CDC officials added that while they have not seen any geographical clustering, they have seen seasonal clustering, with most cases occurring in the late summer and fall, dating back to when the CDC first noticed an uptick in the illness in 2014.
"We understand that people, particularly parents, are concerned", said Dr. Nancy Messonnier, director for the CDC's National Center for Immunization and Respiratory Diseases, during a teleconference call with reporters. Parents should seek medical care immediately if their child develops sudden weakness or loss of muscle tone in the arms and legs.
The cause of most of the AFM incidents is unknown, according to the CDC, as are the long-term effects.
Doctors say that most of the reported cases of AFM involve those under the age of 18. Another spike came in 2016. The U.S. has had 386 confirmed cases since August 2014.
The agency has not yet confirmed whether the New Hampshire patient, a juvenile from Rockingham County, contracted AFM or if it is another illness, said Jake Leon, a spokesman for the state Department of Health and Human Services. But Messonnier cautioned that it would be "premature" to be confident that this year will be the same as the earlier years. CDC experts say the overall rate of AFM is about one in a one million.
Some patients recover quickly, while others experience paralysis and require ongoing care.
In most cases, the patient's arms or legs become weak, similar to the results of infection with polio.
The condition remains very rare.
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Messonnier said that officials have not been able to find the cause of the illness. None have tested positive for poliovirus.
In 2014, a large AFM epidemic coincided with a national outbreak of severe respiratory illness caused by enterovirus D68 (EV-D68), but in-depth testing of patient samples hasn't consistently found a common cause. "For some of the previous cases we've identified one pathogen or another, but we have no unifying diagnosis".
There is no treatment specifically for AFM, but affected children can undergo physical and occupational therapy to maximize their strength and adapt to their limitations.
Benjamin Greenberg, a neurologist who has treated children with AFM at the University of Texas Southwestern in Dallas, said AFM is "exquisitely rare".
The CDC encourages people to prevent the disease by staying up to date on vaccines, washing hands and protecting against mosquito bites. Some children paralyzed by AFM have eventually regained their ability to walk, but need time.
So far, the CDC hasn't seen any geographic patterns based on reporting from states.
The following year, there were 22 confirmed cases in 17 states, and 2016 saw 149 cases in 39 jurisdictions, including D.C. In 2017 there were 33 confirmed cases in 16 states. So far, it's been reported in 22 states.