An international collaboration between doctors may have uncovered the cause of Acute Flaccid Myelitis (AFM), a relative of polio that leads to muscle weakness and paralysis and primarily affects children. The research, published in The Lancet in February, led doctors to a previously identified airborne virus called Enterovirus D68 (EV-D68), which had previously been thought of as relatively harmless.

For almost four years now, doctors have been perplexed by medical cases in which healthy people, primarily children, were developing an illness similar to polio, which led to paralysis. Cases of paralysis that result from viral infections have been almost eradicated since the success of the polio vaccine, with only a few other conditions, such as West Nile virus, causing the same effects and even then, only rarely. Then, in 2014, doctors started reporting cases of acute, rapid onset of muscle weakness, paralysis, and spinal cord damage in paediatric patients without clear cause.

The condition, known as Acute Flaccid Myelitis, spread quickly, with 176 confirmed cases in 2014 across the UK and 34 US states. By 2016, AFM had spread to 39 US states, with 149 cases being reported that year. In October of 2016, it was reported that a six year old boy with AFM had died as a result of the condition.

The only link that doctors were able to determine between the patients was that some of them had been infected with EV-D68 shortly before falling ill. The virus, first discovered in the 1960s, had been implicated in very few medical cases before the mid-2000s, and was generally considered to be a very mild infection. However, researchers noticed that in 2014 and 2016, years that both saw an influx of paralysis cases, the US had large outbreaks of the virus that roughly correlated with the appearance of AFM patients. In contrast, 2015 had almost no cases of EV-D68 infections and correspondingly low occurrence of AFM.

While this evidence indicated that EV-D68 could be linked to AFM, there was reason to doubt the connection. Importantly, there were a number of cases of AFM where the patient had never been found to have EV-D68, suggesting that the condition could develop independently of the virus. Doctors were also unsure as to whether the virus was capable of travelling to the spinal cord to inflict the damage they had observed in patients.

Nonetheless, the doctors behind this most recent work believe that there is sufficient evidence to conclude that the virus is causative of AFM. According to the team, the data they have collected from over the previous 4 years is able to fulfil the Bradford Hill criteria, the checklist that is used to determine whether or not a pathogen is a cause for a specific condition.

Further, some members of the team have been able to demonstrate that a strain of the virus collected from AFM patients is capable of causing spinal cord lesions in mice, leading to paralysis. Their research also found that the strain of EV-D68 they were using shared many more characteristics with the poliovirus than previous strains had, indicating why AFM might not previously have developed in EV-D68 patients.

The researchers behind this study acknowledge that more work needs to be done to ensure that the link they have identified is accurate, but they believe that this could be the insight needed into AFM to help better treat patients. At present, cases of AFM have now been confirmed in 14 countries across 6 continents, meaning that doctors are keen to develop a deeper understanding of the condition. Disease monitoring agencies, including the US CDC, have been less willing to officially recognise the link between the virus and the disease, however, until more research becomes available.