FORT WORTH, Texas — Health officials say a spring-fed lake in Fort Worth has tested positive for a parasite that causes intestinal problems.

The Centers for Disease Control and Prevention in Atlanta confirmed that cryptosporidium was found in water samples from Burger’s Lake.

Tarrant County Public Health officials released those results Wednesday, about a week after the lake was closed after some swimmers reported a loss of appetite, nausea, vomiting and diarrhea.

More than 600 people have reported being ill after swimming there in the last month or so, but health officials have only confirmed 20 cases of the disease so far.

It is resistant to normal chlorine levels.