A California disease surveillance program has helped provide clues to a mysterious polio-like illness that has struck 59 people, mostly children, in the state over the past three years. But the cause of the illness remains unknown, according to a new report in the Journal of the American Medical Association

The syndrome, called acute flaccid myelitis, is characterized by muscle weakness or paralysis and reduced muscle tone. Symptoms include respiratory and gastrointestinal distress, fever and muscle pain. Out of the 59 cases identified between June 2012 and July 2015, the median age was 9 years old. In 50 of the cases, the afflicted were under 21 years old. Two immune-compromised adults died within two months of symptom onset, and 38 patients had long-term weakness as a result of the illness, according to the JAMA report.

The California Department of Public Health received three separate reports of acute flaccid paralysis in fall 2012. No such cases had been reported in the past 14 years. In response to the initial cases, the CDPH implemented enhanced surveillance to catch other emerging cases and possibly identify an underlying cause.

Researchers at Stanford University examined the cases reported to the CDPH but were unable to determine an underlying cause. They noted that the number of cases rose significantly during a national enterovirus D68 outbreak from August 2014 to January 2015, compared to other periods.

The Stanford researchers, led by Keith Van Haren, M.D., a pediatric neurologist, concluded that more surveillance is necessary to better understand the illness.

“To our knowledge, the California surveillance program for acute flaccid paralysis is the first to use specific case criteria and report subsequent incidence data for the subset of paralysis cases attributable solely to acute flaccid myelitis,“ the authors wrote in JAMA. “And may serve as a guide for similar surveillance efforts in the future.”