A strange illness has been killing thousands of young people each year, and recently it has started claiming even more victims in Africa.

Called nodding disease, it usually strikes children at the age of 4 or 5 years and starts with occasional bouts of uncontrolled nodding. As the disease progresses through adolescence, the nodding often buds into full-blown epileptic seizures, and victims lose developmental ground, often becoming unable to care for themselves, communicate or even avoid simple accidental death by drowning or burning.

Since it was first described in 1962 in Tanzania, the frequently fatal disease has been blamed, variously, on viruses, pesticides, fungi, vitamin deficiency, monkey meat and parasites. A new special report, published online April 12 in Science, details the more recent outbreaks of the condition in South Sudan and northern Uganda and helps to refine the list of possible causes.

"We have a long list of things that are not causing nodding disease," Scott Dowell, director of the division of global disease detection and emergency response at the U.S. Centers for Disease Control and Prevention, told Reuters earlier this year.

Dowell and his team have made many trips to Uganda to further investigate this strange syndrome. They used EEGs and MRIs to study the brains of patients while they were going through a head-nodding bout. "Something is badly wrong with the brains of these kids, and it's physiological," he told Science. But these tools did not lead them to a definite answer, although most viruses, prion disease (from eating monkey meat), fungi and pesticides seem to be losing steam as likely explanations.

The researchers haven't yet ruled out a vitamin B-6 deficiency. Some people with a particular genetic mutation that reduces B-6 uptake have severe epilepsy. Although kids with nodding syndrome didn't have the lowest levels of B-6, the CDC team is planning to include supplements as part of a forthcoming clinical trial that is also slated to test out anti-seizure medications.

Another possible cause is the parasite Onchocerca volvulus, a worm that also causes river blindness disease (also known as onchocerciasis). "The puzzling thing is that [the worm] is widespread, but nodding is not," Dowell told Science. The disease also seems to occasionally be present where the parasite is not. A 2008 study found, for example, that out of 51 patients with head nodding (some with just nodding, others with more advanced seizures), 43 had traces of O. volvulvus in their bodies, but none of them had evidence of it in their spinal or brain fluid. And a 2010 study found that of 300 people in Tanzania, more severe O. volvulvus infections did not mean a higher risk for epilepsy.

Nevertheless, some researchers are still intrigued by this parasitic worm as a possible cause of the condition. "I am convinced that somehow it is connected," Andrea Winkler, a neurologist at the Technical University of Munich and who worked on both the 2008 and 2010 studies, told Science.

The ultimate answer might lie multiple factors. "Epilepsy is very often multi-factorial," Michel Boussinesq, of the University of Montpellier, told Science. "Onchocerciasis could be a related factor but not sufficient to provoke the condition." Kids who are already vitamin deficient or suffering from other conditions could be rendered more vulnerable to the parasite's attacks.

Dowell's team is still obtaining samples from patients and healthy kids to test for vitamin levels as well as other potential environmental exposures. And they are currently in the process of getting approval to begin the first, 80-child round of clinical trials.