MINNEAPOLIS / ST. PAUL (02/17/2026) — A groundbreaking study from the University of Minnesota Twin Cities has demonstrated how strategic changes in water treatment effectively halted a deadly outbreak of Legionnaires’ Disease in Grand Rapids, Minnesota. The study, published in Emerging Infectious Diseases, marks the first documented case of an outbreak being controlled by introducing disinfection to previously untreated groundwater.
Legionnaires’ disease, a severe and often fatal form of pneumonia caused by the bacterium Legionella pneumophila, has become the most prevalent waterborne disease in the developed world. It typically thrives in environments like cooling towers and water heaters. In 2023 and 2024, Grand Rapids experienced 34 confirmed cases and two fatalities due to the disease. The research team investigated the city’s infrastructure and discovered alarming levels of Legionella in buildings throughout the community.
Community-Wide Water System Connection
“This study is one of the first to directly connect a Legionnaires’ disease outbreak to an entire community water system,” said Molly Bledsoe, a University of Minnesota environmental engineering alumnus and lead author on the paper. To combat the spread, the city implemented chloramine disinfection. Since adopting this system, Legionella levels have dropped to undetectable levels, and the city has reported zero new cases of the disease.
Tim LaPara, professor in the University of Minnesota Department of Civil, Environmental, and Geo-Engineering and a senior author on the paper, emphasized the significance of these findings. “Our findings highlight a critical gap in public health safety,” he stated. “Many smaller or rural towns rely on undisinfected groundwater, assuming it is naturally protected. High levels of ‘assimilable organic carbon’ (AOC)—a type of nutrient for bacteria—in the groundwater may have fueled the Grand Rapids outbreak.”
Understanding Assimilable Organic Carbon (AOC)
AOC is a water quality parameter not commonly measured, particularly in smaller towns and rural areas. The presence of AOC can provide a nutrient-rich environment for bacteria like Legionella to thrive. The research team aims to continue their investigation to determine whether other groundwater-supplied community water systems also contain elevated levels of AOC, potentially posing similar risks.
Implications for Public Health and Future Research
The study’s implications extend beyond Grand Rapids, suggesting a need for broader public health strategies in managing waterborne diseases. The research was supported by the United States Environmental Protection Agency, underscoring the importance of federal involvement in addressing water safety issues.
In addition to Bledsoe and LaPara, the University of Minnesota team included graduate student Maya Adelgren, post-doctoral researcher Apoorva Goel, and project co-leader Raymond Hozalski. Their collaborative efforts have set a precedent for how communities might approach similar public health challenges in the future.
“By the Numbers: 34 confirmed cases, 2 fatalities, and zero new cases post-disinfection.”
Looking Ahead
The findings from Grand Rapids may prompt other communities to reassess their water treatment strategies, especially those relying on untreated groundwater. As research continues, the hope is to establish more comprehensive guidelines for monitoring and managing water quality, potentially preventing future outbreaks of Legionnaires’ disease and other waterborne illnesses.
As the study gains traction, it could influence policy changes at both state and national levels, encouraging investment in water infrastructure and safety measures. The University of Minnesota’s research team remains committed to advancing understanding in this critical area, aiming to safeguard public health across the nation.
