Similar viral illnesses have paralyzed children in Europe and Asia but scientists don't know if those outbreaks are linked to the one in California.
California doctors investigating cases of polio-like illnesses in children have been "flooded" with calls but are no closer to knowing what is behind the illnesses.
"We're working with the California Department of Public Health to go through the cases, to look for a cause and to figure out how many patients are affected," said Keith Van Haren, a pediatric neurologist at Lucile Packard Children's Hospital in Palo Alto, Calif.
He will present the cases of five of the children at the American Academy of Neurology's upcoming annual meeting in Philadelphia.
The initial report about the cases was released on Sunday and since then doctors and patients from around the country have contacted the hospital and the state's department of public health.
"Physicians and public health officials who have encountered similar illnesses have submitted 20 reports to California Department of Public Health, and CDPH has conducted preliminary tests on 15 of these specimens," said Gil Chavez, California's state epidemiologist.
All the cases are in children. Sofia Jarvis, 4, was the first case of acute flaccid paralysis, as the syndrome is called, that the Stanford doctors came across.
So far no common causes have been found to suggest that the cases are linked, but "investigation is ongoing," Chavez said.
The Centers for Disease Control and Prevention has not received reports from any other states, said Danny Feikin, epidemiological branch chief in CDC's division of viral diseases. The CDC is working with the California Department of Public Health to study the illnesses.
All cases have several things in common. They come on suddenly, they result in paralysis of one or more limbs and an MRI of the spine shows an injury to the central part of the spinal cord, said Van Haren. There is no known cause and no prospect of regaining use of limbs.
California has enhanced its surveillance for acute flaccid paralysis and will continue to seek, test and review specimens and information to help protect public health, he said.
What they're seeing is "likely to be a very rare manifestation from a rare virus," said Van Haren.
The numbers are no higher than would be expected by normal background cases, said Feikin.
According to the World Health Organization, about one in 100,000 children experience acute flaccid paralysis (AFP) each year.
It is a description of a syndrome, not a disease itself, said Feikin. Just as pneumonia can be caused by many different pathogens, AFP can have multiple causes.
Given those numbers, the CDC would expect to see about 78 cases of acute flaccid paralysis in the United States each year.
So while the cases are devastating for the children and their families, they do not constitute an outbreak or a cluster, Feikin said.
However, pediatric neurologists on the front lines in California are concerned they might be seeing something different. "This is not something we're used to seeing in the pediatric population," he said.
The initial concern was that the children might have polio, but testing quickly ruled that out, Feikin said.
"We haven't had polio in the United States for decades. Today it is only endemic in three countries: Pakistan, Afghanistan and Nigeria."
The doctors who first described the cases, at Stanford University Medical School, believe it might be caused by a virus but can't say for certain. Testing found one type of enterovirus in two of the affected children, but not others.
Polio is one of more than 200 enteroviruses in the Picorna family, Feikin said. These viruses also cause hand, foot and mouth disease, a common childhood ailment, as well as the common cold.
"In general these viruses are asymptomatic. You don't even know you have them. Sometimes they cause mild illnesses, sometimes a rash," Feikin said.
It is estimated that there are between 10 million and 15 million cases of symptomatic enterovirus in the United States each year, "almost all of them just mild, cold-like illnesses," Feikin said.
One virus in the family, enterovirus 71, was responsible for several outbreaks of disease in California in the 1960s and later Bulgaria, Hungary. In the 1990s it sickened tens of thousands of children in Malaysia, Taiwan and China. Some victims developed a similar polio-like paralysis.
None of the recent California cases tested positive for enterovirus 71, Van Haren said.
Clinicians and doctors are doing their job as guardians of public health when they bring attention to possible illnesses so that outbreaks can be spotted early, said Feikin.
In this instance, it doesn't appear to be an outbreak, Feikin said.
"We're not trying to minimize, but there's no evidence right now that there is an outbreak or clusters," Feikin said.