"We have not confirmed the cause for the majority of these cases", the agency said.
"Despite extensive laboratory testing, we have not determined what pathogen or immune response caused the arm or leg weakness and paralysis in most patients", Messonnier added.
"We have [had] two patients we have treated in CHEO for [acute flaccid paralysis] since the summer", said Dr. Sunita Venkateswaran, a pediatric neurogologist with the children's hospital. They've been seeing an increase in cases since 2014. "I know many parents want to know what the signs and symptoms are that they should be looking for in their child". Of these, 62 cases have been confirmed by the CDC in 22 states.
Although the symptoms of AFM have been described as "polio-like", polio has been ruled out as a cause in the USA cases, CDC said in a press conference.
"I am frustrated that despite all of our efforts we haven't been able to identify the cause of this mystery illness", said Messonnier. Brain Institute and at Children's Health told Dallas News.
The CDC is open to the possibility that it's not a virus that is causing the condition, but can not find any other plausible cause, either. A child may get a runny nose or a slight fever, and in most cases there would be no reason for parents to worry - until the tell-tale symptoms of a droopy eyelid, or weak arm, or trouble standing showed up.
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It's unclear what's causing the outbreak, which can develop after a viral infection, environmental toxins, and genetic disorders.
Most of the AFM cases that the CDC has studied have no indication of any particular infection.
Messonnier said other viruses besides EV-D68 have been found in patients with AFM. "That's something that we're still really learning about", Jones said.
"CDC is now providing the number of patients still under investigation so people can better anticipate increases in confirmed cases over the coming months", the agency said in a statement.
The CDC has been clear on one thing: none of the patients has tested positive for polio. Parents can also help protect their children by encouraging hand washing, staying up to date on recommended vaccines and using insect repellent to prevent mosquito bites.
The rare but serious condition affects people's nervous system, particularly the area of the spinal cord called gray matter.