Health officials implicate cooling towers in Legionella outbreak

The City of Milwaukee Health Department reported 58 clinically diagnosed cases of Legionnaires’ disease in Milwaukee County from June 1 to Sept. 30, 2013, according to a report in MMWR.
This is more than double the normal number of annual cases from the previous 5 years, according to the report. Most of the cases (78%) were in the city of Milwaukee. All but one of the cases in the county were hospitalized. The health department received one report of a death related to legionellosis.
The health department conducted environmental sampling for detection of Legionella at 11 sites throughout the city. They collected 39 swab and bulk water specimens. Three cooling towers were positive for L. pneumophila. Investigations using pulsed-field gel electrophoresis on lower-respiratory specimens from cases and tests at the 11 sites found six distinct strains of L. pneumophila, but no patient strains matched strains from environmental samples.
Thirty-one of the patients (53%) lived within 3 miles of one of the positive cooling towers. The investigators found an RR of 1.6 (95% CI, 0.9-2.7) for Legionella exposure within one of these towers, but the association was not significant. Investigators with the health department met with cooling tower contractors and identified a gap in maintenance due to weather conditions.
“This investigation underscores the need for local public health authorities to be prepared to rapidly enhance surveillance, deliver appropriate public risk messaging and coordinate with the private sector to mitigate environmental transmission of Legionella within a community,” they wrote.