Legionnaires' Disease Study Shows Water Treatment Success and High Mortality Risks
Research shows chloramine disinfection successfully stopped a Legionnaires' disease outbreak in Minnesota, while clinical data reveals Legionella pneumonia has 11.9% 30-day mortality and disproportionately affects immunocompromised patients. The studies highlight both prevention strategies and diagnostic challenges.
A new study demonstrates that strategic changes in water treatment effectively stopped a deadly Legionnaires' disease outbreak, while separate research reveals the severe clinical impact of Legionella pneumonia with high mortality rates. For the first time, research published in Emerging Infectious Diseases provides evidence of an outbreak being stopped by introducing disinfection to previously untreated groundwater.
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. To combat the spread, the city implemented chloramine disinfection. Since implementing this system, Legionella levels have dropped so low that they could no longer be detected and the city has had zero new cases of Legionnaires' disease.
The findings highlight a critical gap in public health safety. 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.
Meanwhile, a separate study in Clinical Infectious Diseases provides a snapshot of 344 episodes of Legionella pneumonia managed through the Mayo Clinic from January 2019 to September 2025. Median age was 66.6 years with 45.1% immunocompromise. Intensive care unit admission occurred in 36.1%, and mechanical ventilation in 22.7%. Thirty-day and 90-day mortality were 11.9% and 16.6%, respectively.
Cirrhosis was the strongest risk factor for 30-day mortality, with an odds ratio of 10.2. Age, immunocompromise and lymphopenia were also independent risk factors. Gastrointestinal symptoms were reported in 27.6% and pleural effusion in 64.1%. Higher incidence was observed in summer/early fall.
Levofloxacin was the final antibiotic in 48.6%, followed by azithromycin in 36.9%. Legionella was identified by urinary antigen in 51.5%, with PCR testing of respiratory specimens in 52.9% and culture in 25%. Among the 121 patients that were either PCR-positive or culture-positive and had urinary antigen testing, the urinary antigen was positive in only 31 (25.6%). PCR and sputum positivity were high among patients who underwent bronchoscopy with bronchoalveolar lavage: 156/162 (96.3%) and 30/31 (96.7%), respectively.
The utility of Legionella urinary antigen, which only detects L. pneumophila serotype 1, appears poor compared to PCR and/or culture from bronchoalveolar lavage. 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.