BUFFALO (Reuters) - State health officials have determined stress likely caused a dozen female high school students to suddenly experience tics and other neurological symptoms associated with Tourette Syndrome, they said on Friday.

The Le Roy Central School District, about 50 miles east of Buffalo, scrambled to conduct environmental testing for air quality and mold after 12 students developed tics and impulsive verbal behavior over the course of the last three months.

But state health investigators ruled out environmental factors, latent side-effects from drugs or vaccines like Guardasil, trauma or genetic factors.

The girls were all treated by doctors and most are improving, school officials said.

"Stress is often attributed to these kind of symptoms," Jeffrey Hammond, a spokesman with the New York State Department of Health, said on Friday, echoing the opinion of neurologists who have treated the girls.

"The Le Roy school is safe," Hammond said. "The environment or an infection is not the cause of the students' tics. There are many causes of tics-like symptoms."

Dr. David Lichter, clinical professor of neurology at the University at Buffalo, said he evaluated one of the girls who exhibited involuntary movements as well as periods of unresponsiveness.

Lichter said a phenomenon called "mass psychogenic illness," once called mass hysteria, is likely the cause.

He said high levels of stress may increase the chances.

"Subjects turn subconscious psychological stresses into physical symptoms, and they do it without being conscious of it," he said.

"I don't think the girls in this particular school are more stressed. The thing about this outbreak - and it has been documented around the world - is there may be one or two who manifest a true organic disorder, and then modeling behavior takes place," Lichter said.

The health department continues to monitor each case. No new cases have been reported.

(Editing By Barbara Goldberg and Paul Thomasch)

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