Enterovirus D68. The new polio?

News from CDC is that there have been more cases of a strange paralytic syndrome in children in which limbs go limp following what appears to be an upper respiratory illness (http://emergency.cdc.gov/han/han00370.asp). Some, but not all, of these children have tested positive for Enterovirus D68 (EV-D68). The cluster announced yesterday is in Colorado and involves 9 children. Earlier this year California reported twenty cases of the same illness that had occurred over a 2-year period (http://www.nytimes.com/2014/02/26/health/unknown-form-of-paralysis-strikes-california-children.html?_r=0).

So, has polio returned in a new guise?

I am not old enough to remember polio. I remember receiving the oral vaccine and seeing old black and white newsreels of children in iron lungs, but no one I knew had contracted the disease. Even during my training I didn’t see a case. Although the disease is believed to have been around since Greco-Roman times, it wasn’t until the late 19^th and early 20^th centuries that humanity was plagued by epidemics.  Like EV-D68, polio belongs to the enterovirus family and is transmitted person to person via the fecal-oral route.  Believe it or not the epidemics of last century are attributed to improved hygiene. With less exposure to the virus during infancy there was a decline in immunity that resulted in a more severe illness later in life when exposed. Most polio infections (95%) are inapparent, meaning they don’t result in any symptoms or paralysis, yet the person is capable of transmitting the virus. Among those with symptoms only about 1% get paralysis. To any individual the risk was low, however, on a population scale this was disastrous. During the peak years there were about 20,000 cases of paralytic polio per year in the US. After vaccine was introduced in the 1950s there was a dramatic decline and polio in the United States was all but forgotten.

I was speaking to a public health historian the other day. She told me that when she was in graduate school she was told that infectious diseases had been conquered and that it was pointless to study them. Chronic disease was the new frontier.  Short sighted to say the least. Ebola, Legionnaire’s disease, HIV, Hantavirus, SARS, Nipah and MERS have all emerged since this professor was told to forget about the field. West Nile virus moved into a new hemisphere, we’ve had an influenza pandemic and Chikungunya exploded in the Caribbean. Forget what you were told, infectious diseases will be with us as long as there still is a planet earth.

Consider this. Your 4-digit PIN number has 10,000 possibilities. There are ten options for the first digit, i.e., zero through nine. Since one can repeat numbers there are also ten options for the next three digits. That’s 10x10x10x10, or 10,000.  A computer program can run through every possible PIN number very quickly. Supposing a thief had a program that could submit the guessed PINs to your account. It wouldn’t take long before he or she was staring at your account balance.

Now consider the ever-changing world of nature. Instead of PIN numbers we are talking about the genetic code, large sequences of DNA or RNA. Each location is like the digit of a PIN number. Nature is like a huge supercomputer, changing the genetic code constantly. The change here is a mutation and instead of allowing access to your bank account the mutation of concern is one that improves the organism’s chance of survival or increases virulence. Millions and millions of mutations don’t, but by chance alone, sooner or later, a mutation is going to result in is a more virulent organism or one that can cross species. This is why there will always be new infectious diseases on the horizon. EV-D68 is a RNA virus. RNA viruses are more prone to mutation (influenza for example). But has EV-D68 somehow mutated to become a paralytic virus like its cousin polio? We don’t know yet. Right now the occurrence of paralysis is quite rare.