A Polio-Like 'Mystery Illness' Is on the Rise, and Health Officials Are Concerned. Here's What You Should Know About Acute Flaccid Myelitis

Cases have increased since 2014, mostly among children.

The U.S. Centers for Disease Control and Prevention (CDC) has received an unusually large number of reports of polio-like illnesses in people—mostly children—across the United States in recent months. In 2018, 228 cases of the rare disease, called acute flaccid myelitis (AFM), were confirmed in 41 states, with an additional 150 reports under investigation. There have been 25 more reports in 2019, including four confirmed cases, according to the CDC.

So what is acute flaccid myelitis? AFM is a serious condition that affects the nervous system, causing sudden weakness, and sometimes pain or paralysis, in the arms and legs. It can also cause drooping eyelids, slurred speech, and difficulty swallowing. News reports and home videos of children afflicted with these symptoms have circulated online, adding to the country’s growing concern.

“We know this can be frightening for parents,” said Nancy Messonnier, MD, director of the CDC’s National Center for Immunization and Respiratory Diseases, said in a briefing in October. “I know many parents want to know what the signs and symptoms are that they should be looking for in their child.” Here are a few takeaways about AFM, including what to look for and how worried experts really are.

RELATED: How Long Does It Take for the Flu Shot to Be Effective?

Acute flaccid myelitis has been on the rise since 2014

Dr. Messonnier said that the CDC has been testing for and monitoring reports of AFM since 2014, when an increase in cases was first noted. Since that time, most confirmed AFM cases have been among children.

Among AFM patients confirmed in 2018, the average age is 4. More than 90% of recent cases have been in patients 18 and younger. Often, patients experience a mild illness—which can include a fever, headache, and stiff neck—before developing a sudden onset of muscle weakness.

Most AFM cases occur in the fall

Based on previous years’ reporting, AFM strikes most patients in late summer and fall, and 2018 was no different: The number of cases reported in October was "a substantially larger number than in previous months this year,” Dr. Messonnier said, due to an increase in reports of patients whose symptoms started in August and September.

That doesn’t mean that we’re in the midst of an unprecedented outbreak, however. “The number of cases reported in this time period in 2018 is similar to what was reported in the fall of 2014 and 2016,” Dr. Messonnier said. It’s not unheard of for a number of cases to be reported all at once, either: In 2016, eight children were diagnosed with the condition in Washington State within the same week. The number of reported cases has declined so far in 2019, according to the CDC, although it's still too early to know how this year will ultimately measure up.

RELATED: Can You Get the Flu From the Flu Shot? A Survey Shows Many Parents Think So, but Here's What an Expert Says

The cause is unknown, but it’s definitely not polio

The CDC has tested stool samples from every confirmed AFM patient, and none has tested positive for poliovirus. Other viruses (like enterovirus, rhinovirus, and West Nile virus) and environmental toxins can cause AFM, but so far the CDC has not been able to identify a cause for most of the cases recently diagnosed. “[I]f you are having the peaks of disease every late summer and early fall, you would think we are finding a single agent,” Dr. Messionnier explained. “That is what we are not finding.”

Dr. Messonnier also said that she's frustrated “that despite all of our efforts we haven’t been able to identify the cause of this mystery illness." The CDC is continuing to investigate these illnesses, she added, to better understand its risk factors and possible causes of the increase in recent years.

Since the October briefing, the CDC has begun to suspect a virus is causing AFM. "Clinical, laboratory, and epidemiologic evidence to date suggest a viral association," according to a November 2018 Morbidity and Mortality Weekly Report.

Some patients recover quickly, while others have long-term damage

“We don’t fully understand the long-term consequences of AFM,” Dr. Messonnier said. “We know that some patients diagnosed with AFM have recovered quickly and some continue to have paralysis and require ongoing care.” In the most serious cases of AFM, patients can experience respiratory failure when the muscles that affect breathing become weakened. In 2017, one child died from AFM.

There is no specific treatment for AFM, but doctors may prescribe immunosuppressant drugs, steroids, and blood-replacement procedures. They may also recommend physical or occupational therapy to help patients get their strength and mobility back after experiencing muscle weakness or paralysis.

RELATED: 7 Conditions That Feel Like the Flu—but Aren't

Parents shouldn't freak out—but they should take basic precautions

“Parents need to know that AFM is very rare, even with the increase in cases that we are seeing now,” said Dr. Messonnier. (Overall, since 2014, the risk of developing the illness is less than one in a million.) Nevertheless, she added, “we recommend seeking medical care right away if you or your child develop sudden weakness of the arms or legs.”

Until more is understood about what’s causing these cases, Dr. Messonnier said in October that parents can help protect their children from pathogens by washing their hands and staying up to date on vaccines. "As a mom, I know what it’s like to be scared for your child, and I understand parents want answers," she said. "CDC is a science-driven agency. Right now the science doesn’t give us an answer. That’s why we at CDC, along with all our partners, will keep looking for answers."

To get our top stories delivered to your inbox, sign up for the Healthy Living newsletter

This post was originally published on October 17, 2018 and has been updated for accuracy.