As the fall 2018 viral season ended, there was hope that the national outbreak of acute flaccid myelitis (AFM) in children would follow historic patterns and fade out as well.
“Most patients [have] had onset of AFM between August and October, with increases in AFM cases every two years since 2014,” the Centers for Disease Control and Prevention reports.1 “At this same time of year, many viruses commonly circulate, including enteroviruses, and will be temporally associated with AFM.”
As of Dec. 3, 2018, there were 134 confirmed cases of AFM in 33 states. Another 165 possible cases were under investigation.
In a recent report2 on 80 of the confirmed cases, the CDC detected some viruses previously linked to the condition (i.e., enterovirus) but not in sufficient numbers to explain the outbreak.
Most AFM cases were children between two and eight years old, with an upper limb afflicted in roughly half of the cases.
“CDC has tested 125 spinal cord fluid respiratory and stool specimens from 71 of the 80 confirmed AFM cases,” the agency reported. “Of the respiratory and stool specimens tested, about half were positive for enterovirus or rhinovirus, including EVA-71 and EVD-68. The spinal cord fluid was positive in two cases. One had evidence of EVA-71, and one had evidence of EVD-68.”
AFM emerged in 2014 during a large national outbreak of EVD-68 and has peaked and ebbed in alternative years every fall since then.
“Most of these patients have a fever and/or a respiratory infection three to 10 days before the onset of limb weakness,” said Nancy Messonnier, MD, director of the CDC National Center for Immunization and Respiratory Diseases, said at a recent press conference.
“And fever and respiratory infections are certainly classic symptoms of respiratory viral disease.”
It is entirely possible at this point that AFM may fade back again without the CDC finding the “unifying diagnosis” that has proved elusive.
“It may be one of the viruses that we’ve already detected,” she said.
“It may be a virus that we haven’t yet detected. Or it could be that the virus is actually triggering an autoimmune process. Those are all hypotheses we’re looking closely at.”
It is not as simple as concluding AFM is caused by different respiratory viruses, though that could end up being part of the answer.
“Certainly, finding a pathogen in the spinal fluid points to that pathogen as causing the disease,” Messonnier said.
“But other fluids — for example, nasal pharyngeal swabs — can harbor lots of pathogens even in children that are mostly healthy. We would find other things that they’re carrying that aren’t making them sick.”
On the other hand, despite extensive testing, no pathogens have been found in the spinal fluid of many AFM cases. “This may be because the pathogen has been cleared by the body or it is hiding in tissues that make it difficult to detect,” she said.
“Another possibility is that the pathogen triggers an immune response in the body that causes damage to the spinal cord.”
- CDC. AFM investigation. Dec. 3, 2018. Available at: https://bit.ly/2f1r2X2.
- McKay SL, Lee AD, Lopez AS, et al. Increase in Acute Flaccid Myelitis — United States, 2018. MMWR 2018;67:1273–1275.