Both EV-D68 and EV-A71 are enteroviruses, and they are in the same family of viruses as polio. Polio is notorious for causing neurological complications, including near complete paralysis but also limb weakness. Polio once caused regular epidemics until vaccination wiped it out across most of the world.
The other viruses such as EV-D68 and EV-A71 cause much milder disease or no symptoms at all. But in very rare cases, they can damage the nerves.
"We know that enteroviruses, especially EV-A71, have been associated with acute flaccid myelitis," Herlihy told NBC News.
EV-A71 is a common cause of hand, foot and mouth disease, an uncomfortable and highly contagious childhood illness marked by blisters or spots on the palms of the hands, soles of the feet, in the mouth and elsewhere.
In Colorado, EV-A71 has caused neurologic disease in at least 38 very young children, the health department said.
“This includes cases of meningitis, encephalitis, and acute flaccid myelitis (AFM) as well as children experiencing myoclonus (abnormal muscle jerking) and ataxia (loss of balance). Cases are all under age 3 years with no underlying medical conditions,” the state said in a health alert to doctors last June.
"Parents described their kids as dizzy, wobbly and having jerky movements," Herlihy said. Most of the children have recovered, she added.
Minnesota officials have not yet identified a particular virus in the six cases there. “All recent Minnesota cases have been in children under 10 years old and all were hospitalized. Cases have been reported from the Twin Cities, central Minnesota and northeastern Minnesota,” the state health department said in a statement.
Symptoms of acute flaccid myelitis include sudden onset of weakness in the arms or legs, as well as drooping facial muscles, including the eyelids, and difficulty moving the eyes. Most patients must be hospitalized.
“We don’t yet have any effective treatment for the virus or for the condition,” Messacar said.
“But we do know that over time, with rehabilitation therapies, many of the children can regain function.”
Affected muscles may remain weak, but it’s possible to strengthen surrounding muscles so kids regain movement, Messacar said. But a few still have partial paralysis and depending on which muscles are affected, children may need ventilators to help them breathe and may have to use wheelchairs.
One victim, 5-year-old Carter Roberts of Chesterfield, Virginia, died last month after developing AFM in 2016. He never recovered.
Messacar said it's important is for parents to seek immediate medical help if their child shows symptoms of weakness in the muscles of the face or throat. Acute flaccid myelitis is tricky to diagnose and may require tests of spinal fluid, as well as magnetic resonance imaging (MRI).
But there’s not much else that parents can do to protect their children. There’s no vaccine against EV-D68 or EV-A71, so the same precautions apply as for any other infectious disease: people should wash their hands frequently, stay home when they are sick, cover coughs and sneezes and stay away from people who are coughing and sneezing.
Almost any viral infection can cause neurological symptoms, although it’s extremely unusual, so the CDC says it is also important to be fully up to date on vaccinations.