Last fall, the United States experienced a nationwide outbreak of enterovirus D68. It caused respiratory illness in 1,153 people, most of whom were children, and left 115 paralyzed despite being a non-polio enterovirus. How could this happen? A new study has found the virus that infected these kids was a mutated form, making it resemble polio more than other common strains.

Typical symptoms of enterovirus D68 infections include fever, runny noses and sneezing, body aches, and sometimes wheezing and difficulty breathing. But since August, as more children appeared in hospitals across 49 states and the District of Columbia with polio-like paralysis in their arms and legs, called acute flaccid myelitis, health care providers started to suspect the enterovirus was responsible.

Now, the study from the University of California, San Francisco, has found that the virus could indeed be the reason some of these kids became paralyzed. Researchers analyzed the genetic sequences of the virus from 25 children who developed paralysis, 16 of whom were from California and nine of whom were from Colorado.

They found that all of them had a strain of the virus called B1, which was first discovered about four years ago. This strain, they said, had mutations making it resemble the poliovirus as well as the nerve-damaging enterovirus D70. While they found it in both respiratory fluid and blood, they didn’t find it in cerebrospinal fluid — a finding consistent with other nerve-damaging viruses — which suggested “the neurological symptoms are coming from an aberrant immune response to recent EV-D68 infection, and not because the virus is directly invading neurons,” said Dr. Charles Chiu, an associate professor or laboratory medicine at UCSF.

What’s more, of two siblings who became ill with identical forms of the virus, only one experienced paralysis. “This suggests that it’s not only the virus, but also the patients’ individual biology that determines what disease they may present with,” Chiu said. “Given that none of the children have fully recovered, we urgently need to continue investigating this new strain of EV-D68 and its potential to cause acute flaccid myelitis.”

Source: Greninger A, Naccache S, Messacar K, et al. A novel outbreak enterovirus D68 strain associated with acute flaccid myelitis cases in the USA (2012–14): a retrospective cohort study. The Lancet Infectious Diseases. 2015.