For Kelly Wooldridge of St. Louis, the change in her son's behavior was so abrupt, it was like someone flipped a switch.
Overnight, Brendan, now 10, went from being an easy-going, "huggy-kissy" kid to a rageful child plagued with tics, compulsions and obsessions, she said.
"He would walk up and choke kids at school, or pick up a chair and throw it at them," recalled Wooldridge, 37. Brendan developed facial tics, constant throat clearing, some humming.
"He was just miserable in his own skin," his mother said.
The shift first occurred when Brendan was 3, just after several recurring bouts of strep throat. The disturbing behaviors lingered, seeming to wax and wane for the next few years with no clear cause or explanation.
It wasn't until last year that Wooldridge -- like a growing number of parents, pediatricians and researchers -- finally connected the dots between the common childhood infection and the sudden onset of some forms of mental illness.
"Last spring, we learned about PANDAS," said Wooldridge. "I thought it sounded a little crazy, but it totally fit."
PANDAS -- or Pediatric Autoimmune Neuropsychiatric Disorders Associated with Streptococcal Infections -- is the unusual diagnosis given to a group of children who abruptly develop Obsessive Compulsive Disorder or tic disorders such as Tourette's Syndrome - but only after contracting infections such as scarlet fever or strep throat caused by Group A streptococcus bacteria.
"Parents describe it as a 'possession' or an 'explosion' of symptoms," said Dr. Susan Swedo, a leading PANDAS expert and senior investigator in neuroscience at the National Institutes of Health.
PANDAS turned out to be the cause of non-stop sneezing for Lauren Johnson, a 12-year-old girl featured on NBC's TODAY show, and it was linked for a time to outbursts of tics and twitches suffered by more than a dozen school-aged girls in LeRoy, N.Y.
On Thursday, the International OCD Foundation, or IOCDF, warned that mental illnesses such as OCD can be triggered by infections in children.
"If a parent recognizes these symptoms developing seemingly overnight, along with a glaring change in their child's personality and/or behavior, they should immediately have their child tested for strep," agency officials said.
Quick treatment with antibiotics can be the key to reversing OCD, tics and other symptoms, said Swedo.
"If you treat it acutely with antibiotics, it will help quite quickly," she said, noting that symptoms from the first episode eventually will subside. "But if a child gets another trigger, symptoms will return. If you have three episodes, the symptoms will become chronic."
It's not clear how many children may be affected by the disorder. About 1 percent of all children have OCD, but Swedo says there has been no research to determine the prevalence of sudden onset of the disorder preceded by strep or other infections.
But both she and Dr. Michael Jenike, a Harvard Medical School psychiatrist who's also been studying the problem, say in-progress research and the comments they hear from desperate parents every day indicate that PANDAS could be far more common than anyone suspects.
"It could be dozens of kids a day in this country," said Jenike, who also chairs the IOCDF scientific advisory board.
Some children may have a genetic vulnerability to the disorder and parents of some sufferers believe that it may run in families.
Now, new research suggests that the problem may extend beyond sudden OCD sparked by strep bacteria to include other infections, including Lyme disease, chicken pox -- even the flu.
This month, Swedo and other researchers have published a paper in the journal Pediatrics & Therapeutics that expands the definition of PANDAS to PANS, or Pediatric Acute-onset Neuropsychiatric Syndrome, which encompasses more potential causes.
The goal is to urge doctors and other clinicians to consider a full range of signals when they're faced with abrupt changes in a child's behavior or mental state.
"It's clinically very, very distinct," Swedo said. "Your only job is to find a treatable trigger."
The trigger is believed to lie with the antibodies that are normally produced by the body to fight infection. Usually, in the case of a bacterial infection, the antibodies attack the invading bacteria and help elminate them from the body. But in the case of PANDAS, for instance, researchers believe patients mount an unusual immune response in which the antibodies may mistakenly attack the part of the brain that controls emotions, behaviors and physical movements.
Parents of children with PANDAS have been a galvanizing force for education and awareness, said Lynn Johnson, Lauren's mother, who founded the nonprofit PANDAS Resource Network after her family's experience.
To such parents, there's no question that strep and other infections are involved. Meghan Sherman, 15, of Plano, Texas, was diagnosed with PANDAS only recently, but she's had OCD symptoms that rise and fall with infections since she was 7, says her mother, Jen Sherman.
"When we treated the strep, the symptoms would go away. I've seen it with Fifth disease and with flu and with other illnesses," said Jen Sherman. Meghan now takes precautionary antibiotics during the winter cold and flu season to ward off infections -- and OCD.
When Kelly Wooldridge found out about PANDAS, she asked a doctor to test Brendan's blood for strep, even though he didn't seem sick. It came back positive.
Now she's taken his care a step further, opting for intravenous immunoglobulin or IVIG treatments that are thought to suppress harmful inflammation in the body.
The change in her son, she says, is remarkable. He's no longer in trouble for behavior at school and he's been able to make friends.
"He's had not one trip to the principal's office," she said. "The biggest thing now is that he's happy."
Not everyone agrees about the PANDAS diagnosis, which was first described in 1998. Parents who suspect their kids have the disorder say they're routinely discouraged by doctors who aren't aware of the diagnosis or who actively reject it. There has been considerable debate about the role of strep A infections in the development of the disorder. Some experts believe that PANDAS may be a version of a better-known related disorder, Sydenham's chorea, in which antibodies attack the part of the brain that controls movement. The American Academy of Pediatrics is wary of preventive administration of antibiotics for strep infections, advising treatment of active illness.
But Swedo said experts do agree that there is a subgroup of children who develop sudden, dramatic cases of OCD, accompanied by a wide range of neural and psychiatric symptoms.
The new PANS label will generate additional research, even as Swedo and other PANDAS researchers continue to focus on the effects of strep. She hopes the increased attention will only expand awareness of the disorder among reluctant doctors.
"You have a treatable and perhaps preventable form of mental illness," she said. "While we're debating, these kids could be being treated."