Hennepin County Public Health is investigating an illness outbreak among people who were on Lake Minnetonka over the Fourth of July holiday.

Public Health Epidemiology Manager Dave Johnson says more than 100 people have called to report vomiting or diarrhea after being on the lake in recent days, especially in the Big Island area. The department has confirmed about 30 cases of illness.

"I would say that the symptoms are consistent with a water or food exposure and so we've been interviewing the ill persons to evaluate all potential exposures," Johnson said. "So we're asking them about, were they in the water, were they swimming, did they potentially accidentally ingest some water."

Johnson said the county does not yet know how many cases of illness are connected to an exposure on Lake Minnetonka.

A few people have reported seeking medical care for dehydration. "Some have reported needing to get fluids intravenously which is consistent with vomiting and diarrhea, sometimes people can get dehydrated in those situations," Johnson said.

The Minnesota Department of Health is running tests to identify the pathogen causing the illness, and Johnson said results are expected later this week.

Hennepin County is testing the lake water for pathogens.

Johnson said there have been similar outbreaks of illness related to Lake Minnetonka in the past. The most recent he could recall was in 2013, when three people became ill from E. coli bacteria in the water, according the Health Department records.