Causes of AFM

AFM can be caused by viruses, including enteroviruses.

Evidence that viruses cause AFM

CDC has been tracking AFM since 2014. This has contributed important evidence to support a role for viruses, including enteroviruses, in AFM.

Most patients had respiratory symptoms or fever

  • More than 90% of patients with AFM had a mild respiratory symptoms  or fever consistent with a viral infection before they developed AFM.
  • Respiratory symptoms and fever from viral infections such as from enteroviruses are common, especially in children, and most people recover.
  • CDC continues to work toward learning why a small number of people develop AFM after having a virus, including if
    • A virus directly infects motor neurons (nerves that make the muscles move)
    • A virus leads to an inflammatory or immune response directed toward motor neurons
    • Genetic factors cause certain patients to be more susceptible than others.

Detecting the causes of AFM

CDC has tested many different specimens from AFM patients for a wide range of pathogens (germs) that can cause AFM.

  • We have detected coxsackievirus A16, EV-A71, and EV-D68 in the spinal fluid of a small number of patients with AFM, which points to the cause of their AFM. For all other patients, no pathogen (germ) has been detected in spinal fluid to confirm a cause.
  • Studies have shown that AFM patients had antibodies specific for enteroviruses in their spinal fluid more often than persons without AFM. Having antibodies for enterovirus means that a person was previously infected with the virus.
  • All the stool specimens from AFM patients that we received tested negative for poliovirus. This means that the cases of AFM since 2014 are not caused by poliovirus.

Often, despite extensive testing of AFM patients, no pathogens are found in the spinal fluid. This may be because the body has cleared the pathogen, or it is hiding in tissues that make it difficult to detect. Another possibility is that the pathogen triggers an immune response in the body that causes damage to the spinal cord.

Increase in cases between August and November

  • Most patients developed AFM between August and November, with increases in AFM cases in 2014, 2016, and 2018.
  • At this same time of year, many viruses, including enteroviruses, commonly circulate in the U.S.
  • The 2014 AFM outbreak coincided with a national outbreak of severe respiratory illness caused by EV-D68. Smaller EV-D68 outbreaks have also been detected in 2016 and 2018.
  • CDC is working with national partners to understand the annual circulation of enteroviruses, including EV-D68, and their association with AFM.
Page last reviewed: December 29, 2020