This year, more than half of all U.S. states have had confirmed or possible cases of acute flaccid myelitis, the polio-like illness that can cause paralysis and mostly affects children, according to an exclusive CNN analysis.
The Centers for Disease Control and Prevention (CDC) has confirmed cases in 22 states overall.
Messonnier added that health officials are considering other potential causes, such as West Nile virus and environmental toxins, though none of the cases have involved them.
Since the condition was first recognized by CDC in 2014, the agency has confirmed 362 cases.
More than 90 percent of cases are in children.
"For some previous cases, we have identified one pathogen or another, but we have no unifying diagnosis", said Dr. Nancy Messonnier of the National Center for Immunization and Respiratory Diseases. Although the cause remains a mystery in the majority of cases, the 2014 jump coincided with "a national outbreak of severe respiratory illness among people caused by enterovirus D68", though it wasn't found in all patients, according to the CDC.
In addition to viruses, potential causes may include environmental toxins and genetic disorders, according to the CDC, and it "can be hard to diagnose because it shares numerous same symptoms as other neurologic diseases". But officials haven't been able to find a single agent that would explain the clusters of cases that occur around the same time.
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"This is a mystery so far", Messonnier said, describing AFM as a "pretty dramatic disease", which preys on a child's nervous system.
Symptoms include arm and leg weakness, loss of muscle tone and reflexes, in addition to facial drooping or weakness, difficulty moving the eyes, drooping eyelids, difficulty with swallowing or slurred speech. Fifteen states said they'd confirmed cases this year.
Some patients recover quickly, while others experience paralysis and require ongoing care. But mysteriously no other country has reported the emerging every-two-years pattern seen in the U.S., Messonnier said.
There have been no confirmed cases of AFM in Maine this year, according to Emily Spencer, spokesperson for the Maine Department of Health and Human Services and the Maine CDC. Though AFM has not claimed any lives this year, there was one death in 2017.
For example, the CDC doesn't know who may be at higher risk for developing AFM or why some are at higher risk, she said.
More broadly, she noted, "there is a lot we don't know about AFM".
Because officials don't know the cause of AFM, they can't recommend a specific way to prevent it. States are not required to provide this information to CDC, but they have been voluntarily reporting their data. Working with local and state health departments and hospitals, the CDC has been able to confirm a number of these cases faster, she said.