Environmental poliovirus surveillance during oral poliovirus vaccine and inactivated poliovirus vaccine use in Córdoba Province, Argentina

Abstract

This study compares the presence of environmental poliovirus in two Argentinean populations using oral poliovirus vaccine (OPV) or inactivated poliovirus vaccine (IPV). From January 2003 to December 2005, Córdoba City used IPV in routine infant immunizations, with the exception of intermittent OPV use in August 2005. Between May 2005 and April 2006, we collected weekly wastewater samples in Córdoba City and the province's three major towns, which continued OPV use at all times. Wastewater samples were processed and analyzed for the presence of poliovirus according to WHO guidelines. During the months of IPV use in Córdoba City, the overall proportion of poliovirus-positive samples was 19%. During an intermittent switch from IPV to OPV, this proportion increased to 100% within 2 months. During the 3 months when IPV was reintroduced to replace OPV, a substantial proportion of samples (25%) remained positive for poliovirus. In the OPV-using sites, on average, 54% of samples were poliovirus positive. Seventy-seven percent of poliovirus isolates showed at least one mutation in the VP1-encoding sequence; the maximum genetic divergence from the Sabin strain was 0.7%. Several isolates showed mutations on attenuation markers in the VP1-encoding sequence. The frequency or type of virus mutation did not differ between periods of IPV and OPV use or by virus serotypes. This study indicates that the sustained transmission of OPV viruses was limited during IPV use in a middle-income country with a temperate climate. The continued importation of poliovirus and genetic instability of vaccine strains even in the absence of sustained circulation suggest that high poliovirus vaccine coverage has to be maintained for all countries until the risk of reintroduction of either wild or vaccine-derived poliovirus is close to zero worldwide.

FIG. 1.

Percentage of wastewater samples positive for PV by month and site. Córdoba City (CC) used OPV during August to September 2005 and from January 2006 onwards. Sites using exclusively OPV at all times were Rio Cuarto, Villa Maria, and San Francisco.

FIG. 2.

Mutations observed on VP1 attenuation markers. Above each nucleotide, the column height indicates the percentage of all PVs of a given serotype isolated during the study that feature a mutation at this position. Below each codon, the circle shows the distribution of the amino acid substitutions resulting from the mutation. The capital letters below the nucleotides and in circles represent conventional abbreviations for amino acids. syn., synonymous mutation.

FIG. 3.

Recombinant sites in three intertypic recombinant Sabin-like PV isolates. Shown is an alignment of nucleotide sequences of isolates 05/124L2P3, 06/205L1P3, and 06/256L1P3 and S1, S2, and S3 strains at the crossover junctions. The recombination sites observed in the sequence of each of our isolates are highlighted in red. (A) Recombination sites identified in the capsid (VP1). In sequences of S2 and S3 strains, the recombination site reported previously by Martin et al. (23) is shaded blue and the one reported by Blomqvist et al. (1) and Dedepsidis et al. (6) is shaded yellow. (B) Recombination site identified in the 3D gene of isolate 05/124L2P3. (C) Recombination site identified in the 3D gene of isolate 06/205L1P3. Numbering for panel A is according to VP1 nucleotide sequences; numbering for panels B and C is according to the full-length genomic nucleotide sequences.