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Abstract
Oral polio vaccine has been used successfully as a powerful tool to control the spread of wild polioviruses throughout the world; however, during replication in under immunized children, some vaccine viruses revert and acquire the neurovirulent phenotypic properties. In this study, we describe the evolution and circulation of Vaccine-Derived Polioviruses (VDPVs) in Helmand province of Afghanistan. We investigated 2646 AFP cases of Afghan children from June 2009–February 2011 and isolated 103 (04%) vaccine viruses, 45(1.7%) wild type polioviruses and six (0.22%) type 2 circulating vaccine-derived polioviruses (cVDPVs). These cVDPVs showed 97.7%–98.2% nucleotide and 98%–98.7% amino acid homology in VP1 region on comparison with Sabin type 2 reference strain. All these cVDPVs had two signature mutations of neurovirulent phenotypes and 12 additional mutations in P1 capsid region that might also have contributed to increase neurovirulence and replication. Phylogenetic analysis revealed that all these viruses were closely related and originated from previously reported Sabin like 2 virus from Pakistan which did not conform to the standard definition of VDPVs at that time. It was also observed that initial OPV dose was administered approximately 9 months prior to the collection of first stool specimen of index case. Our findings support that suboptimal surveillance and low routine immunization coverage have contributed to the emergence and spread of these viruses in Afghanistan. We therefore recommend high quality immunization campaigns not only in affected district Nad Ali but also in the bordering areas between Pakistan and Afghanistan to prevent the spread of cVDPVs.
Highlights
Poliovirus (PV), the causative agent of acute paralytic disease poliomyelitis, is a member of enterovirus genus of Picornaviridae family and has three distinct serotypes (PV1, PV2 and PV3) [1]
We describe the characterization of six cVDPV2 strains isolated from Acute Flaccid Paralysis (AFP) cases from Nad Ali district of Helmand province in Southern Afghanistan that remained in circulation for at least one year (2010 to 2011)
All Sabin-like (SL) viruses were screened for Vaccine-Derived Polioviruses (VDPVs) using reverse transcriptase polymerase chain reaction (rRT-PCR) and 12 positive cases were detected which were sequenced for viral protein 1 (VP1) gene
Summary
Poliovirus (PV), the causative agent of acute paralytic disease poliomyelitis, is a member of enterovirus genus of Picornaviridae family and has three distinct serotypes (PV1, PV2 and PV3) [1] It contains a positive sense 7.5 kb long single stranded RNA genome enclosed by icosahedral capsid which is made of four structural viral proteins VP1, VP2, VP3 and VP4. OPV viruses are genetically unstable [2] and vaccine strains may revert to pathogenic phenotypes during replication in human gut due to high error frequency during RNA synthesis (approximately 1024 per base pair per replication cycle) [3] These substitutions accumulate at an approximate rate of 1% per annum in viral protein 1 (VP1) region of genome [4] and can be used for estimation of evolutionary intervals between poliovirus isolates
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