Doctors are warning of the emergence of a mysterious polio-like syndrome that has caused paralysis in more than 20 Californian children in little more than a year.
The condition, which causes rapid paralysis and severe weakness, has left some patients paralysed in all four limbs.
In some instances the illness has not improved despite treatment.
Medical experts in the US have identified up to 25 cases so far. The syndrome appears to be affecting children between two and 16 years of age.
Some of them suffered paralysis and others also experienced respiratory difficulties. In all the cases those affected had been vaccinated against polio.
Doctors investigating the illness found that the patients' spinal cords showed patterns of damage similar to that found in polio sufferers.
The Los Angeles Times reports that state public health officials have been investigating the illness since a doctor requested polio testing for a child with severe paralysis in 2012.
Dr Carol Glaser, leader of a California Department of Public Health team investigating the illnesses, called the doctor's request "concerning" because polio has been eradicated in the US and the child had not traveled overseas.
Since then, similar cases have sporadically been reported throughout the state, however, doctors say they do not expect an epidemic of the polio-like virus and that the infection remains extremely rare.
Dr. Keith Van Haren, a pediatric neurologist at Stanford University's Lucile Packard Children's Hospital who has worked with Glaser's team, will present the cases of five of the children at the American Academy of Neurology's upcoming annual meeting.
He said all five patients had paralysis in one or more arms or legs that reached its full severity within two days. None had recovered limb function after six months.
"We know definitively that it isn't polio," Van Haren added, noting that all had been vaccinated against that disease.
Glaser wouldn't provide the number of illnesses. Van Haren said he was aware of around 20. She urged doctors to report new cases of acute paralysis so that investigators can try to figure out a possible cause.
In the west polio is now largely eradicated. During the 1940s and 50s outbreaks of the disease crippled tens of thousands of children across the world.Reuse content