Public health officials have traced the source of an outbreak of intestinal illness that surfaced last week to the swimming pool at the Ashland YMCA.

At least 35 people were infected with the diarrheal illness caused by Cryptosporidium, a microscopic organism that spreads easily in water. Other symptoms included abdominal cramps, nausea, and vomiting.

The outbreak surfaced when two physicians noted a number of young children had been ill with severe diarrhea. Sixteen of the 17 patients who were ill had been swimming at the Ashland Y.

Investigators interviewed 85 people in the process of determining that the Ashland pool was the source of the illness, said Hank Collins, Jackson County’s director of Health and Human Services.

Collins said a water sample taken when the outbreak surfaced indicated the Y pool’s chlorine level was acceptable. The Y drained its pool and raised the chlorine level when it was refilled.

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Owners of nearly two dozen other pools open to the public also "hyper chlorinated" their pools as a precaution. Collins said their action may well have prevented the illness from spreading any further.

He said the outbreak shows how easily "crypto" can spread if an infected person uses a public swimming pool.

He noted that Cryptosporidium spores can survive many days in water with low concentrations of chlorine. Chlorine levels must be raised four to 20 times above the levels that are normally found in public pools, and maintained for at least eight hours, to kill the parasite.

"This could happen to any swimming pool," Collins said. He noted that the Ashland Y has "one of the most sophisticated chlorination systems" in the region.

"If it could happen to them it could happen to anyone."

Messages left at the Ashland YMCA seeking comment were not immediately returned Thursday.

Cryptosporidium produces an illness that’s much more severe than an overnight case of diarrhea. In some people, symptoms can persist for 10 to 14 days.

"Some of these little kids were pretty sick," Collins said.

He said the best way to keep the disease from spreading is for people who are suffering from diarrhea — especially young children — to stay out of public swimming pools. "It’s real hard to keep a child like that from shedding these bugs when they’re in the pool."

People who have had symptoms should stay out of public pools for 14 days after their last episode of diarrhea, he said.

Reach reporter Bill Kettler at 776-4492, or e-mail bkettler@mailtribune.com.