Over 100 cases of possible norovirus have been reported in Kumamoto Prefecture, the prefectural government said Tuesday, with a local river identified as a potential culprit.

On Aug. 13, seven high school students who were playing near Todoroki Falls in the city of Amakusa experienced vomiting and diarrhea after playing in the river nearby, and similar cases were reported in the following days. All reported symptoms were mild, according to the city.

The city has since then set up signs near the river, calling on visitors to refrain from playing in the water.