No Increase in Acute Flaccid Myelitis Despite Rise in Enterovirus Infections

Increases in enterovirus infections in recent years were not associated with a rise in confirmed cases of acute flaccid myelitis (AFM), a rare polio-like neurologic condition, according to a CDC report.

A total of 172 confirmed AFM cases were reported to the CDC from January 2020 through December 2025, with annual case totals ranging from a low of 17 in 2025 to a high of 48 in 2022. That range is below the numbers seen in 2014, 2016, and 2018, when annual confirmed cases ranged from 120 to 238 cases per year.

AFM has been associated with enteroviruses, particularly enterovirus D68 (EV-D68). Data on pediatric acute respiratory illnesses from the New Vaccine Surveillance Network showed a seasonal spike in EV-D68 infections in 2018 that corresponded with a spike in confirmed AFM cases.

But subsequent EV-D68 case spikes that triggered CDC alarms weren’t matched by AFM case spikes in 2022, 2024, and 2025. Only one AFM patient in 2025 had EV-D68 detected in a respiratory specimen, reported Adriana Lopez, MHS, of the CDC’s National Center for Immunization and Respiratory Diseases, and colleagues in the Morbidity and Mortality Weekly Report.

Despite the recent disconnect between EV-D68 infections and AFM cases, clinicians must remain vigilant, Lopez and team cautioned.

“Given the unexpected EV-D68 increase in 2025, clinicians should remain alert for patients suspected of having AFM and report these cases to their local or state health department,” they wrote. “Obtaining polio vaccination status, collecting stool specimens from patients suspected of having AFM, and retaining high polio vaccination levels are critical for detecting and maintaining the elimination of polio in the United States.”

Clinically and radiologically indistinguishable from polio’s acute flaccid paralysis, AFM is characterized by acute onset of flaccid weakness in one or more limbs, as well as spinal cord gray matter abnormalities.

The reason for fewer AFM cases despite rising EV-D68 infections isn’t clear, the authors noted. One hypothesis: Newer circulating EV-D68 strains in 2022, 2024, and 2025 may have been less neurovirulent. Recent research in mice showed predominant EV-D68 strains lacked key proteins tied to neurovirulence.

About 75% of confirmed AFM cases during 2020-2025 were in patients younger than 18, with median ages ranging from 7 years in 2022 to 12 years in 2023. History of respiratory illness or fever fluctuated by year. While 72% to 79% of patients with confirmed AFM had those symptoms in 2022, 2024, and 2025, only 42% to 62% of patients did in 2020, 2021, and 2023. Nearly all patients (98% to 100%) during 2020-2025 were hospitalized, but no patients died in that period.

Up to half of the 172 AFM cases in 2020-2025 had confirmed enterovirus or rhinovirus infections, with positive rates ranging from 26% in 2023 to 55% in 2024.

Collecting stool specimens from AFM patients “is critical to rule out polio,” Lopez and team wrote. Only about half of patients with confirmed AFM cases in 2020-2025 had such specimens collected and tested for polio, a percentage that fell short of the 80% rate recommended for AFM surveillance. However, 75% to 100% of confirmed-AFM patients had received at least three polio vaccine doses and thus were considered to be at low risk for the condition.

Study limitations included unknown polio vaccination status in some patients, which could have led to underestimates of vaccination percentages. Suspected AFM cases are passively reported to the CDC, which could lead to underestimation of cases.