Web EpiMonitor

CDC released its latest report on the 2018 outbreak of Acute Flaccid Myelitis (AFM) in mid-November in the Morbidity and Mortality Weekly Report describing 80 confirmed cases so far this year. This number represents a threefold increase over the 33 confirmed cases in 2017, but a number below the cases reported in 2014 and 2016. 

The US has experienced AFM outbreaks every two years since 2014 between August and October:

Year                Cases

2014                120

2015                22

2016                149

2017                33

2018                80*
*(An updated report not included in the MMWR article reveals 116 cases from 31 states as of late November.)

Beginnings

The following report was compiled from the MMWR article as well as other documents made available by CDC, including a transcript of a telebriefing given by Nancy Messonnier, Director of the National Center for Immunizations and Respiratory Diseases.

The 120 cases in 2014 were reported in conjunction with a national outbreak of severe respiratory illness caused by enterovirus-D68. Since enteroviruses are known to cause paralysis, this virus has been a prime suspect. However, viral isolates from cerebrospinal fluid which can be considered etiologic have been limited since only 4 viral isolates have been made from the 404 confirmed cases since 2014. These were coxsackievirus A16, enterovirus A71, and enterovirus D68.

Findings

Since 2014, more than 90% of the cases have had a mild respiratory illness or fever consistent with a viral infection before they developed AFM. About half of the cases with respiratory or stool specimens tested in 2018 were positive for enterovirus or rhinovirus. However, the significance of the positive findings in respiratory specimens is unclear. Because many viruses circulate at the same time of the year, the viruses found will be temporally even if not causally associated with AFM. While the evidence to date suggests a viral association, as noted by CDC, “the absence of a pathogen in most AFM cases means we haven’t found the definitive cause yet.”

Why No Pathogen

And the reasons for failing to find a pathogen may be because the virus has been eliminated by the time the specimens are taken at or near onset of paralysis, or the pathogen is difficult to detect, or testing for the right pathogen is not being carried out. All the stool specimens since 2014 have tested negative for poliovirus.

Broader Thinking

Also, the cases in 2018 have not been associated with an increase in EVD-68 or EVA-71 so CDC is broadening its thinking to consider not only specific viral etiologies but also to investigate what is triggering AFM in a small number of infected persons. It could be that one of the viruses detected so far or another yet to be found is triggering an autoimmune process. In that case, the implicated virus would be an indirect rather than a direct cause of AFM.

Hypotheses

Given the wide distribution of AFM cases in multiple states, a toxic cause of AFM is not considered likely but has not been ruled out, according to Messonnier. Vaccines have been mentioned on social media as a possible cause and are going to be investigated but they have not been implicated and CDC continues to recommend that all children receive their vaccinations.

Risk

AFM is considered a rare condition with CDC estimating that less than 1-2 in a million children will get AFM every year. Since virus circulation is common during the months of the outbreaks and many children are presumably being infected, what is most mysterious is why some children are developing AFM while others presumably infected are not. In short, there is intense focus on what this missing trigger might be.

Descriptive Epidemiology from MMWR 11/16/18

   ♦  Most cases are in children with a median age of 4 years but ranging from 7 months to 32 years.

   ♦  All confirmed cases have been hospitalized but no deaths reported

   ♦  Cases have been reported from 25 states.

   ♦  Males accounted for 59% of cases.

   ♦  Whites accounted for 86% of cases with available race information.

   ♦  Most cases (47.5%) had only upper limb involvement but 28.8% had both upper and lower limbs affected. At least half of the cases don’t recover.  ■

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