State conservation officials are warning swimmers to stay out of Walden Pond, after a routine test there revealed a high level of potentially dangerous bacteria.
A weekly screening on Tuesday turned up unusual amounts of enterococcous, a pathogen that in severe cases can cause digestive ailments including vomiting and diarrhea. It can also cause fever and ear, eye, nose, and throat infections, according to Troy Wall, a spokesman for the Department of Conservation and Recreation.
The state's "red flag" warning for the pond cautions people not to swim, but does not prohibit them from going into the body of water in Concord.
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Lifeguards will still be stationed at the pond for anyone who chooses to swim despite the warning.
The department said it will test Walden Pond again Thursday, and may update the warning at that time.
Sarah Roberts can be reached at sarah.roberts@globe.com. Follow her on Twitter @heysarahroberts