Obsessive Compulsive Disorder and tics could be triggered in people who are exposed to certain strains of strep, say researchers backing a controversial idea.

The hypothesis is that in some children who probably have a genetic susceptibility, certain strains of strep may trigger the serious mental illness almost overnight, Toronto science journalist Alison Motluk tells CBC Radio's Quirks & Quarks.

OCD is characterized by repetitive unwanted thoughts, images and impulses, such as non-stop handwashing, checking that the door is locked, or elaborate rituals like tapping a leg.

Kelly O'Donnell of Ottawa describes what happened to her then seven-year-old daughter back in 2012.

Alison Motluck

Science journalist Alison Motluck explores the provocative and controversial theory behind a childhood condition known as PANDAS, where young children suddenly develop OCD and severe tics almost overnight. (CBC)

One school night, she came home from school tearful. At a swimming lesson, her dad noticed she couldn't stop looking over her shoulder, O'Donnell said. Within 48 hours, she progressed to a point where everything she did required a repetitive behaviour.

"We thought she was having a nervous breakdown," O'Donnell recalled.

The O'Donnells took her to emergency. The doctor on duty remembered a rare disorder caused by strep and ordered a throat swab.

Listen to "Can you catch OCD?" on CBC Radio's Quirks & Quarks, Sat. Oct. 5 at noon

"They called, and said that she tested positive for strep, which we were surprised by. But they called in the prescription for penicillin and we picked it up by noon. And by noon the next day her symptoms were 95 per cent gone."

Motluck speaks to Susan Swedo, the pediatrician who worked for years at the U.S. National Institute for Mental Health and first suggested the disease, Pediatric Autoimmune Neuropsychiatric Disorder Associated with Streptococcus or PANDAS might be a disease in its own right.

Swedo said the best theory for PANDAS is that it's a variant of rheumatic fever.

A bad strep in an unlucky child can lead to a cross-reactive autoimmune reaction where the body's anti-strep antibiodies mistake a person's own cells in part of the brain for strep and launch an immune attack, Motluck explains.

However, a study by neurologist Roger Kurlan that followed children over time, found not all children with PANDAS symptom tested positive for strep.

"Despite very compelling cases like O'Donnell's, so far no one has shown in a really convincing way that PANDAS is always preceded by strep  and that strep is what makes PANDAS symptoms worse," Motluck cautions.

Motluk's own daughter developed sudden emotional outbursts and bizarre obsessive thoughts immediately after a serious ear infection, although her case is less clear cut than O'Donnell's.