Although sporadic cases of non-polio infantile paralysis have been reported in the medical literature in the past, it was not until 2012 in California and 2014 in Colorado when unusual but significant clusters of pediatric patients with a polio-like illness started rising quickly in the USA. This prompted the CDC to begin collecting surveillance data on the disease we now call AFM. Between August 2014 and June 2020, there have been 626 confirmed cases across the USA. AFM cases have also been reported in multiple regions worldwide including Canada, Europe, Asia, Australia, and South America.
Thanks to the epidemiological surveillance efforts, it was noted that this disease usually follows a particular seasonal biennial pattern, with notable peaks of cases between August and October every 2 years (2014, 2016, 2018, and maybe 2020!).
Over 90% of cases of AFM occur in children (median age of 6 years), and no racial or ethnic predispositions have been clearly identified. A slight predisposition to males has been reported.
^ Confirmed AFM cases by CDC from August 2014 through June 5, 2020, with onset of condition by March 31, 2020. Case counts are subject to change. The CDC updates the case count on a regular basis, please click here to see the latest numbers: https://www.cdc.gov/acute-flaccid-myelitis/cases-in-us.html
We are still learning about what causes this unusual illness, and how is it causing the paralysis.
There is evidence that supports a connection between the recent outbreaks of AFM with non-polio enterovirus infections as potential causative agents. The most commonly associated virus is enterovirus D68, but other enteroviruses like enterovirus A71, and some coxsackie virus strains have also been reported.
First, individuals affected by AFM usually experience a febrile respiratory illness a few days prior to the onset of weakness. Enteroviruses are known for typically causing a respiratory infection. Second, there seems to be a temporal and geographic correlation between enterovirus D68 circulation and the outbreaks of AFM. Third, enterovirus D68 is the most commonly identified pathogen in respiratory or stool specimens in patients with AFM across many regions, and it is hypothesized that new strains of the virus may have acquired the ability to cause the flaccid paralysis.
Although this reinforces the idea of a virus as a driving agent of the disease, the mechanism by which the virus might cause injury to the spinal cord is not well understood. Direct neuronal infection and indirect immunological response have been proposed as potential explanations.
Finally, in many patients with AFM, no viruses are detected in routine samples. This could be explained by shortcomings of the tests themselves, but also by the possibility of other viruses that we still have not identified.