Almost all kids come down with strep throat, and the vast majority of them suffer nothing more than a few days of fever, sore throat, and missed school. Some kids, though, seem to recover then suddenly develop a tic, extreme anxiety, or symptoms of obsessive-compulsive disorder (OCD) — a condition known as PANDAS (Pediatric Autoimmune Neuropsychiatric Disorders Associated with Streptococcus). "Parents have used the word 'possessed,'" says leading PANDAS researcher Susan Swedo at the National Institute of Mental Health (NIMH), one of the first people to link strep to OCD in the mid-1990s. "Their sweet, wonderful child turns into a monster seemingly overnight." Here's a guide to the puzzling, controversial disorder:

What exactly is PANDAS?
An autoimmune reaction to an infection, usually strep, that causes a child to suddenly develop OCD, tics, or neurological problems like anxiety, hyperactivity, and even anorexia. Though the disorder is not well understood, a new and growing body of research is lending wider legitimacy to the idea that mental illnesses can be caused by common bacterial infections.

How could a strep infection affect the brain?
Infections like strep trigger antibodies that are supposed to help fight bacteria, but in some cases — like the now-rare, strep-linked rheumatic fever — the antibodies go haywire and attack the body. The theory with PANDAS is that the antibodies attack the brain, causing the OCD and other problems. That theory is bolstered by a 2003 study in Nature Medicine in which immunologist Madeleine Cunningham showed that strep antibodies can attach to neurons that then release large amounts of dopamine and other neurotransmitters believed to cause OCD symptoms. 

What is PANDAS like in practice?
In one case, a mother describes how her second-grade son, coming home from school one day not long after a strep infection, started running maniacally through the house shutting off lights and the TV because they were spreading "radiation" and "poison." Another mother says her just-recovered 11-year-old son woke up one day so terrified of germs that he refused to return to school, showered compulsively, and would only wear a sheet or freshly microwaved clothes. "He had never been like this before. Ever," she tells the Los Angeles Times. "He just woke up with it."

How is PANDAS diagnosed?
There's no test — yet. The National Institute of Mental Health defines the disorder as a handful of tic outbreaks or OCD episodes that occur shortly after a strep infection. PANDAS is more common in families with a history of rheumatic fever, and scientists think some kids are genetically predisposed to the disorder and other strep-related autoimmune problems.

How common is it?
About 1 percent of U.S. children have OCD and maybe 20 percent have at least temporary tics. But nobody knows how many of those cases have any connection to strep, how long PANDAS can lie dormant after a strep outbreak, or even how common ODC is. "I used to think it was exceedingly rare," says OCD expert Michael Jenike at Harvard Medical School. "Now I think it's exceedingly common."

How do you treat PANDAS?
The medical community is split. Some doctors treat PANDAS like other cases of OCD or tics, typically with antidepressants or cognitive-behavioral therapy. Others use antibiotics to douse any remaining strep infection, or turn to a more invasive method called intravenous immunoglobin (IVIG), which involves transfusing plasma from healthy donors to shut down the wonky antibody response. 

Do either of those work?
The research is mixed. A 1999 study in the British journal the Lancet found IVIG treatments to be effective for children with strep and OCD, but another study the next year found no improvement. A new study is in the works at the NIMH. Until there is more definitive evidence that either experimental treatment works, the American Academy of Pediatrics recommends antidepressants and therapy. That doesn't still well with all PANDAS parents, says Melinda Beck in The Wall Street Journal, many of whom "are incensed that doctors are willing to prescribe psychotropic drugs to their young children but not 10 days of penicillin."

Might antibiotics really cure OCD?
Some specialists swear by them as a treatment, if not a cure. "I've been working in psychiatry for 20 years and I've never seen psychotropic medications turn a child around as quickly as I've seen antibiotics turn a PANDAS child around," says Tanya Murphy at the University of Florida's Rothman Center for Pediatric Neuropsychiatry. But symptoms can flare up again, often more severe, if the person is exposed to strep again. Still, researchers are excited about the possibilities, for OCD and a host of other psychological disorders. "If you can prevent lifelong suffering by using antibiotics or some acute intervention, that would be huge," says Jenike

What are the larger implications?
If researchers can definitively show that strep infections can cause OCD, that implies that other neurological disorders like autism, schizophrenia, and eating disorders might also be tied to errant antibodies from normal infections. It also opens up the possibility that those conditions could be treated with antibiotics or other medical interventions. "The whole area of mental illness caused by infections is being looked at more closely because of PANDAS," Harvard's Jenike tells the Los Angeles Times.

Sources: Daily Mail, Los Angeles Times, Wall Street Journal