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Immunization Branch

Acute Flaccid Myelitis Q&A

What is AFM?

AFM is a rare neurological condition involving sudden weakness or paralysis of limbs caused by damage to the spinal cord. AFM most often affects children and is typically preceded by a fever and respiratory illness.

In 2012, California was the first state to recognize the condition that is now known as AFM and the California Department of Public Health (CDPH) has conducted surveillance for AFM ever since that time. Nationally, there was an increase in cases in 2014, and the Centers for Disease Control and Prevention (CDC) requested that all states report suspect cases. Although cases have occurred every year since 2012, increases in cases have been noted every two years, including 2016 and 2018.

What is the cause of AFM?

According to the CDC, because AFM is a complex condition, it is difficult to determine why some people develop AFM. CDC has tested many different specimens from AFM patients for a wide range of pathogens (germs) that can cause AFM. CDC has detected certain enteroviruses, which typically cause mild illness, including coxsackievirus A16, enterovirus A72 (EV-A71), and enterovirus D68 (EV-D68) in the fluid that surrounds the spinal cord of four AFM cases out of 596 confirmed cases since 2014, which points to the cause of their AFM. For all other patients, no pathogen has been detected in their spinal fluid.

Most patients had onset of AFM between August and October. At this same time of year, many viruses commonly circulate, and CDC, CDPH, and other experts on AFM are investigating the role that pathogens, including enteroviruses, play with AFM. The large number of AFM cases identified in 2014 and 2016 coincided with a national outbreak of severe respiratory illness among people caused by EV-D68, and some experts think that EV-D68 might have an important role with AFM.

Why are children most often affected?

While the cause of AFM has not yet been definitively established, if AFM is caused by an enterovirus infection, it would be expected that most cases would be in children. This is because enteroviruses are common and spread via the respiratory and fecal-oral routes, typically causing asymptomatic or mild illness.

Once a person has been infected with a particular enterovirus strain, they are immune to that strain. Since most people are infected with enteroviruses as children, fewer adults are susceptible. For a particular enterovirus strain to be spread widely there must be enough susceptible people to sustain transmission.

Why is this happening now?

If a viral etiology, such as EV-D68, is responsible for the increases in cases of AFM, changes may have occurred in the virus that have increased neurovirulence. It's also possible that EV-D68 may not have been circulating widely until recently, and that when more people were infected, the rare complication of AFM was detected. Scientists continue to research the causes of AFM.

If enteroviruses are common, why is AFM rare?

There are more than 50 different types of enteroviruses, and most enterovirus infections are mild or asymptomatic. There is a possibility that certain host factors, including genetic or immune factors, may be associated with development of AFM. If we look back at poliovirus, which occurred before poliovirus vaccines were introduced, most infections were asymptomatic and fewer than 1% of infections were associated with paralytic disease. More research needs to be done to identify potential host factors that increase risk of AFM.

Do children with AFM recover?

Recovery varies. Some children fully recover, others regain some use of their paralyzed limb(s) and others remain paralyzed. In severe cases, the muscles used for breathing may be paralyzed, and such patients require the use of a ventilator. Patients with severe paralysis are at increased risk for additional complications such as pneumonia, respiratory failure, and death. Research is needed regarding the best immediate and long-term treatment of AFM patients.

What is CDPH doing about AFM?

CDPH investigates every suspected AFM patient reported by health departments and conducts enhanced surveillance to better understand the potential causes, optimal treatment, and outcomes of AFM. Enhanced surveillance includes viral testing at CDPH Viral and Rickettsial Diseases Laboratory (VRDL) to identify causes of AFM. Additionally, CDPH continues working with CDC, the Council of State and Territorial Epidemiologists (CSTE) and other states to learn more about AFM.

For more information on AFM, please visit:

CDC Acute Flaccid Myelitis page
CDPH Acute Flaccid Myelitis Page

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