Outbreaks of an infectious, polio-like disease have popped up every other summer in the U.S. since 2012. This year, the viral illness would have been expected to surge yet again—but the widespread measures taken to slow the spread of the coronavirus may also prevent large spikes of the paralyzing condition it can cause, known as acute flaccid myelitis.
In 2012, doctors in California came across a small group of children who were suddenly stricken with paralysis of their arms and legs, along with nerve damage to their spinal cord. Their symptoms resembled what used to happen to tens of thousands of children every year in the U.S. during the mid-20th century, back when the poliovirus was a routine childhood menace. But it couldn’t have been polio, because that virus is now nearly eradicated from the world thanks to widespread vaccination and has been extinct in the U.S. since 1979.
The handful of cases in California were an early warning. Two years later, doctors reported more than a hundred children across the country who developed acute flaccid myelitis, or AFM (flaccid describing the weakness of the affected muscles, and myelitis meaning inflammation of the spinal cord). Every two years since, there’s been a spike in children getting AFM during the late summer months of August to October—a spike that’s gotten bigger over time. In 2014, according to the Centers for Disease Control and Prevention, there were 120 total confirmed cases of AFM in people under 22; by 2018, that number was 238.
Doctors and researchers have long known that other medical conditions and viruses besides polio, like the mosquito-borne West Nile, can cause AFM, but never in the reported numbers that they were now seeing. And they were quick to suspect that a close cousin of the poliovirus, called EV-D68, was the primary cause of these recent spikes of AFM.
Both EV-D68 and polio are enteroviruses, a broad group of viruses that either infect us through fecal-oral transmission, as with polio, or from breathing it in, as with EV-D68. Most children with AFM had symptoms of a typical respiratory infection before their paralysis began, the kind of common cold that EV-D68 causes.
Public health agencies like the CDC have suspected that EV-D68 causes AFM since 2014. But for several years, the CDC’s wording was more cautious about a causative link between the two, to the frustration of some scientists. One reason for that hesitance is that it’s been hard to find intact traces of the virus in the spinal cords of AFM patients, which would provide direct evidence of infection. However, following the 2018 wave of AFM, the CDC’s position is now more definitive.
“CDC believes that enteroviruses, particularly EV-D68, are likely responsible for the peaks in AFM cases that have occurred every two years since 2014,” Janell Routh, lead of the CDC’s AFM and Domestic Polio Team as well as a member of the CDC’s National Center for Immunizations and Respiratory Diseases, told Gizmodo via email. Routh cited five years of clinical, epidemiological, and lab data collected by the CDC and other researchers.
There are still lots of important questions about AFM left to be solved. We don’t know exactly why these cases have spiked since 2014, for one. EV-D68 was discovered in the 1960s but was always considered a mild cold bug. It’s likely that something about the virus’s genetics has changed in recent years to make some strains more dangerous to a child’s nervous system. EV-D68 is probably also not the only culprit behind these spikes—another enterovirus called A-71 was linked to a cluster of children in Colorado developing neurological symptoms, including AFM, in 2018. And more relevant right now, we don’t know how 2020 will shake out for AFM in the U.S., thanks to the current pandemic.
The novel coronavirus has infected at least 11 million people and killed over a half million worldwide as of early July. But these numbers aren’t the worst-case scenario, since governments enacted or encouraged restrictions on people coming into close contact with one another in an effort to slow down its spread. By some estimates, these measures may have prevented millions of deaths.
Covid-19 isn’t the only infectious disease spread by close contact, though. There’s already evidence that the lockdowns in the U.S. are dramatically limiting the spread of common viruses like the respiratory syncytial virus, relative to the baseline seen in past years. In other countries, including South Korea, where enteroviruses like EV-D68 spread at different times of the year than the U.S., there’s also been evidence of reduced transmission.
Logically, you would expect the same to happen for EV-D68 this summer and AFM in turn. But there are a few important wrinkles to consider. One is that lockdowns in the U.S. are being lifted at varying speeds, even as some states continue to experience troubling increases in new covid-19 cases. Right now, many states are planning to reopen their elementary schools this fall. By late summer and fall, when the virus is most active, there’s no telling just how much distancing people will be practicing, especially the children who are vulnerable to AFM.
“This year really depends on how strictly the measures are put into place and if people comply with them. But there is a good chance, based on the data that’s been analyzed and just thinking theoretically, that we could social-distance away an EV-D68 outbreak this year, which would be fantastic,” Kevin Messacar, a pediatric infectious disease physician and researcher at Children’s Hospital Colorado and the University of Colorado, told Gizmodo. Messacar was also one of the first doctors to study the emergence of AFM when it started spiking in 2014.
This reprieve may not come without a cost, though. Researchers like Messacar strongly suspect that the seasonal, every-other-year spikes of EV-D68 and similar enteroviruses are driven by immunity, or rather, the lack of immunity in young children.
Put simply, enteroviruses like EV-D68 need lots of fresh bodies to spread widely, which then cause spikes of disease. Once it’s exhausted its readily available supply of non-immune people, the virus retreats back into the shadows, only causing a low level of infection in the general population. It then takes about two years for enough people without immunity to be born that it can again erupt into large outbreaks of illness. Enteroviruses also seem to prefer the warmer climate of the summer and fall, explaining their seasonality during those months.
Cases of EV-D68 and AFM still happen during off-years, just not to the extent they do in peak years. So if children aren’t catching EV-D68 as much this summer due to social distancing, that doesn’t mean the virus disappears. Whenever the conditions for it to spread widely in the community arrive again, it should show up, and maybe with greater force than usual.
“With all of these respiratory viruses, you have a bunch of people who haven’t gotten infected in a while,” Messacar said. “So you potentially could have even a larger susceptible group in upcoming seasons. And nobody really knows what would happen in that scenario.”
Messacar still thinks that a 2020 summer without EV-D68 is ultimately for the best, even considering what could happen down the road. An extra off-year is more time for doctors and researchers to better understand the virus and how it causes AFM. It’s also more time to find and develop promising treatments that can improve children’s prognosis or even a preventative vaccine that can hopefully protect kids from getting AFM at all.
“With AFM, it’s a very prolonged, if not lifelong, debilitating condition. So any case we can avoid is a success,” he said. “You’re essentially buying time if you put off an outbreak, which is good news.”
AFM can be fatal, especially if the paralysis reaches the lungs. Roughly 70 percent of cases still have lingering symptoms like limb weakness or permanent paralysis a year after diagnosis. But what has changed for the better in recent years is that doctors are more aware of the condition and able to find it earlier in affected children. The earlier that supportive treatments like physical therapy start, the less severe any long-term complications tend to be.
The importance of early detection means it’s crucial for doctors and health agencies to be able to track any potential clusters of EV-D68 and AFM that may arise, even during a pandemic. For their part, the CDC says it has increased its capacity to respond to outbreaks of AFM, including providing educational programs for physicians so they can better recognize it.
So far, Messacar hasn’t received the sort of phone calls he typically does every two years from doctors newly encountering AFM in their hospitals or offices. But it’s still too early to know whether it will take a break this year. Whatever ends up happening with AFM this summer, it’s another reminder that covid-19 isn’t the only disease we have to worry about out there.