(CNN) -- Health officials are looking for the cause of a neurologic illness that's affected nine children in Colorado.
The children were hospitalized with muscle weakness in their limbs between August 9 and September 17, according to the Centers for Disease Control and Prevention; MRI tests spotted abnormalities in the children's spinal gray matter. Most of the children experienced a respiratory illness before being admitted to the hospital.
The CDC is still investigating the cause of these symptoms. Health officials do not believe that the cases were caused by polio, as eight of the nine children are up to date on their polio vaccinations.
"We don't know, at this point, if there is any association between the enterovirus EV-D68 that's circulating and the paralytic conditions some of the children in Colorado are experiencing," CDC spokesman Tom Skinner said.
Tests of the children's cerebrospinal fluid came back negative for enteroviruses and West Nile virus. But a test of their nasal passages found enterovirus in six out of eight patients who were tested.
Of those six, four tested positive for enterovirus D68, which has been sending children across North America to the hospital with severe respiratory illnesses. The other two test results are pending.
"It could be something else. That doesn't prove cause and effect, but it's circumstantial evidence," said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases. The cause "still remains a puzzle."
A cluster of children with similar paralysis symptoms was identified in California last year. Samples from two of the children tested positive for enterovirus D68.
Enterovirus D68 is part of the Picornaviridae family, which also includes the poliovirus, other enteroviruses and rhinoviruses. Enteroviruses are very common, especially in late summer and early fall. The CDC estimates that 10 million to 15 million infections occur in the United States each year.
These viruses usually appear like the common cold; symptoms include sneezing, a runny nose and a cough. Most people recover without any treatment. But some types of enterovirus are more serious. These can cause hand, foot and mouth disease; viral meningitis; encephalitis (inflammation of the brain); an infection of the heart; and paralysis in some patients.
This year, enterovirus D68 seems to be exacerbating breathing problems in children who have asthma. The virus has infected at least 277 people in 40 states, according to the CDC. Cases have also been reported in Canada.
CNN affiliate WCBS reported Friday that a New Jersey toddler died last week from a severe respiratory illness and that the CDC will be testing samples to see whether he had enterovirus D68.
The CDC is asking other hospitals across the country to be on the lookout for similar cases and to send in information on any patients with these symptoms.
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