Minnesota health officials have confirmed nine cases of Legionnaires' disease in Hopkins — up from five late last week — in the state's largest outbreak of the disease since the mid-1990s.

Investigators with the Minnesota Department of Health (MDH) are homing in on the source of the bacteria, which has infected only people who live and work in Hopkins. Health investigators from Hennepin County and the state are looking at several Hopkins-based businesses, including a Supervalu warehouse, a plastics facility called Thermotech and an outdoor Cargill fountain, among other sources, said Health Department spokesman Doug Schultz. Cooling units used for air-conditioning on large structures are often the culprit.

Legionnaires' disease, which resembles a serious case of pneumonia, is spread by inhalation of fine spray from water sources containing Legionella bacteria. It can be treated with antibiotics and is rarely fatal. The disease is not spread person to person, and cannot be contracted from drinking water. Hopkins drinking water is safe, authorities said.

Five people became ill between Aug. 4 and Sept. 1, the state Health Department said. Another person developed Legionnaires' disease last week and three more cases were confirmed on Thursday. At least four people are currently hospitalized. Two others were treated and have since recovered. The patients are all over age 50 except one, who may have had preexisting conditions, officials said.

Many of those who fell ill live within a 2-mile radius of the possible sources, Schultz said.

The outbreak is part of an uptick in cases this year — both in Minnesota and nationally. Minnesota typically sees 50 to 60 cases of Legionnaires' disease annually, but more than 60 cases already have been reported in the state so far this year.

No clusters have been reported in outstate Minnesota. Outbreaks are more likely among vulnerable adults in hotels, long-term care facilities and hospitals, according to the Centers for Disease Control and Prevention. They're often linked to cooling towers, cooling misters, decorative fountains or plumbing systems.

Each of the Hopkins businesses has already taken steps to disinfect its facilities, Schultz said. Sometimes that involves "super-chlorination." Samples taken from each water source will not yield results for another week or two.

Other cases are likely to develop, Schultz said, because the incubation period of the bacteria lasts around 10 days.

The Cargill fountain was shut off Tuesday as a precautionary measure, though the company regularly tests its water systems for any related contaminants, said spokesman Pete Stoddart.

"We have not received any indications from the tests that we do or the inspections being carried out by health officials that our fountain is the source," he said.

At Thermotech, the plastics company, an internal memo notified about 245 employees of the local health concern, reassuring them that owners were taking "every possible precaution" to keep them safe, a manager said.

None of the company's workers has become ill or shown symptoms, said Brent Weaving, Thermotech's environmental health and safety specialist. Thermotech routinely "shocks the system" with chlorine baths, and ordered the most recent round this weekend. "Our water is actually cleaner than the water that comes from the cities," he said.

Supervalu also maintains weekly cleaning programs for its cooling and refrigeration systems and has implemented additional safety measures, said Jeff Swanson, vice president of communications.

Individual risks vary

Minnesota's last serious outbreak of Legionnaires' disease occurred in 1995, when the infection killed two people and infected dozens more in Luverne and Mankato.

Most people exposed to Legionella bacteria don't get sick. People over 50, smokers or those with certain medical conditions — including weakened immune systems, chronic lung disease or other chronic health conditions — are at increased risk.

Symptoms of Legionnaires' disease include muscle aches, chills, shortness of breath, headache, fatigue, loss of appetite and coughing. These symptoms may be followed by high fever (102-105 degrees), pneumonia and occasionally abdominal pain and diarrhea.

Liz Sawyer • 612-673-4648