Marshalltown outbreak: local health officials report 34 cases, trace source toward cooling towers

State and local public-health officials told the Marshall County Board of Supervisors on Sept. 10 that they have identified 34 cases and one death tied to a single Legionnaires outbreak centered in Marshalltown and are investigating industrial cooling towers as the most likely source.

"For Marshalltown so far, we found 34 cases and 1 death that we believe are tied into and associated with a single outbreak here in Marshalltown," said Matt Donahue, an internal medicine physician and state epidemiologist with Iowa HHS. Donahue said Legionella is a bacterium that multiplies in warm, man-made water systems and can be spread when contaminated water is aerosolized.

The state team, local public-health staff and the Centers for Disease Control and Prevention have coordinated the response, Donahue said. "It's most likely coming from a cooling tower," he said, and teams have tested nine of the most likely cooling towers and asked those facilities to disinfect the systems even before culture results return. "We disinfect those cooling towers," Donahue said, explaining that disinfection should reduce the risk of airborne spread while laboratory cultures are pending.

Health officials described the outbreak investigation timeline: the first local cases were reported Aug. 24, investigators completed patient interviews to track exposures and clinicians received a Health Alert Network advisory to increase testing. Donahue said environmental testing for Legionella requires culture and can take about seven days; he identified the State Hygienic Lab in Iowa City as the facility performing environmental cultures.

Board members asked whether Legionella transmits person-to-person. "There is no person to person transmission of Legionella," Donahue said, adding that devices that heat and spray water (such as improperly cleaned humidifiers or hot tubs) can pose risks if not maintained.

Public-health officials emphasized that most people exposed to Legionella do not become ill but urged anyone in Marshalltown with pneumonia-like symptoms to tell their clinician about the possibility of Legionella so proper testing and treatment can begin. Donahue said some environmental cultures may not grow even when bacteria were present, so teams are prioritizing disinfection of likely sources now rather than waiting for a definitive laboratory 'smoking gun.'

The board thanked Public Health Director Sydney Gruwell and state staff for their work. Next steps are pending culture results and completion of the remaining cooling-tower testing and disinfection; health officials said they will monitor for a 14-day period without new cases to consider the outbreak under control.