Photo Credit: CDC Public Health Image Library ID #11148 by Janice Haney 2009; Edited with cropping. The prevalence of Legionella bacteria in drinking water and distributions systems has gained notice over the past several years due to its increasing rate of infection in the United States. Inhalation or aspiration of small aerosolized Legionella bacteria from water can cause Pontiac fever and Legionnaires’ disease most frequently in sensitive or immunocompromised populations. Between 2000 and 2015, the National Notifiable Diseases Surveillance System (NNDSS) reports that the incident rate of Legionnaires’ disease in the U.S. increased from approximately 0.42 cases per 100,000 persons to 1.89 cases per 100,000 persons. According to the Ohio Department of Health, potential reasons for this change in rate might include increased monitoring and awareness, higher population susceptibility, climate change, water-saving fixtures, and/or aging infrastructure. As of 2019 Legionnaires’ disease is reported to afflict and kill more people in the U.S. than any other waterborne disease. Existing research indicates that, though Legionella bacteria can be found in all parts of the water treatment system, they amplify best inside protozoan hosts and near the biofilm typically found within premise plumbing or drinking water systems . The resiliency of biofilm to disinfection acts as a protective barrier for Legionella while creating an environment abundant in nutrients. Protozoan hosts also offer defense against extreme temperatures and treatment technologies. A 1994 study by Kramer and Ford found that hundreds of Legionella bacteria can be contained within a single amoeba vesicle. L. pneumophila , the species responsible for most human infections, can also differentiate into various life cycle forms that alter susceptibility to water treatment. This symbiotic relationship with other microorganisms complicates Legionella disinfection. Hot spots for growth include showerheads, faucets, plumbing systems, cooling towers, hot tubs, fountains, and distribution systems where water stagnation, insufficient disinfectant residual, warm temperatures (77-124°F), or excess nutrients foster biofilm formation. As a result, the most frequent outbreaks from Legionella have been documented in hotels and healthcare facilities. Management of outbreaks can start at the site of these impacted buildings as well as the treatment plant. Drinking water utilities can participate in prevention by understanding the conditions that favor propagation and the methods to control growth. The U.S. EPA established a Maximum Contaminant Level Goal (MCLG) for Legionella at zero microorganisms . While this is not an enforceable limit, the Agency believes that if Giardia and other viruses are removed or inactivated as required under the Surface Water Treatment Rule, Legionella will also be controlled . Requirements to manage bacterial contamination under the Revised Total Coliform Rule and Ground Water Rule also contribute to Legionella management. Though some systems may routinely monitor for Legionella bacteria, testing methods can often yield both false positives and false negatives. Given the complications of environmental monitoring as well as the cost, management generally starts in response to outbreaks or sporadic cases. Ongoing research has identified that potential drinking water treatment methods for Legionella include chlorination, copper-silver ionization, ultraviolet (UV) light, ozonation, and thermal disinfection. Among these technologies, chlorine, chlorine dioxide, chloramine, and ozone are the most widely used disinfectants. A combination of these techniques offers the most effective defense against recolonization and biofilm formation. To inactivate individual bacteria as well as those contained within biofilm, operators should also pay attention to the contact time and concentration of disinfectant used during treatment. Equally important to contact time is the maintenance of disinfectant residuals throughout distribution. The National Academy of Sciences’ Management of Legionella in Water Systems details the recommendations for proper disinfection using free chlorine, chlorine dioxide, monochloramine, and technologies more commonly used by building water systems. To effectively manage Legionella in drinking water, utilities must also collaborate with impacted buildings. Facilities that have experienced outbreaks can develop their own management plan using the Center for Disease Control’s (CDC) Developing a Water Management Program to Reduce Legionella Growth & Spread in Buildings and the World Health Organization’s Legionella and the Prevention of Legionellosis . This literature, along with the CDC training on Legionella Water Management Programs and the other resources linked within this guide will ensure that your community members, especially those at greater risk to illness, are protected from Legionella .