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 19 February 2020

• “Polio is one of the few diseases we can eradicate in the world in the next few years. This is a priority of the United Nations and I am extremely happy to see it is a clear priority for the Government of Pakistan.” – António Guterres, United Nations Secretary General during his recent visit to Pakistan. Read more…
• Summary of new viruses this week (AFP cases and environmental samples): 
  • Afghanistan: two cVDPV2 positive environmental samples
  • Pakistan: five WPV1 cases and six WPV1 positive environmental samples
  • Angola: one cVDPV2 case
  • Côte d’Ivoire: six cVDPV2 positive environmental samples
  • Ethiopia: one cVDPV2 case
  • Ghana: two cVDPV2 cases
  • Philippines: one cVDPV2 positive environmental sample