A Legionnaires' disease cluster in Hopkins continues to grow and is now approaching an outbreak record for the state set two decades ago.
Public health investigators confirmed three more cases of the respiratory infection Tuesday, bringing the Hopkins outbreak total to 20.
One person has died in the outbreak. Sixteen people have been or are currently hospitalized. Most of the illnesses have been among men, which is consistent with other Legionnaire's outbreaks.
The affected individuals range in age from 29 to 97 years old. All of them live or work in Hopkins, or visited the city during the 10 days before they became ill.
In 1995, a Legionnaires' disease cluster in Luverne climbed to 24 confirmed cases — the most ever in Minnesota. That same summer a separate outbreak in Mankato sickened 17 people.
The Minnesota Department of Health said the Hopkins case totals are probably just the tip of the iceberg. There are likely more people who were infected but not identified, said Richard Danila, assistant state epidemiologist.
"We're seeing the sickest people, most of them whom were hospitalized and who get tested for Legionnaires' disease," said Danila. "There are probably other cases out there of pneumonia who did not get tested for Legionnaires' disease."
More illnesses are possible in Hopkins, according to Danila. But the risk of ongoing transmission is dropping because suspect water sources in the city have been identified and disinfected.
Legionnaires' disease is contracted by inhaling mist from water containing Legionella bacteria. Contaminated cooling towers are often identified as a breeding ground for the bacteria. That was the source of the 1995 outbreaks in Luverne and Mankato.
So far, the Minnesota Department of Health hasn't detected the bacteria in its Hopkins water sampling investigation. But that doesn't mean it wasn't there said Danila.
"Testing water is very, very laborious, tedious and difficult to do. It's not an easy bacteria to grow," he said.
In addition to the agency's own testing, the Health Department has asked the New York state diagnostic laboratory to also test water samples from Hopkins. The New York lab has more recent experience with Legionella bacteria testing following an outbreak in New York City last summer.
However, it's possible the bacteria will never be found and the source of this outbreak will remain a mystery, Danila said.