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Officials for the McHenry County Department of Health (MCDH) in northeast Illinois announced that the McHenry County Legionnaires’ disease outbreak has increased from nine to 12 victims. In addition, department officials said they have narrowed their search for a common source.
Working with the Illinois Department of Public Health (IDPH) and the Centers for Disease Control and Prevention (CDC), the MCDH said it had determined that six people lived, worked or spent time within a 1½-mile radius of the intersection of Route 176 and Walkup Road in Crystal Lake.
The movements of the six other people sickened in the outbreak showed no connection. Of the three new cases reported in this update, two are from outside the county, and one is an out-of-state resident who visited the county.
McHenry County Legionnaires’: investigation ongoing
The radius around the Crystal Lake intersection will serve as the focal point of the investigation, said MCDH community information coordinator Keri Zaleski. “Coming up with this case definition is very important,” she said.
The CDC is examining environmental samples from the area to determine the source of the Legionella bactera.
“Many environmental samples have been collected,” acting MCDH administrator Joe Gugle said in a news release. “However, it is often the case that a single source is never found, which is not surprising or unusual considering that Legionella bacteria are pervasive in our natural environment,”

The McHenry County Legionnaires’ disease outbreak has increased from nine to 12 victims. Officials said they have narrowed their search for a common source to a 1½-mile radius of the intersection of Route 176 and Walkup Road in Crystal Lake.
McHenry County Legionnaires’: more cases unlikely
Seen more frequently in hot weather, Legionella are transmitted through mist or small droplets of water from freshwater environments such as lakes and streams.
Zaleski said that because the 10- to 14-day incubation period for Legionnaires’ disease has passed, the chances of future cases have decreased considerably.
McHenry County Legionnaires’: first report
This outbreak first made headlines on July 11, when the MCDH confirmed that nine residents of McHenry County had been sickened with Legionnaires’ disease in less than a month.
Residents of the cities of Algonquin, Crystal Lake, Huntley, McHenry and Wonder Lake were diagnosed with Legionnaires’ disease between June 7 and July 1. The ages of those sickened ranged from 46 to 82 years old.
“I can tell you eight people were hospitalized, (and) seven have been released,” Zaleski said in an email to the Woodstock Independent. “(Federal privacy) laws prevent any further information about this.”
None of the initial nine people who became ill were residents of long-term-care facilities, but those facilities, other health-care providers and local municipalities all have been alerted “as a precautionary measure,” Zaleski said.
McHenry County Legionnaires’: LD increase
This outbreak represents a large spike in the incidence of Legionnaires’ disease in McHenry County. Last year, there were only four cases of Legionnaires’ disease in the county, compared to nine in 2016 and three in 2015.
The northern-most border of McHenry County abuts Wisconsin. The county’s population is a little more than 300,000, which makes it the sixth-most populous county in Illinois.