In this Monday, Sept. 8, 2014 photo, Melissa Lewis, of Denver, helps her son, Jayden Broadway, 9, as he coughs in his bed at the Children's Hospital Colorado in Aurora, Colo. He was treated for the enterovirus 68 and released, but his asthma made the illness more difficult to fight. (AP Photo/The Denver Post, Cyrus McCrimmon)

Several children in Colorado, including some that have tested positive for the Enterovirus 68 respiratory illness, also reported neurological symptoms including muscle weakness and paralysis.

Colorado health officials say nine children were identified between Aug. 8 and Sept. 17 after they developed neurological symptoms that are not commonly associated with Enterovirus 68, which causes severe breathing problems particularly in children with pre-existing asthma or respiratory problems.

That virus has been confirmed in the District of Columbia and all but 10 states, according to the Centers for Disease Control and Prevention, and it has sickened more than 277 people, mostly children.

The CDC said it is investigating the cases in Colorado and has called on health officials nationwide to be on the lookout for similar symptoms.

They do not believe that the symptoms are related to polio, which is caused by a viral infection that leads to paralysis, because most of the children -- eight out of nine -- are fully up to date on their polio vaccinations. Of the eight that were tested by the CDC, four tested positive for Enterovirus 68, and two others were positive for different unidentified strains of an enterovirus.

"This is a very small number of patients," Larry Wolk, director of the Colorado Department of Public Health and Environment told the Denver Post. "That's why we're asking if there are other cases. People shouldn't panic."

The children are from the Denver area and are being treated at Children’s Hospital Colorado in Aurora, Colo., and they all reported symptoms of fever and breathing problems two weeks before developing limb weakness, according to the CDC. None have fully recovered from their symptoms, Wolk said.

Outbreaks of enteroviruses and rhinoviruses, which cause the common cold, are common in the fall, and the often go undiagnosed if the symptoms are not severe. But this year, the outbreak has been worse than usual and children with asthma or other pre-existing breathing problems have been particularly hard hit. The CDC said that it expects the number of cases to wane later in the fall.

The CDC's advisory asked doctors to be watchful for any acute onset of limb weakness and evidence an magnetic resonance imaging (MRI) test of the spinal cord that shows large lesions on the gray matter of the spinal cord.

None of the children experienced seizures or an altered mental state.

While it isn't necessarily common for non-respiratory symptoms to occur with Enterovirus 68 infections, Wolk told the Denver Post that myelitis, or the infection of gray matter of the spinal cord, can occur as a complication of viral infections.

"We've seen this in the past and have studied these kinds of clusters in the past," Wolk said.

Earlier this year, health officials in California investigated 25 cases of polio-like symptoms in children, some of whom tested positive for Enterovirus 68.