MINNEAPOLIS / ST. PAUL (02/17/2026) — A breakthrough new study from researchers at the University of Minnesota Twin Cities shows how strategic changes in water treatment effectively treated a deadly outbreak of Legionnaires' Disease.
For the first time, the study published in Emerging Infectious Diseases, provides evidence of an outbreak being stopped by introducing disinfection to previously untreated groundwater.
Legionnaires' disease is a severe, often fatal form of pneumonia caused by the bacterium Legionella pneumophila. While it is known to grow in places like cooling towers and water heaters, it has become the most common waterborne disease in the developed world.
In 2023 and 2024, Grand Rapids, Minn., had 34 confirmed cases of the disease and two fatalities. The research team conducted an investigation into the city's infrastructure and found concerning levels of Legionella in buildings throughout the community.
"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 implementing this system, Legionella levels dropped so low that they could no longer be detected and the city has had zero new cases of Legionnaires' disease.
"Our findings highlight a critical gap in public health safety," said Tim LaPara, professor in the University of Minnesota Department of Civil, Environmental, and Geo- Engineering and one of the senior authors on the paper. "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."
AOC is a water quality parameter that is not commonly measured, especially in smaller towns and rural areas. The team is hoping to continue this research to explore whether other groundwater-supplied community water systems also contain elevated levels of AOC.
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.
The research was supported by the United States Environmental Protection Agency.
Read the full paper entitled, "Effect of Chloramine Disinfection of Community Water System on Legionnaires' Disease Outbreak, Minnesota, USA, 2024," on the U.S. Centers for Disease Control and Prevention website.
