How many cases of obsessive-compulsive disorder in children are caused by bacteria? The US National Institute of Mental Health intends to find out, and to test whether antibodies will cure it.

Streptococcus bacteria, which cause “strep throat”, scarlet fever and other infections, evade the immune system by making surface proteins that mimic human ones. The immune system eventually catches on and makes antibodies to the proteins – but these can then attack human tissue including the heart, joints and brain. This has long been known to cause heart disease and a nervous disorder called Sydenham’s chorea.

In children, it emerged several years ago that this attack on the brain can inflame brain structures called the basal ganglia, and may precipitate a syndrome whose symptoms include obsessive thoughts and compulsive rituals, identical to the psychological condition obsessive-compulsive disorder.

The NIMH has now launched a project to find such cases and improve diagnosis and treatment, including a study to see how well IVIG, a human antibody treatment used to dampen autoimmune reactions, reverses the syndrome.

It also wants to find out how many such cases are related to strep infections, or if there are other causes. The NIMH has now put out guidelines for diagnosing Sydenham’s chorea whenever it appears, whether or not the child is known to have had a recent strep infection. Tracking all cases this way should allow researchers to investigate causes more thoroughly, as well as diagnostic signs, and treatments.