About 30 students and 10 staff members at Lake Forest Elementary School reported having a rash-like reaction, possibly from bug bites, last week, officials said.
Those affected notified the Saddleback Valley Unified School District on Sept. 15, said Tammy Blakely, assistant to the superintendent. Many were part of an after school program on campus.
The Orange County Mosquito and Vector Control District has launched an investigation, but officials have not determined what is causing the irritation, said spokesman Jared Dever.
“It is a big mystery and it’s one that we’re hoping to resolve,” Dever said.
"We have done extensive surveys on stinging and/or biting insects that could possibly have caused the reaction that we’re seeing in the students. However our investigations have really not turned up any conclusive evidence."
Officials have ruled out mosquitos and are trying to narrow down a list of possibilities based on the symptoms and reports.
He explained that the symptoms point to a smaller insect that something that one might not notice, like a mite or a chigger. The concern is that smaller insects are able to travel quicker.
"You can inadvertently be moving them to your property without your knowledge," Dever said.
District officials had the campus treated over the weekend with a pesticide and plan to do so again next weekend.
Officials said the district reacted quickly after receiving reports and believe the pesticide treatment is working.