A rare and mysterious polio-like illness may have afflicted up to 25 children in California, several of whom have suffered limb paralysis, and health experts were struggling to identify the cause of the ailment, medical researchers said on Monday.

Since 2012, between 20 and 25 previously healthy children from across California have shown signs of the illness, possibly caused by an infectious virus, the American Academy of Neurology said in a statement detailing the research of two California neurologists.

One of the children remains in serious condition but none have died from the syndrome, researchers said.

Stanford University pediatric neurologist Keith Van Haren said in a statement that the cases could indicate the possibility of an "emerging infectious polio-like syndrome in California," although federal health officials said there were too few cases to consider the spread of the ailment as an imminent threat.

Polio, eradicated in the United States over three decades ago, is an infectious virus that can permanently paralyze or kill victims within hours of infection. A vaccine, developed in the 1950s nearly wiped out the disease worldwide, although it remains endemic in Pakistan, Afghanistan and Nigeria.

A study of five California children affected by the mystery ailment found they experienced the sudden paralysis of one or more limbs, with the symptoms reaching the height of severity within two days, according to findings of research by Van Haren and University of San Francisco neurologist Emmanuelle Waubant.

"Although poliovirus has been eradicated from most of the globe, other viruses can also injure the spine, leading to a polio-like syndrome," Van Haren said in a statement.

Children vaccinated against polio

Scans of the bodies of the five children included in the study also showed white spots on their spinal chords, indicating undefined damage, according to the research that is set to be released at an American Academy of Neurology meeting in Philadelphia in April.

Despite the illness being similar to polio, all five of the children had been vaccinated against poliovirus before their symptoms began, the research showed.

The additional 15 to 20 children showing signs of the syndrome were not closely analyzed in the study, but initial blood tests and other information collected showed they had suffered from sudden paralysis or extreme limb weakness and possible spinal chord damage, Waubant told Reuters. All the children were between the ages of 2 and 16.

Two of the five children in the study tested positive for a strain of enterovirus, which has been linked to polio-like outbreaks in children in Asia and Australia over the past decade, while three tested negative, Waubant said.

She said the children who were negative could have had the virus but were not tested in time for it to show up in their blood.

The test results of the broader group of children with polio-like symptoms were pending, Waubant said. She said it was too soon to determine whether that particular virus had affected the bulk of the children.

United States Centers for Disease Control and Prevention researcher Dr. Jane Seward said the center was aware of the study but said there were too few cases of the illness to suggest there was an imminent threat of the illness spreading across the state or the U.S.

The California Department of Public Heath said it would release a statement about the study and its investigation into the reported illnesses later on Monday, and declined comment ahead of the planned statement.

Source: ANTARA