An extremely rare, polio-like disease has appeared in more than a dozen California children within the past year, and each of them suffered paralysis to one or more arms or legs, Stanford University researchers say. But public health officials haven’t identified any common causes connecting the cases.

The illness is still being investigated and appears to be very unusual, but Dr. Keith Van Haren at Lucile Packard Children’s Hospital at Stanford University warned that any child showing a sudden onset of weakness in their limbs or symptoms of paralysis should be immediately seen by a doctor.

“The disease resembles but is not the same as polio,” he said. “But this is serious. Most of the children we have seen so far have not recovered use of their arm or leg.”

But doctors are not sure if it’s a virus or something else, he said. Dr. Van Haren says he has studied five cases from Monterey up through the San Francisco Bay Area, including two who were identified with the disease enterovirus-68, which is from the same family as the polio viruses. He said there have been about 20 cases statewide.

“We want to temper the concern, because at the moment, it does not appear to represent a major epidemic but only a very rare phenomenon,” he said, noting similar outbreaks in Asia and Australia.

Dr. Van Haren said polio vaccines do not protect children from the disease, but stressed that it is still important for children to receive that vaccine.

The California Department of Public Health has not identified any common causes that suggest that the cases are linked, said Dr. Gil Chavez, the deputy director of the Centre for Infectious Disease and state epidemiologist.

“Physicians and public health officials who have encountered similar illnesses have submitted 20 reports to CDPH, and CDPH has conducted preliminary tests on 15 of these specimens,” he said. “Thus far, the department has not identified any common causes that suggest that the cases are linked.”