A rare and mysterious illness capable of causing paralysis in young children is growing in the United States, and doctors say the clock is ticking until it strikes again.

Acute flaccid myelitis (AFM) is a condition that affects the nervous system, specifically the gray matter of the spinal cord. It can weaken and ultimately paralyze the limbs of the body by damaging the cells in the spine that process signals related to motor control and movement.

According to the most recent statistics from the Centers for Disease Control and Prevention (CDC), published this month, 233 confirmed cases of AFM occurred in the United States in 2018 — the most ever reported since the CDC began surveillance of the disease in 2014.

AFM first appeared on the radar in a big way in 2014 with 120 confirmed cases across 34 states.

Since then, the illness has followed a biannual trend — spiking every two years — with the overall number of cases growing with each outbreak as well as the number of states affected.

Doctors believe that the next major outbreak is set to occur in 2020, and they want to be ready for it.

There is a problem, however: Although AFM can be diagnosed, it’s still unclear what actually causes it.

A search for the cause of AFM

The leading candidate, according to experts in the field, is known as enterovirus-D68 (EV-D68), although the link between the virus and AFM hasn’t been conclusively established.

“That will be the one that most folks expect might be the culprit, but we don’t know yet for sure,” said Dr. William Schaffner, an infectious disease expert at Vanderbilt University Medical Center in Tennessee.

“We’re kind of where we were years ago with Legionnaires’ disease: For the longest time we could define the illness, but we didn’t have the cause. That turned out to take a little bit longer before the laboratory gave us that answer,” he added.

The presentation and severity of AFM also has researchers looking at another deadly virus, one already eradicated in the United States: poliovirus. In fact, read anything about AFM and chances are you’ll see it described as a “polio-like” illness.

Polio is a highly infectious neurologic disease transmitted by the poliovirus that can cause limb weakness and paralysis.

Before the advent of the polio vaccine in 1955, polio was responsible for more than 15,000 cases of paralysis annually in the United States alone.

Given the similarities between polio and AFM, researchers looked to see if the poliovirus was the culprit. It wasn’t.

The data remains inconclusive, but further research should uncover whether EV-D68 is the real culprit.

“The reality is that at this time we don’t have a way to determine which children may be at an elevated risk for developing AFM, or the underlying explanation that predisposes them to develop AFM in the first place. The search for a biomarker from the blood or cerebrospinal fluid would be helpful to identify those at elevated risk,” said Dr. Robert Glatter, an emergency physician at Lenox Hill Hospital, NYC.

Who’s at risk?

The risk of AFM remains very low, but there is still important information that parents should be aware of.

AFM can affect people of all ages, but it occurs most frequently among children, similar to polio. According to the CDC, among those confirmed to have contracted AFM in 2018, the median age was around 5 years old.

Also just like polio, EV-D68 is an intestinal (enteric) virus that is primarily spread through fecal bacteria coming in contact with the mouth. So good hygiene, including frequent handwashing, is a practical measure for keeping you and your family healthy.

“Young children are less hygienic than older persons, and these enteric germs obviously spread more readily among children because they spread it easily among each other,” said Schaffner.

What are the signs of AFM?

Symptoms of AFM may not be immediately obvious, and can resemble a mild respiratory infection at first. More severe symptoms, which include muscle weakness, facial drooping, difficulty swallowing, and slurred speech, need to be taken seriously. AFM can result in paralysis, but the severity of its effects may differ from one individual to the next.

“The long-term effects of AFM are unclear at this time. While some patients have recovered rapidly, others have remained paralyzed and require a high level of ongoing care to support their breathing and monitor for neurological deterioration,” said Glatter.

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There is no specific treatment for AFM, but basic preventative measures should be taken by parents and children alike.

“It’s important to adhere to standard precautions and steps to reduce the risk of developing AFM by staying up-to-date on vaccinations, washing your hands thoroughly, and taking precautions to reduce the risk of mosquito bites,” said Glatter.

This article first appeared on HealthLine.com.