlorine dioxide.

Legionellosis, manifesting as either Legionnaires’ disease or Pontiac fever, is a serious concern in the United States. Water providers and property owners must do their part to control Legionella.

A conference held in 2018 focused on the current challenges of monitoring and preventing occurrence of Legionella; Robust guidelines and regulations are needed to establish a uniform, sufficient standard of care.