While polio has been eradicated across much of the earth, other viruses from the same family continue to circulate. Many infections are mild or even asymptomatic, but occasionally the viruses can cause the paralysis associated with polio. Over the past 18 months, Stanford University researchers have identified 20 possible cases in California.
The cases have prompted a slew of media coverage, but the Centers for Disease Control and Prevention (CDC) has played down fears. So far the Stanford team has confirmed 5 cases of acute flaccid paralysis (AFP)--the symptoms seen in polio cases--and suspects up to 20 children may have been affected over the past 18 months. While expressing concern for the families involved, the Stanford researchers and CDC have sought to reassure the public that California is not facing an epidemic of a polio-like illness.
As AFP is not a notifiable disease in the U.S., it is unclear how many cases occur typically. However, countries that perform regular surveillance find there is one case per 100,000 children aged under 15 years old every year. By applying this rate to California--which has 8 million kids in the age demographic--CDC predicts the state could suffer 80 cases of AFP a year. "Within that context, the number of cases we're aware of is not a cause for public concern," CDC viral disease expert Jane Seward told The Washington Post.
All 5 of the children with confirmed cases of AFP were vaccinated against polio. The cause of the cases is unclear, but the Stanford researchers suspect a virus is playing a role. A rare enterovirus was found in two of the children, but unlike its cousin polio the virus is not associated with causing paralysis. However, viruses can cause surprising symptoms when they interact with their host atypically. There is a long history of this happening but researchers still don't fully understand such rare events.
"If you think about a condition like chickenpox, it used to infect basically every single child in the U.S. With 4 million cases a year a hundred kids died. Why did those kids die? It's really hard to explain," Seward told Scientific American.