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Minnesota might face a future of more beach closures and waterborne illnesses as heavier rains contaminate lakes, but state health officials cautioned that last week’s unusual Big Island outbreak may have more to do with human debauchery than climate change.

While high levels of E. coli have temporarily closed beaches around the Twin Cities this month, largely due to rain runoff tainted with animal feces and other contaminants, public health investigators are exploring other theories in the Lake Minnetonka outbreak that sickened at least 116 people.

“Our investigation is focusing more on human sources, as opposed to runoff contamination,” said Allison Thrash, a spokeswoman for Hennepin County Public Health.

With hundreds of partygoers wading among boats packed in Cruisers Cove on July 4, the source of contamination could have been anything from a boater carelessly pumping out a marine toilet to a single person with norovirus defecating or vomiting in the water, public health experts said.

“You have to understand the Big Island experience,” said Michael Osterholm, a former state epidemiologist and director of the University of Minnesota’s Center for Infectious Disease Research and Policy. “It is like having one huge child-care facility, operating with a number of people challenged by their [levels of] chemical intake.”

Health officials said the Big Island outbreak shouldn’t make people fearful of swimming in lakes, but urged them to observe beach closures and take precautions.

Sucking up water and pretending to be a fountain probably isn’t a good idea, said Trisha Robinson, supervisor of the waterborne diseases unit at the Minnesota Department of Health. Neither is allowing lake water to splash into a can or other beverage.

“Then you certainly are drinking up the water,” she said.

The problem doesn’t appear to be getting worse, according to water testing data from the City of Minneapolis and Hennepin County.

The county has reported 11 beach closures so far this year, including the current closure of Excelsior Commons on Lake Minnetonka due to high E. coli levels. (E. coli is tested as the “proxy,” meaning that other health hazards could be in the water as well.)

The number of closures peaked at 49 and 45 in 2015 and 2016, respectively, when summer rainfalls were heavier than average.

Last week the Minneapolis Park Board closed the Thomas and 32nd Street beaches at Bde Maka Ska/Lake Calhoun, and the Lake Hiawatha Beach. But historical data since 2006, when E. coli thresholds were updated, show that the city issues one to five beach closures each year.

“Oftentimes geese will kind of pick a beach, hang out there, poop on it, and that beach will generally have problems,” said Rachael Crabb, a city parks water resource supervisor.

The Lake Nokomis beaches were closed for three days in 2008 because someone tipped over a portable toilet and the sewage spilled into a storm drain that fed into the lake.

It’s likely that climate change will produce heavier and more intense rainfalls, and those could contribute to more beach closings in the future, said Osterholm, who has studied waterborne illness in the streams of the Boundary Waters Canoe Area Wilderness. “Clearly, climate change is going to factor into this as time goes on.”

Lakes safer than pools

For now, swimming pools remain bigger risks for waterborne illnesses because of a parasite known as cryptosporidium, which can come from human feces and survive in chlorinated environments, Robinson said. Of 56 recreational water illness outbreaks in Minnesota since 2009, 32 occurred in swimming pools and nine took place in lakes and rivers, state health data show.

Robinson urged people to continue to swim for the health benefits, but to take preventive steps to protect others.

People should refrain from swimming when sick, and shower before entering the beach to clean off any fecal material that could contaminate the water.

Testing has limits. Most of Minnesota’s rural lakes and beaches aren’t tested at all. And Minneapolis’ testing is conducted each Monday of swimming season, so that any closures and retesting can be completed in time to reopen the beaches by the following weekend, said Deb Pilger, the Park Board’s director of environmental management.

The downside, Robinson said, is that E. coli testing “tells us today that we should not have gone swimming yesterday. So it’s not a perfect science, but it’s the best that is available.”

The state Health Department monitors beaches along Lake Superior and uses predictive modeling to close them based on the temperatures, currents and other factors meant to predict contamination.

Robinson said that approach isn’t practical for community lakes.

Some people sickened at Big Island have provided fecal samples for testing in an effort to determine the cause of the outbreak, but health officials said the culprit might never be found.

“Just one or two people vomiting out on Big Island ... could explain this entire situation,” Osterholm said.

“People might think it was because they drank too much alcohol, but it’s actually (that they were sick with) noro­virus.”

Staff writer Greg Stanley contributed to this report.

Jeremy Olson • 612-673-7744

Riskiest beaches

These five Hennepin County beaches, not including Minneapolis city beaches, have had the most closures since 2013:

Fort Snelling: 29

Round Lake Park: 26

Nelson Lakeside Park: 25

Excelsior Commons: 20

Wayzata: 19