Since August, 115 children in 34 states have developed limb paralysis (leg or arm) the cause of which is still largely a mystery. Now, doctors believe a new strain of a common respiratory virus may be responsible for the sudden lapses.

Most of the 100 children hospitalized with acute flaccid myelitis haven’t recovered from paralysis and weak limbs. CC BY-NC 2.0

After the first cases were reported, some researchers and doctors claimed the acute flaccid myelitis (AFM), as the sudden paralysis is called, may be due to  an enterovirus called EV-D68 since at the time U.S. emergency rooms experienced an unprecedented wave of children coming in with severe EV-D68. The paralyzed children, however, didn’t have any traces of the virus in their spinal chord fluid – where doctors expected to find evidence of B1, the name of the new virus strain. The fact that  enteroviruses are so common doesn’t help at all in pinpointing the root of AFM.

Scientists at the University of California, San Francisco (UCSF) believe however that they have found significant evidence linking B1 to AFM. The doctors drew nasal samples from 25 children and found 12 of them tested positive for the B1 strain. The scientists then genetically sequenced the virus and found it shared certain mutations that resemble those found in the poliovirus genome. Using a technique called molecular clock analysis, the team determined the strain must have mutated four and half years ago – around the time the first cases were also reported.  Only one child had the virus in his blood also, after doctors at the  Children’s Hospital Los Angeles carried out the tests. This is significant because poliovirus also enters the bloodstream before invading the central nervous system.

“We were fortunate to detect it,” said  Dr. Charles Chiu, the study’s senior author and an associate professor of laboratory medicine at University of California, San Francisco., adding that usually, “we are diagnosing these cases after the fact, when the sample you want is one taken when they start developing symptoms.”

Since the child was tested much earlier than other children who typically came in the hospital after symptoms were already fully developed, the researchers believe this is why they haven’t been able to find the virus in other blood samples. What’s interesting is that children infected with the new strain will not all develop paralysis. One case of note involves a pair of siblings, a boy and girl who both contracted the virus.  The girl suffered paralysis in both arms and her trunk, but her brother experienced no lasting effects. This suggests that genetics have a major role to play. Next, the doctors plan on isolating cell cultures from each sibling and infecting them with the EV-D68 new strain to see what happens.

Findings were reported in the Lancet Infectious Diseases.