A mystery illness that causes polio-like systems has been spreading across the US and around the world. Known as acute flacid myelitis (AFM), the neurological illness causes weakness in limbs, paralysis, and severe respiratory symptoms. First seen in California, 2014 saw the biggest outbreak yet, with clusters occurring across the US, Canada, and Europe. Now, researchers think they know what caused it.

Before 2012, the illness was considered very rare, with perhaps one case presenting every five years. In 2014, more than 120 children presented with the illness in the US alone, across 49 states. 

"The three cases since August really look like each other. They have severe arm flaccidity and no mental status changes," a case report in Neurology Today from 2014 reads. "All of them have similar spine MRIs showing gray matter involvement. You could lay all three MRIs on top of each other and they look almost the same. It's pretty striking."

Neurologists described the illness as a polio-like syndrome; a "disease indistinguishable in most respects from traditional polio, in terms of its symptoms,” one case study read.

"Almost all of the patients have an increase in their white blood cells in the cerebrospinal fluid. Some of the patients have brainstem findings and cranial-nerve findings. This is all the same as what polio does. None of us has ever seen anything like this before, with few exceptions." 

Now researchers have concluded that the polio-like illness is most likely caused by a virus that was also doing the rounds at the time, publishing their study in infectious disease journal Eurosurveillance.

The researchers from the University of New South Wales (UNSW), Australia, suspect Enterovirus D68, which usually causes runny noses, coughs, muscle aches, fever, and difficulty breathing, as a possible cause for the disease.

During the AFM outbreak of 2014, there were 2,280 reported cases of Enterovirus D68 in the same regions. The team at UNSW analyzed the literature on AFM and Enterovirus D68, and applied the Bradford Hill criteria to it, the same scientific method that was used to prove that smoking causes cancer.

“The scientific method Bradford Hill used to prove that smoking caused cancer is now an accepted tool to determine causality,” team leader Professor Raina MacIntyre said in a statement.

“We are first to use this approach to analyse the relationship between EV-D68 and acute flaccid myelitis. Our results show that it is very likely that EV-D68 is the cause of the mystery illness and the paralysis of children."

The virus was previously suggested as a cause for AFM but had not been shown to cause paralysis before. The team says that the link between the virus and AFM needs to be acknowledged so that governments can focus on strategies that prevent the spread of the virus when it occurs.

“The incidence of EV-D68 infections is increasing worldwide, and a genetically distinct strain has recently evolved," said MacIntyre. "There is no treatment or vaccine for the polio-like illness caused by EV-D68, which makes it important to act quickly to stop outbreaks.”

The team recommended strategies similar to preventing the spread of common colds, including washing your hands with soap and water for 20 seconds.