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Full recovery has eluded almost all of the children stricken by polio-like paralysis during last year’s outbreak of serious viral respiratory illness in Colorado and across the country.

Scientists’ is the key to unlocking the mystery of the paralysis cases.

From Aug. 2 through Thursday, the U.S. Centers for Disease Control and Prevention confirmed 107 pediatric cases in 34 states. In these cases, the sudden onset of paralysis affected one or more limbs and sometimes facial muscles.

The patients’ ages ranged from infant to 21 years old, but the median age was 7. There have been no fatalities among these cases, called acute flaccid myelitis, but symptoms generally have been severe and persistent, CDC officials say.

In Colorado, where doctors first sounded the alarm over paralysis cases, the count rose to 12. Two additional cases at Children’s Hospital Colorado fit some, but not all, of the CDC criteria and are excluded from the count. Two children didn’t have limb weakness, with paralysis affecting only cranial nerves, Children’s Dr. Sam Dominguez said.

“A lot of the children are coming back later this month or next month for examinations,” Dominguez said of the local patients. He’ll know more about their progress then.

“We haven’t been at their bedsides, but what we’ve heard from clinicians is that these children experienced substantial weakness,” said Dr. Jim Sejvar, the CDC’s lead investigator.

About two-thirds of the children observed have reported some improvement, but a third have not, he said. Only one child is known to have recovered fully. Yet recovery could be possible in the months or year ahead.

Clinicians at Children’s were the first to alert public health officials in August that an outbreak of EV-D68, alongside other viral cases, appeared to be spawning some cases of severe limb weakness — apparently stemming from distinctive abnormalities of the central spinal cord.

Only one cerebrospinal fluid sample out of almost 100 specimens tested nationwide has been positive for EV-D68, but there were certain issues with the quality of the sample that call it into question, CDC officials said.

And, while federal and local scientists remain puzzled that they can’t confirm the presence of enteroviruses or other pathogens in the cerebrospinal fluid, something else is pointing them toward EV-D68, Sejvar said.

It’s really about the timing — the EV-D68 outbreak and the incidence of acute flaccid myelitis coincided, beginning in August and subsiding around late October.

“What is pointing toward a connection is the temporal overlap,” Sejvar said. Dominguez agrees.

“It’s definitely still the leading hypothesis,” Dominguez said. “It’s clearly associated with EV-D68.”

During the outbreak, more than 40 patients’ upper respiratory tracts were sampled for enterovirus/rhinovirus, and more than 40 percent of them tested positive. It’s inconclusive data. Viruses can clear the body before sampling can occur. Testing for other pathogens in these samples is ongoing.

“We’d like to have some additional evidence,” Sejvar said. “It remains a mystery.”

Children’s physicians found, using magnetic resonance imaging, that their patients suffering partial paralysis had developed lesions in the gray matter of their spinal cords after bouts of respiratory illness, high fever or both.

The gray matter is the central region of the spinal cord surrounding the central canal. It consists of cell bodies and motor neurons, which send signals to muscles, and of interneurons, which carry information between motor and sensory neurons.

Colorado health officials alerted the CDC on Sept. 12 about the Denver cluster of paralysis cases. Two weeks later, the CDC issued a health alert asking state and local health departments across the country to and to send specimens related to the nationwide outbreak of severe respiratory disease caused by EV-D68.

Not only was CDC officials said, it was behaving strangely in children in terms of the severity of respiratory symptoms and in its association with the inexplicable paralysis.

“Strains of the virus that have been associated with this particular season don’t seem to be substantially different (genetically) from strains that have circulated in the past,” Sejvar said. “It’s perplexing.”

The virus was detected in specimens from 13 patients who died. In Colorado, there were no pediatric deaths. Adult deaths in cases of viral respiratory illness are not tracked by the state.

Dr. Eyal Leshem said the CDC is developing several new research projects to learn more about acute flaccid myelitis, including its baseline rate, or more typical incidence outside of massive viral outbreaks such as 2014’s.

Electa Draper: 303-954-1276, edraper@denverpost.com or twitter.com/electadraper