Updated 3:55 p.m. ET
A rare condition causing weakness in the arms or legs — and sometimes paralysis — has been confirmed in 62 children so far this year, the Centers for Disease Control and Prevention said Tuesday.
One child has died of the condition, called acute flaccid myelitis, or AFM.
At least 65 more cases are under investigation, said Dr. Nancy Messonnier, director of the CDC's National Center for Immunization and Respiratory Diseases. So far, a common cause linking these illnesses has not been found.
"There is a lot we don't know about AFM," Messonnier said during a teleconference for reporters. "I am frustrated that despite all of our efforts, we haven't been able to identify the cause of this mystery illness."
The average age of the children is about 4, she said, and 90 percent of cases the CDC has been studying since 2014 have involved patients 18 or younger.
Messonnier said scientists don't fully understand the long-term consequences of the illness: "We know that some patients diagnosed with AFM have recovered quickly and some continue to have paralysis and require ongoing care."
Since the condition was first recognized by CDC in 2014, the agency has confirmed 386 cases through Oct. 16, mostly in children. AFM appears to be seasonal, occurring mostly in the late summer and fall, but appears in greater numbers every other year.
The number of cases in 2018 is on track to match a similar number of cases in 2014 and 2016. But Messonnier cautioned that it would be "premature" to be confident that this year will be the same as the earlier years.
It's possible that some milder cases haven't been reported by doctors to their state health department or the CDC, but Messonnier believes that number would be small.
"This is actually a pretty dramatic disease," she said. "These kids have a sudden onset of weakness and they are generally seeking medical care and being evaluated by neurologists, infectious disease doctors and their pediatricians and coming to public health awareness."
Possible causes being considered include viruses that affect the digestive system called enteroviruses, and possibly strains of rhinoviruses, which cause the common cold, she said. The CDC is also considering the possibility that environmental toxins could be triggering the sudden muscle weakness. And it is not ruling out possible genetic disorders.
Media reports in recent weeks have suggested that a "polio-like virus" might be triggering the condition, elevating fears that it might be polio itself.
"Right now, we know that poliovirus is not the cause of these AFM cases," Messonnier said.
She said that CDC has tested every stool specimen from AFM patients. None have tested positive for poliovirus. She also said West Nile virus hasn't been linked to any of these cases, either.
"As a parent myself I understand what it's like to be scared for your child," Messonnier said. "Parents need to know that AFM is very rare, even with the increase in cases that we are seeing now. We recommend seeking medical care right away if you or your child develop sudden weakness of the arms and legs."
Messonnier stressed the rarity of the condition, emphasizing that it happens in fewer than one in a million children in the U.S. So far this year, cases have been confirmed in 22 states, based on findings from MRI studies and the cluster of symptoms a child has.
The CDC says disease prevention steps should be followed, including staying up to date on vaccines, washing hands and using mosquito repellant.
NOEL KING, HOST:
All right. The CDC is investigating a mystery childhood illness that is on the rise. So far this year, the CDC says there are confirmed cases in 22 U.S. states. This disease can come on suddenly and cause muscle weakness and paralysis in arms and legs. NPR's Allison Aubrey has the story.
ALLISON AUBREY, BYLINE: The condition is known as acute flaccid myelitis, or AFM for short. It can start with symptoms such as droopy eyelids and difficulty swallowing. The CDC has confirmed 62 cases in the last few months, mostly in young children. Since the CDC began tracking the disease four years ago, a total of 386 cases have been confirmed. One case was a 7-year-old boy, Evan. His mother, Susan Coyne, says her son came down with an ear infection. This was back in the fall of 2014.
SUSAN COYNE: So I took him to the pediatrician, and she put him on an antibiotic.
AUBREY: But within a few days, Evan complained of severe pain in his back and legs. He became extremely lethargic and went downhill quickly.
COYNE: He lost total muscle control in both his arms and his legs. You know, one minute you have a healthy kid, and the next minute your kid is totally paralyzed. It was beyond horrifying to us.
AUBREY: It's not clear what caused Evan's sickness, and the CDC's Nancy Messonnier says, unfortunately the cause of this condition is still unknown.
NANCY MESSONNIER: I'm frustrated that despite all of our efforts, we haven't been able to identify the cause of this mystery illness. We don't know who may be at higher risk for developing AFM or the reasons why they may be at higher risk.
AUBREY: Messonnier says given the rise in cases, the CDC is escalating its response and consulting with a variety of experts to solve the mystery. They do know that some kids who get it have also had viruses such as enterovirus. Another possible risk factor is West Nile virus. But so far, there's no common pathogen that links all the cases.
MESSONNIER: Despite extensive laboratory testing, we have not determined what pathogen or immune response caused the arm or leg weakness and paralysis in most of these patients.
AUBREY: Given that the symptoms are similar to polio, a disease that has been almost completely eradicated, Messonnier pointed out that this is not polio.
MESSONNIER: Right now we know that polio virus is not the cause of these AFM cases. CDC has tested every stool specimen from the AFM patients. None of the specimens have tested positive for polio virus.
AUBREY: The CDC says despite the increase in cases, AFM is a very rare condition affecting fewer than one in a million children. But she says it's good to be aware of the signs and symptoms. Though 90 percent of those affected are children, there are some cases in adults.
MESSONNIER: We recommend seeking medical care right away if you or your child develop sudden weakness of the arms or legs.
AUBREY: And though the CDC can't give any specific information about what's causing AFM, Messonnier says it's good to take general precautions.
MESSONNIER: Parents can help protect their children from serious diseases by following prevention steps like washing their hands, being up to date on recommended immunizations and using insect repellent to prevent mosquito bites.
AUBREY: In the case of Susan Coyne's son, Evan, he's healthy again. He was treated with steroids, and he was in occupational and physical therapy for 18 months.
COYNE: Evan is fully recovered. He has regained all movement and strength in all four limbs.
AUBREY: But this is not always the case. Some kids who've been diagnosed with AFM continue to have paralysis and require ongoing care. Allison Aubrey, NPR News. Transcript provided by NPR, Copyright NPR.