In a recent study published in The Lancet, researchers investigated molecular properties of type 2 poliovirus (PV) isolates obtained from sewage samples (SS) to detect PV transmission in communities.
PV transmission has affected all nations and has been designated an international public health emergency by the World Health Organization (WHO). The global PV outbreak risks could be aggravated in nations using inactivated polio vaccines, which confer robust protection against paralytic polio but have lesser efficacy than oral vaccines against PV shedding, probably enabling undetected PV circulation over a long period.
The present study's authors have previously conducted PV environmental surveillance in the United Kingdom (UK) from 2014 onwards. They found that vaccine-like PV isolates could be detected sporadically in wastewater (WW) samples, most probably imported from nations using oral polio vaccines for vaccinations and generating immune responses.
The authors had previously identified two, two, and eight vaccine-like PV isolates of type 1, 2, and 3, respectively, from 2017 to 2021, none of which were genetically related, and all comprised mutations, with no related viruses detected in subsequent WW samples, indicative of no local PV transmission.
About the study
In the present study, researchers extended their previous analysis by performing an environmental surveillance analysis for detecting PV type 2 isolates in sewage systems in London.
Ribonucleic acid (RNA) was extracted from SS and subjected to polymerase chain reaction (PCR) analysis using PV–enterovirus primers and sequenced by whole-genome sequencing. SS were tested using WHO-recommended techniques, including concentration, viral isolation in L20B mice and rhabdomyosarcoma cells, and molecular characterization. DDNS (direct detection by nanopore sequencing) and phylogenetic analyses were also performed.
In addition, a new nanopore sequencing protocol was used for direct PV detection in stool samples and rapid generation of whole genome PV sequences to improve understanding of their geographical transmission and history. In the modified protocol, type 2 PV isolate-targeted primers were used to improve viral detection sensitivity by decreasing the VP1 sequence background from the co-circulating C enteroviruses species obtained using the original detection protocol.
The sampling procedure was performed extensively to localize the region where PV transmission may occur. Beckton sewage treatment works were subcategorized into small sub-catchment regions. Sampling was also extended to larger London sewage treatment works plants beyond Beckton to investigate probable type 2 PV presence in wider geographical areas.
Two PV type 2 vaccine-like isolates were identified in a sewage sample obtained from London Beckton sewage treatment work on 8 February 2022. In addition, three and six type 1 and type 3 PV vaccine-like isolates were identified in 52 SS obtained from London between 11 January and 4 July 2022, all of which comprised a few mutations without any genetic relationships except for the parent Sabin strain.
In total, 118 genetically-associated PV isolates genetically linked to the Sabin 2 vaccine strain (with six same mutations, including A481G and VP1-I143V site reversions) were identified in 21 (out of 52) sequential SS obtained in London between 8 February and 4 July 2022. The isolates acquired 11 more mutations between 8 February and 12 April 2022, at differing nt positions, increasing PV genetic diversity.
The isolates were recombinants with a species C enterovirus with a crossover at nucleotide (nt) position 5139. Out of 118 isolates, 20 met the criteria for vaccine-derived PV, with six to ten nt alterations in the VP1 capsid protein-encoding gene. The 20 isolates included vaccine-derived PV type 2 obtained from neighboring sewage areas with non-overlapped drainage populations, indicative of local PV transmission.
Only two samples (ENV-22-117 and ENV-22-027) comprised PV isolates with similar sequences. Genetically associated PV type 2 isolates were detected in increasing concentrations in the cell culture experiments performed on samples obtained from the London Beckton region from 12 April 2022 onwards.
Positive sampling locations identified indicated that PV was present mainly in a few regions within Beckton and several boroughs located in the east and north of London. When using type 2 PV isolate-specific primers in the second nested PCR test, increased sensitivity was observed. Type 2 PV isolates could be detected from stool samples reported as negative by viral isolation experiments.
Overall, the study findings showed that environmental surveillance enabled the detection of PV importation (and circulation) in London in the early stages, allowing swift public health responses, including increased surveillance and an inactivated polio vaccine campaign in one-year to nine-year-old children. The viruses appeared to undergo evolution in several directions originating from a common source, with multiple London sewage PV type 2 isolates sharing only one or similar VP1 nt alterations from the Sabin 2 strain, indicating that genetic associations between the isolates were not only based on VP1 sequences.
Therefore, it may be critical to refine vaccine-derived viral organism definitions to ensure the rapid and early detection of PC transmission events, especially in nations free of polio for several years.