A Legionnaires’ disease cluster was reported in Marshalltown, Iowa, on September 4 by the Iowa Health and Human Services. As of September 12, 2025, 38 people are infected with Legionnaires’ disease, and 1 has unfortunately passed away. The main source of this outbreak is suspected to be cooling towers.
Legionnaires’ Disease is caused by a bacterium called Legionella pneumophila. The water in cooling towers is at an ideal temperature for Legionella bacteria to grow. The risk of Legionella is heightened if the water is left untreated, uncleaned, and untested. A cooling tower can spread the Legionella that has been growing in its water. It does this when fans with high air flows encounter water particles, aerosolizing the water. This water aerosol containing Legionella can infect people in the building it is in or people walking nearby.
The recent New York City Legionnaires’ disease outbreak, centered in Harlem, from late July had progressed into 114 confirmed cases and 7 deaths according to the NYC Department of Health. This is a high number of cases, although it is worth noting that around 190,000 people are living in Harlem compared to around 28,000 in Marshalltown, Iowa. This means that as of September 12, 2025, 0.06% of the population of Harlem has been infected with Legionella compared to 0.136% of the population of Marshalltown. This means that this outbreak infected twice the concentration of people. The outbreak in Harlem has been confirmed to be from cooling towers at Harlem Hospital and the City Public Health Laboratory.
It is not yet confirmed whether cooling towers are the source of the Legionella outbreak. Tests have been taken on 10 suspected cooling towers in Marshalltown. The results are yet to be known. It will take 14 days since the last confirmed case of Legionella for the outbreak to be considered complete.
Testing for Legionella should be done on a routine basis. The test involves sampling water source(s) in the cooling tower. There are various testing methods, such as plate types and nucleic acid amplification. When the results of the test come back, the CDC recommends different courses of action based on how much Legionellabacteria is present in the sample(s):
If you live in Marshalltown, Iowa, and are suddenly experiencing the following symptoms, it is important to get immediate medical attention:
Legionella occurs naturally in nature, and over time, an untreated cooling tower is likely to harbor Legionella. Chardon Labs offers chemical water treatment services to help control Legionella in cooling towers. All our current service managers are ASSE 12080 certified for Legionella risk management. We monitor your cooling tower and feed chemicals based on its water chemistry. Additionally, we provide fast testing/routine testing. We can also help with the remediation of Legionella in cooling towers. You can schedule a free Legionella survey for regular cooling tower water treatment, order testing, or remediation here.
