Just sixty years ago polio was one of the most dreaded childhood diseases in the U.S. Vaccination campaigns effectively stamped out domestic cases of the disease, with the last cases of naturally occurring paralytic polio in the U.S. in 1979. But news that a small number of children have developed polio-like symptoms in California has fueled instant public interest and concern. Keith Van Haren, a pediatric neurologist at Stanford University, said in a report released February 23 that five children between August 2012 and July 2013 had developed paralysis reminiscent of polio. 
The children had previously been vaccinated against polio virus. And although the children were afflicted with paralysis and severe weakness, physicians have concluded the children do not have polio. Still, none of the children fully recovered limb function after six months. The jury is still out on exactly what caused their condition. Van Haren’s findings will be presented at the American Academy of Neurology’s annual conference in April. Physicians and public health officials have submitted 20 reports to the California Department of Public Health of similar cases. Thus far, the CDPH has not identified any common causes that suggest the cases are linked.
Polio virus is part of a larger family of enteroviruses, and the different types each carry a small degree of paralysis risk. Two of the children did test positive for one type of rare enterovirus, enterovirus 68. More common enteroviruses, however, are associated with respiratory conditions including pneumonia. Scientific American spoke with CDC's Deputy Director of the Division of Viral Diseases Jane Seward to get further insights.
[An edited transcript of the interview follows.]
These children did not test positive for polio, but two of the children did test positive for a different kind of enterovirus, enterovirus 68. What’s your theory for what’s going on here?
Acute flaccid paralysis, that’s acute paralysis of parts of the body–in this case the limbs, can result from a variety of viruses including polio virus and non-polio enteroviruses including enterovirus 68, West Nile Disease, echovirus and adenoviruses. Most people who get enteroviruses have mild symptoms and no testing is ever done on them. I think we’re looking at a rare outcome in these children.
Are we seeing these symptoms in places other than California?
Acute flaccid paralysis is not a nationally notifiable disease in this country so we’re really not able to assess the significance of this number of cases. Our understanding is that what happened here is these cases came from people that came in at first for testing of polio virus, or at least some of them did. That’s great. We don’t want physicians to forget about polio virus and they need to keep testing for it and be alert for it because polio could come from parts of the world where polio is endemic.
In countries that are performing regular surveillance there is at least one case of acute flaccid paralysis per year for every 100,000 people under 15 years old. We don’t have that same system in place here but if we did the same numbers would apply. We would expect at least 80 cases of acute flaccid paralysis--at least--from California since they have a population of children under 15 of eight million. So, again, if we were conducting surveillance for acute flaccid paralysis we would expect at least 80 cases.
So then are you saying this is expected or we should not be concerned?
These researchers only report on five cases in the abstract, two of which tested positive for enterovirus 68. Based on that we are not unduly alarmed. We are in touch with California but not have been in touch with the researchers at Stanford that put the abstract together.
Have there been other cases where enterovirus 68 causes paralysis?
CDC tracks enteroviruses that circulate around the country using a passive reporting system from labs that happen to test for those viruses. That just gives us no idea about the total numbers. It just tells us which viruses are circulating from year to year. We have had at least one reported case [of paralysis] as you can see in a Morbidity and Mortality Weekly Report from 2006. This is just the absolute tip of the iceberg of what may be occurring around the country though, because the reports don’t reflect the total numbers in the country. We don’t have total surveillance. We are aware of 47 cases of enterovirus 68 since 2000. Those are just the ones we know have been isolated in laboratories and most were respiratory disease.
Other than polio, would enterovirus 68 or any other enteroviruses be more likely to cause paralysis?
No, they are not commonly known to cause paralysis.
The most important thing to know is that enteroviruses  are common and most people don’t get very sick from them. It’s only rare that these cause severe illness. With any case of paralysis a clinician should consider if it’s polio virus and request testing for that.
So what would spark an enterovirus to cause paralysis when that’s such a rare symptom?
It’s probably a combination of how the host interacts with the virus. If you think about a condition like chicken pox it used to infect basically every single child in the U.S. With four million cases a year a hundred kids died. Why did those kids die? It’s really hard to explain. It’s an interaction of the virus and the host. Either genetically they didn’t cope with the virus as well or they got an unusually high dose of the virus, it’s hard to say.
How do we test for enteroviruses to identify them?
It’s usually on the stool or through cerebral spinal fluid testing. It’s better to get it out of the CSF because these viruses are common so if you pick up the virus from a nonsterile site like the throat or stool that may not tell you the child has it since healthy carriers could still shed the virus.
How are enteroviruses other than polio transmitted? Is this an issue where you spread the virus through poor hygiene after interacting with feces?
Definitely a lot of them are transmitted via respiratory secretions, especially this one–enterovirus 68, which is commonly associated with respiratory disease.
So what can people do to protect themselves from this unidentified illness?

The best way to protect yourself is if you have any respiratory symptoms to practice cough etiquette and cover your cough, or your sneeze. You should wash your hands often and stay home if you’re sick.