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Health officials: Illnesses connected to Lake Minnetonka on July 4 still a mystery

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Boaters sail on Lake Minnetonka in July 2014.
Boaters sail on Lake Minnetonka in July 2014. Nearly 200 people reported vomiting and diarrhea after being on Lake Minnetonka near Big Island on July Fourth.
Paul Huttner | MPR News 2014

Nearly 200 people reported vomiting and diarrhea after boating near a popular gathering spot on Lake Minnetonka after July Fourth, but health officials say there's no ongoing threat to the general public. 

State and Hennepin County public health investigators interviewed 225 people since Monday. Hennepin County epidemiology manager Dave Johnson said Friday that 172 of those interviewed reported symptoms. 

Johnson emphasized the increased number of reports is "not the result of ongoing disease transmission or risk to the public. The common factor in these illnesses is being on Lake Minnetonka near Big Island on July Fourth only."

Health officials may never know exactly what made everyone sick. The state tested a stool sample from one person and it came back negative on all 22 of the viruses, bacteria and parasites that typically lead to diarrhea and vomiting. 

The county's environmental health unit goes out every Monday to test E. coli levels. The closure of Excelsior Commons beach happened to coincide with the first reports of sickness from Big Island, but Johnson said there's not necessarily a connection between the two. Beaches that are closer to Big Island were open on Friday. 

"That E. coli that we test for in beach monitoring is simply an indicator organism that there may be some sort of fecal contamination in the water," said Trisha Robinson, who focuses on waterborne diseases for the Minnesota Department of Health. "It is not the same E. coli illness that we talk about when humans are sick."

Robinson said there's no evidence of person-to-person transmission of whatever pathogen is causing people to get sick. 

To avoid infection, Robinson said people who have diarrhea or who've been vomiting should stay out of the water. Swimmers should try not to swallow water, and shower before and after going into lakes and rivers. And if you're boating and there's not a toilet on board, go back to shore and find the facilities.