Vital Signs

Surveillance for Acute Flaccid Myelitis — United States, 2018

Adriana Lopez, MHS; Adria Lee, MPH; Angela Guo, MPH; Jennifer L. Konopka-Anstadt, PhD; Amie Nisler, MPH; Shannon L. Rogers, MS; Brian Emery; W. Allan Nix; Steven Oberste, PhD; Janell Routh, MD; Manisha Patel, MD


Morbidity and Mortality Weekly Report. 2019;68(27):608-614. 

In This Article

Abstract and Introduction


Background: Acute flaccid myelitis (AFM), a serious paralytic illness, was first recognized as a distinct condition in 2014, when cases were reported concurrent with a large U.S. outbreak of severe respiratory illness caused by enterovirus D-68 (EV-D68). Since 2014, nationwide outbreaks of AFM have occurred every 2 years in the United States; the cause for the recent change in the epidemiology of AFM in the United States, including the occurrence of outbreaks and a biennial periodicity since 2014, is under investigation. This report updates clinical, laboratory, and outcome data for cases reported to CDC during 2018.

Methods: Clinical data and specimens from persons in the United States who met the clinical criterion for AFM (acute onset of flaccid limb weakness) with onset in 2018 were submitted to CDC for classification of the illnesses as confirmed, probable, or non-AFM cases. Enterovirus/rhinovirus (EV/RV) testing was performed on available specimens from persons meeting the clinical criterion. Descriptive analyses, laboratory results, and indicators of early recognition and reporting are summarized.

Results: From January through December 2018, among 374 reported cases of AFM, 233 (62%) (from 41 states) were classified as confirmed, 26 (7%) as probable, and 115 (31%) as non-AFM cases. Median ages of patients with confirmed, probable, and non-AFM cases were 5.3, 2.9, and 8.8 years, respectively. Laboratory testing identified multiple EV/RV types, primarily in respiratory and stool specimens, in 44% of confirmed cases. Among confirmed cases, the interval from onset of limb weakness until specimen collection ranged from 2 to 7 days, depending on specimen type. Interval from onset of limb weakness until reporting to CDC during 2018 ranged from 18 to 36 days, with confirmed and probable cases reported earlier than non-AFM cases.

Conclusion: Identification of risk factors leading to outbreaks of AFM remains a public health priority. Prompt recognition of signs and symptoms, early specimen collection, and complete and rapid reporting will expedite public health investigations and research studies to elucidate the recent epidemiology of AFM and subsequently inform treatment and prevention recommendations.


Acute flaccid myelitis (AFM) was initially defined as a distinct entity in 2014 following reports of the occurrence of acute limb weakness in previously healthy children across the United States during an outbreak of severe respiratory disease caused by enterovirus D-68 (EV-D68).[1,2] AFM is a rare but serious illness for which there are no known treatments or means of prevention. It is a recognized complication of infections caused by West Nile Virus, adenovirus, and enteroviruses;[3,4] however, the more recent epidemiology of AFM, including the occurrence of outbreaks, its biennial periodicity since 2014, and the clustering of cases during the late summer and fall, has not been previously observed.

Neuroinvasive enteroviruses have been identified as causes of sporadic cases of AFM, including EV-D68 and EV-A71.[5–8] Extensive testing of AFM cases confirmed as part of national surveillance has detected multiple enteroviruses from sterile sites (i.e., cerebrospinal fluid [CSF] and serum) and nonsterile specimens (i.e., respiratory specimens and stool). Lack of a clear explanation for the emergence, in addition to the overall rarity of this condition, pose substantial challenges for identifying optimal treatment and prevention measures. Increased awareness of AFM by clinicians and timely reporting of persons with symptoms consistent with AFM to public health authorities are essential to identifying cases, improving patient management, and initiating public health investigations to further understand this condition.

This report summarizes and updates surveillance data for suspected cases of AFM reported to CDC,[9] with onset of flaccid limb weakness from January 1 through December 31, 2018. Data from 2018 were also compared with the previous peak of AFM in 2016 to identify opportunities to improve recognition and reporting.