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A cluster of 14 cases of Legionnaires' disease in Grand Rapids, Minn., has been traced to the municipal water supply, a state health investigation has found.

City authorities said Monday that they plan to flush and disinfect the water system to reduce risks of the disease, which is caused by inhalation of water droplets containing the Legionella bacteria. The disease is fatal in as many as 1 in 10 cases. While no one has died in this outbreak, 11 people were hospitalized.

Individuals were infected in multiple homes and buildings in a small area of Grand Rapids, including two buildings where testing found Legionella bacteria in water, said Trisha Robinson, epidemiologist supervisor for the Minnesota Department of Health's waterborne diseases unit. The bacterial strains from both buildings were highly related to each other and to lab samples taken from the patients, including one patient who was in the area but hadn't been in either building, she added.

Investigators looked for other possible sources, but exposure to the city water supply was the only risk factor all 14 people had in common, she said.

Infections rarely occur through drinking water. It usually happens when people inhale water droplets from a shower, faucet, spray bottle or power washer, she said. People aren't at risk if they drink water in Grand Rapids, but should use distilled or filtered water for aerosolizing devices such as CPAP machines.

The state began investigating shortly after the first cases were identified in Grand Rapids in April. While Legionnaires' disease is more common in warmer months, the cases in this outbreak have occurred sporadically across 2023, and include two illnesses in 2024.

Legionella tends to grow and spread in warmer, stagnant water, especially if it isn't treated with chemicals such as chlorine. Grand Rapids operates one of the few community water systems in Minnesota without chlorination, which is allowed for community wells that draw from groundwater, according to the state Health Department.

While a chlorination system is now "being looked into" as one strategy to confront the outbreak, it can be tricky to introduce the disinfectant into a previously unchlorinated system, said Julie Kennedy, Grand Rapids Public Utilities general manager. "Our approach needs to be methodical to ensure we avoid any additional health, safety or distribution system complications. We will be providing local updates and customer notices as that plan develops."

Minnesota reported 134 Legionnaires' cases and six deaths in 2023 — most of which occurred in isolation and didn't trigger state investigations. The Grand Rapids outbreak is a major one for Minnesota, but is not the largest, Robinson said. More than 20 Legionnaires' cases and one death were linked to cooling towers at a beverage plant in Hopkins in 2016.

Risks of health problems from Legionnaires' disease are elevated for people who are 50 or older, smoke and have existing chronic health conditions. The Health Department has urged doctors in Grand Rapids to watch for suspicious symptoms, and for patients to seek medical attention if they develop pneumonia, which could indicate Legionnaires.