De Guzman and Daniel’s father, Jose Ramirez, brought him to Seattle Children’s Hospital, where his condition at first baffled physicians there, his mother said.

“They had no clue what it was,” said De Guzman, 33. “At first they thought it was a bacteria, so they were giving him antibiotics. That didn’t work, and they figured out it was a virus. They were giving him pain medicine, blood pressure medicine and steroids to help with swelling in his brain.”

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With local news reports describing other children in Washington state as having a polio-like condition called acute flaccid myelitis, or AFM, De Guzman said, “I asked the doctor if it’s possible he could have this, because a lot of the symptoms sounded the same.”

As his condition worsened,Daniel was given a tentative diagnosis of AFM.

“They said they could try to put him in an induced coma to let his body fight the virus,” De Guzman said. “All Daniel wanted to do when he got this was to rest, so we chose the coma.”

Despite the efforts of some 30 doctors treating him, Daniel died Oct. 30.

Daniel is one of nine children tentatively diagnosed in recent weeks with AFM across Washington state, and the first person known to have died of the mysterious disease.

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The Centers for Disease Control and Prevention began tracking the disease in 2014, when 120 cases were confirmed between August and December. The condition mimics polio but is not caused by the polio virus. It affects primarily children, causing sudden loss of control of the muscles in limbs and other parts of the body. It’s not clear what causes the disease, and no medical treatment has proved effective.

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This year, reports of new cases in the United States doubled in September compared with August, the CDC reported Monday, with the year-to-date total of 89 cases in 33 states on track to match or exceed the 120 cases confirmed in all of 2014.

Locally clustered cases of AFM have been reported in Philadelphia and, for the first time, in Europe. The first cluster of European cases of the polio-like condition was described in British news reports last month, involving four cases being treated at the Royal Hospital for Sick Children in Edinburgh, Scotland. Two cases of AFM have also been reported in Canada.

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In Philadelphia, nine children were treated for AFM in August and September, said Sarah Hopkins, a pediatric neurologist at Children’s Hospital of Philadelphia. Although the average age of children treated during the 2014 outbreak was 7, the average age of the nine children seen this year by Hopkins has been closer to 5.

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Most of the children treated in 2014 have made a modest recovery, she added, although none is back to their original strength.

The CDC defines AFM as the sudden onset of flaccid paralysis, in which muscles controlling the limbs, face or other body parts become droopy (not stiff), and in which an MRI exam shows damage to the gray matter of the spinal cord. The condition is often preceded by a fever or severe headache. Although some cases involve only a mild paralysis of a single limb, others affect the muscles that control breathing, and patients require mechanical ventilation to survive.

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As the number of cases rises this year, researchers at the CDC say they are more mystified than ever by what is causing the condition.

“It’s a complete puzzle,” said Steve Oberste, chief of the CDC’s Polio and Picornavirus Laboratory Branch. “What’s going on now is really odd. I hope we can assure the public that people are working on it, not just the CDC but many others. There are so many things about this outbreak that are puzzling, and the pieces aren’t fitting together yet. Eventually we’ll know more, but right now it’s pretty difficult.”

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During the 2014 outbreak, a growing line of evidence suggested that the condition may be a rare but serious complication of infection with enterovirus D68. That year, more than 1,000 children across the country were treated for serious respiratory illnesses linked to D68, which is in the same viral family as the polio virus.

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The problem with that hypothesis was that D68 was never found in the spinal fluid of infected children. Even so, many doctors remain convinced by studies showing a strong geographic and chronological association between D68 and AFM, and by the fact that D68 is related to polio.

This year, however, the case for D68 has weakened, because no similar outbreak of respiratory illnesses associated with D68 or any other enterovirus has been seen in the United States, leaving researchers desperately searching for answers. (Both children affected by AFM in Canada, however, tested positive for exposure to D68.)

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“What we saw in 2014 was a temporal relationship between D68 and AFM,” said Manisha Patel, who leads 10 researchers working full time at the CDC investigating the AFM outbreak. “This year, we don’t have that. So we are not ruling anything out. We’re looking at not just viral causes but environmental causes. Yes, enteroviruses are biologically plausible, but there are also other things that could cause this.”

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Some of the apparent rise in cases, Patel said, may be due to increased surveillance by the CDC and rising awareness among physicians and the public. And despite the higher numbers, she emphasized, the condition remains extremely rare.

In the two years and three months since the CDC began tracking the condition, she said, “less than one in a million people have developed AFM. It’s a very serious illness that CDC is taking very, very seriously. But what remains consistent is that AFM is still rare.”

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For all the CDC’s efforts, some doctors think the agency still is not doing enough and should make it mandatory for doctors to report all new cases of AFM and other cases of sudden paralysis to health authorities.

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“A mandatory reporting for non-traumatic paralytic disease should be urgently considered nationwide,” said Benjamin M. Greenberg, a neurologist at the University of Texas Southwestern Medical Center in Dallas, who has treated some of the children afflicted by the disease. Mandatory reporting, he said, would pick up paralysis due not only to AFM but also to Zika, West Nile, Guillain-Barré and a variety of other paralyzing conditions.

“We need to be proactive, not reactive,” he said.

After losing her child, De Guzman said, “we’re still fighting to figure out what happened to Daniel and to warn people about this. He wouldn’t want this to happen to any other kid.”

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As of Thursday morning, a GoFundMe page set up to help pay for Daniel’s medical care had raised $14,317 from 295 people.

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