In July 2017, the Minnesota Department of Health (MDH) investigated an outbreak of suspected viral gastroenteritis among individuals who swam at a Hennepin County park district beach in Maple Plain.

Contact information from campground reservations and boat rentals was obtained from the park. MDH staff interviewed visitors to obtain information on water and food exposures and illness history. Routine beach monitoring results were also obtained from park staff and reviewed to see if levels met the EPA recreational water quality standards.

Twenty-three park visitors were interviewed. Two visitors met the case definition. Both cases reported diarrhea, vomiting, and abdominal cramps occurring between 21 and 38 hours after water exposure. Swimming at the main beach approached statistical significance (2 of 2 cases vs. 3 of 17 controls; odds ratio, undefined; 95% confidence interval, undefined; p=0.059.)

A routine water sample from the main beach collected on July 5 had an E. coli concentration of 19 most probable number (mpn) per 100 milliliters, which is below the EPA single water sample standard of 235 mpn per 100 ml.

Investigators concluded this was a waterborne outbreak with a ill swimmer most likely introducing the virus into the water. Beach contamination can be transient, and the result from the water sample collected on July 5 was likely not representative of the water conditions the day prior.