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.

Case breakdown by country

NA: Onset of paralysis in most recent case is prior to 2017. Figures exclude non-AFP sources. In 2018, cVDPV includes all three serotypes 1, 2 and 3.
For Somalia: 1 cVDPV2 and cVDPV3 isolated from one AFP case.
cVDPV definition: see document “Reporting and classification of vaccine-derived polioviruses” at [pdf].

Polio this week as of 26 February 2020

• Do you wish to become a member of the Polio Transition Independent Monitoring Board? If so, you are invited to send in your application by 11 March 2020. Details of role requirements can be found here.
• To know more about the state of poliovirus surveillance and the opportunities explored to enhance surveillance in priority areas, take a look at the newly released 2019 Global Polio Surveillance Status Report.
• Get up to date on the latest news, funding updates and cases numbers with the latest edition of Polio News.
• Summary of new viruses this week (AFP cases and environmental samples): 
  • • Afghanistan: one WPV1 positive environmental sample
    • Pakistan: two WPV1 cases and 14 WPV1 positive environmental samples
    • Central African Republic: one cVDPV2 positive environmental sample
    • Benin: one cVDPV2 case
    • Ghana: six cVDPV2 cases and two cVDPV2 positive environmental samples
    • Togo: two cVDPV2 cases