Polio is caused by a human enterovirus called the poliovirus. Polio can interact in its host in two ways:
- Infection not including the central nervous system, which causes a minor illness with mild symptoms
- Infection including the central nervous system, which may cause paralysis
Less than 1% of poliovirus infections result in paralysis. The virus is most often spread by the faecal-oral route. Poliovirus enters through the mouth and multiplies in the intestine. Infected individuals shed poliovirus into the environment for several weeks, where it can spread rapidly through a community, especially in areas of poor sanitation.
The poliovirus consists of an RNA genome enclosed in a protein shell called a capsid. There are three serotypes of wild poliovirus type 1, type 2, and type 3 each with a slightly different capsid protein. Immunity to one serotype does not give confer immunity to the other two. Outbreaks of poliovirus were largely unknown prior to the 20th century. However, with improved sanitation in the 20th century, the average age at which individuals were exposed to poliovirus increased. With progressively declining protection from maternal antibodies, poliovirus infection increasingly resulted in paralysis.
Type 2 poliovirus was declared eradicated in September 2015, with the last virus detected in India in 1999. Type 3 wild poliovirus has not been detected anywhere in the world since November 2012.
Testing for polioviruses
All cases of acute flaccid paralysis (AFP) among children under fifteen years of age are reported and tested for wild poliovirus or vaccine-derived polioviruses within 48 hours of onset. AFP is caused by a range of factors. The Global Polio Laboratory Network tests upwards of 100,000 AFP samples a year, of which a very small portion are positive for poliovirus.
.The wild poliovirus as seen through a scan electron microscope. The virus invades the nervous system, causing paralysis in one out of every 200 children.