A new strain of a common respiratory virus , researchers reported on Monday.
Since August, 115 children in 34 states have developed -like in an arm or a leg. The virus, enterovirus 68, has emerged as a leading suspect.
A study published Monday in the journal Lancet Infectious Diseases strengthens that possibility, although many questions remain.
Researchers at the , analyzed genetic sequences of enterovirus 68 cultured from 25 children in Colorado and California with limb , also called acute flaccid myelitis.
The viruses were genetically very similar, the scientists found, sharing certain mutations that resemble those found in the poliovirus genome.
The researchers concluded that the viruses were a novel strain of enterovirus 68, which they called B1. Using a method called “molecular clock analysis,” the team estimated that the B1 strain emerged four and half years ago.
“I don’t think it’s coincidental that it’s around the time the first cases were described,” said Dr. Charles Chiu, the study’s senior author and an associate professor of laboratory medicine at .
Only one child had evidence of the infection in his blood, the study found.
That 6-year-old had been taken to Children’s Hospital Los Angeles complaining of trouble moving his left leg. Dr. Grace M. Aldrovandi, the chief of at the hospital, ordered tests to see if enterovirus 68 could be found in his stool or nasal passages.
The resident who carried out her orders also sampled the boy’s blood, which was faintly positive for the infection. That is significant because poliovirus also enters the bloodstream before invading the central nervous system.
“We were fortunate to detect it,” Dr. Chiu said, adding that usually, “we are diagnosing these cases after the fact, when the sample you want is one taken when they start developing symptoms.”
Dr. Chiu and his colleagues also examined 14 samples of spinal fluid from children with paralysis using next generation sequencing in which millions of DNA strands can be sequenced at the same time and then assembled. The team found no evidence of enterovirus or any other potential infectious cause. “We didn’t find a virus, bacteria, fungus or parasite,” Dr. Chiu said.
On balance, he said, that strengthens the case that the B1 strain of enterovirus — detected in roughly half of the children’s nasal secretions — was linked to their paralysis.
Nine of the 25 children in the study were admitted to Children’s Hospital Colorado from Nov. 24, 2013, to Oct. 11, 2014. Others were seen at Children’s Hospital Los Angeles or were identified as early as June 2012 by the California Department of Health.
The new study also suggests that not every child infected with the new strain will develop paralysis.
One sibling pair — a school-age girl and her younger brother — were both infected with identical B1 strains of enterovirus 68 and got , Dr. Chiu and his colleagues found. The girl suffered paralysis in both arms and her trunk. Her brother experienced no lasting effects.
Dr. Chiu hopes to grow blood cells from the two siblings in culture and infect the cells with enterovirus 68 to “see if there are differences between his cells and her cells,” he said.
Enterovirus 68 may be a contributor to the children’s paralysis, said Priya Duggal, the director of the genetic epidemiology program at Johns Hopkins Bloomberg School of Public Health, who had nothing to do with the study. “But it must not be acting alone, because children with the same virus and siblings with the same clade have different outcomes.”
For another study, Dr. Duggal is enrolling children with paralysis and their siblings to compare their genes to see if there are telling variations.
Recovery for many of these children has been difficult and uneven. One woman, Marie, who asked to be identified by her middle name to protect her family’s privacy, said that six months after her 4-year-old son started experiencing weakness in his arms and legs, he can grab something with both hands or hop awkwardly.
Still, his right shoulder and upper arm are withered and limp, and his neck is crooked, Marie said.
“I can’t emphasize how scary this is,” Dr. Aldrovandi said. “You have a normal child, and then all of sudden they are pretty incapacitated.”