Environmental officials are testing lake water at a popular recreational destination in central Virginia after at least 20 people reported E. coli infections.

The Virginia Department of Health said it had received numerous reports of gastrointestinal illness over Memorial Day weekend, mainly in children, from people who were swimming in Lake Anna. At least nine people have been hospitalized due to the infections, according to the department, which said in a June 6 statement that “all potential causes of illness, including lake water and food exposures, are being investigated.”

The illnesses occurred between May 27 and June 4, according to the health department. An additional 10 cases are still under investigation, but E. coli infections have not been confirmed in those. 

While all those sickened confirmed to health officials that they had swam in or been exposed to water in Lake Anna, the department said it did not yet have enough information to confirm that exposure to the lake was the cause of the illnesses. 

“Water testing of the lake to evaluate present concentrations of bacteria, and to determine whether a public health risk may be ongoing, is being conducted,” the department said. 

The Department of Environmental Quality conducted water testing at the lake on Tuesday, but those results are still pending, according to the health department.

Lake Anna is a roughly 17-mile-long lake that has about 200 miles of shoreline, according to Luisa County. It was formed in the 1970s as a reservoir to facilitate cooling at an adjacent nuclear power plant. As a result, the lake famously has a “cold” side and a “warm” side that is heated by water discharged from the plant to assist in cooling.

The health department said the reported exposures have occurred at many parts of the lake.

The health department issued reminders that swimmers in natural waters, like lakes and rivers, should never drink untreated water and should not swim if their skin has cuts or open wounds.

E. coli infection can cause stomach cramps, diarrhea, vomiting and fever. In severe cases, the infection can damage kidneys, and lead to hemolytic uremic syndrome.

“We hope that those hospitalized continue to recover and can return home to their families soon,” Rappahannock Health District Health Director Olugbenga O. Obasanjo said in a June 6 statement, urging families to take precautions before swimming in pools, lakes or beaches. “Showering before and after swimming, washing your hands before eating, and being sure not to drink the lake water are some of the ways to stay healthy this summer. It is also important not to swim if you have diarrhea.”