"I am frustrated that despite all of our efforts, we haven't been able to identify the cause of this mystery illness", said Dr. Nancy Messonnier, director of the National Center for Immunization and Respiratory Diseases at the Centers for Disease Control and Prevention.
In 2014 the CDC said there was a spike in the AFM virus, with 120 people afflicted with it from August to December of that year in 34 states.
The U.S. Centers for Disease Control is warning about a rare mysterious illness affecting American children, which can cause "polio-like" symptoms.
Fox 2/KPLR 11's Dan Gray talked to an infectious disease specialist who said the disease isn't new but should be considered a growing health concern.
Typical symptoms of AFM are similar to those of a severe respiratory illness, along with a fever, but those often progress into neurological symptoms.
"Despite extensive laboratory testing, we have not determined what pathogen or immune response caused the arm or leg weakness and paralysis in most patients", she said. Most of the cases have occurred in children.
Other countries have reported AFM cases, but not the same seasonal pattern the United States has experienced.
So far this year doctors say there are more than 100 confirmed or suspected cases of AFM in about two dozen states.
Among the cases under investigation are five reported to Maryland health officials in recent weeks, a health department spokeswoman said Tuesday. "We recommend seeking medical care right away if you or your child develop sudden weakness of the arms and legs". The average age of those afflicted is 4, and 90 percent of those with AFM are 18 or younger.
According to CNN, at least 30 states have cases that were confirmed, suspected or being investigated. However, the illness has been on the rise in the USA since 2014.
Lacking an established cause, health officials have confirmed cases through a review of brain scans and symptoms. Although it is too early to understand how the current season compares to previous ones, she noted, the nation is "on track with what was seen in 2014 and 2016" and will probably have the same number of cases. "So I think that should be reassuring to people", Dominguez said.
Parents have reported that the limbs of affected children appear lifeless. Officials said some patients diagnosed with AFM recover quickly, while others need ongoing care.
"[There's] nothing that provides the unifying diagnosis that we'd expect to explain these peaks of disease", Messonnier said.
Generally, most children with AFM see subtle improvements over time, but "most have long-term weakness", she said.
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