Poliovirus - Origin and Serotypes

Origin and Serotypes

Poliovirus is structurally similar to other human enteroviruses (coxsackieviruses and echoviruses), as well as to human rhinoviruses, which also use immunoglobulin-like molecules to recognize and enter host cells. Phylogenetic analysis of the RNA and protein sequences of poliovirus (PV) suggests that PV may have evolved from a C-cluster coxsackie A virus ancestor, that arose through a mutation within the capsid. The distinct speciation of poliovirus probably occurred as a result of change in cellular receptor specificity from intercellular adhesion molecule-1 (ICAM-1), used by C-cluster coxsackie A viruses, to CD155; leading to a change in pathogenicity, and allowing the virus to infect nervous tissue.

The mutation rate in the virus is relatively high even for an RNA virus with a synonymous substitution rate of 1.0 x 10−2 substitutions/site/year and non synonymous substitution rate of 3.0 x 10−4 substitutions/site/year. Base distribution within the genome is non random with adenosine being less common than expected at the 5' end and higher at the 3' end. Codon use is non random with codons ending in adenosine being favoured and those ending in cytosine or guanine being avoided. Codon use differs between the three genotypes and appears to be driven by mutation rather than selection.

There are three serotypes of poliovirus, PV1, PV2, and PV3; each with a slightly different capsid protein. Capsid proteins define cellular receptor specificity and virus antigenicity. PV1 is the most common form encountered in nature, however all three forms are extremely infectious. Wild polioviruses can be found in two continents. As of 2012, PV1 is highly localized to regions in Pakistan and Afghanistan in Asia, and Nigeria and Chad in Africa. Wild poliovirus type 2 has probably been eradicated; it was last detected in October 1999 in Uttar Pradesh, India. Wild PV3 is found in parts of only two countries, Nigeria and Pakistan.

Specific strains of each serotype are used to prepare vaccines against polio. Inactive polio vaccine (IPV) is prepared by formalin inactivation of three wild, virulent reference strains, Mahoney or Brunenders (PV1), MEF-1/Lansing (PV2), and Saukett/Leon (PV3). Oral polio vaccine (OPV) contains live attenuated (weakened) strains of the three serotypes of poliovirus. Passaging the virus strains in monkey kidney epithelial cells introduces mutations in the viral IRES, and hinders (or attenuates) the ability of the virus to infect nervous tissue.

Polioviruses were formerly classified as a distinct species belonging to the genus Enterovirus in the family Picornaviridae. In 2008 the Poliovirus species was eliminated from the genus Enterovirus and the three serotypes were assigned to the species Human enterovirus C, in the genus Enterovirus in the family Picornaviridae. The type species of the genus Enterovirus was changed from Poliovirus to Human enterovirus C.