The CDC received reports of 33 confirmed cases of AFM across 16 states in 2017, down from 149 confirmed cases in 39 states in 2016. In total, there have been 386 confirmed cases since 2014. She said the cases were definitely not caused by the polio virus, which has not been found in any of the stool samples from affected children. The Chinese have developed a vaccine against enterovirus A71, but it is not available in the US, he adds.
Other possible causes, the CDC says, could be the West Nile virus, which is transmitted by mosquitoes, and non-polio enteroviruses, which can develop into a serious condition for infants and those with compromised immune systems.
"We are actually looking at everything". It causes hand, foot and mouth disease.
According to the CDC, there's been a mild uptick in the number of AFM cases in the country since 2014. Between January and August 2018, there were fewer than five cases reported, according to the Public Health Agency of Canada, which they say is within the "normal range" of the disease.
While AFM is not unique to the US, Messonnier said, "no one else has seen seasonal clustering every other year".
The Department of Public Health has confirmed the second MA case this year of a rare but potentially devastating illness that strikes mostly children and causes muscle weakness or paralysis.
Officials said they will be conducting additional analysis on this year's cases.
WATCH: Six children from Minnesota have been diagnosed with a rare condition that causes weakness or even paralysis in the arms and legs.
Depending on which part of the spine is damaged, different muscles can become weak or paralyzed. The disease attacks the nervous system, weakening the body's muscles and reflexes. If it affects the diaphragm and disrupts breathing, patients may be put on a ventilator. Most people recover quickly from a polio infection, but it can cause fever and stiffness. The average age of those afflicted is 4, and 90 percent of those with AFM are 18 or younger. It's also a serious condition.
Dr. Mary Anne Jackson, a pediatric infectious disease specialist and interim dean of the school of medicine at the University of Missouri-Kansas City, said numerous patients she saw were healthy children before falling ill with the disease.
Although symptoms may appear similar to polio, AFM is not polio and it is not contagious.
Nancy Messonnier, a doctor at the CDC, called it a "mystery illness". With the sudden onset of weakness, patients are "generally seeking medical care" and being evaluated by neurologists, infectious disease doctors and pediatricians.
Some children with AFM recover completely, but others are disabled for years.
Kerkering said AFM is most common in children. Rehabilitation therapy can help patients regain function.
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