The role of supplementary environmental surveillance to complement acute flaccid paralysis surveillance for wild poliovirus in Pakistan - 2011-2013

Abstract

Background: More than 99% of poliovirus infections are non-paralytic and therefore, not detected by acute flaccid paralysis (AFP) surveillance. Environmental surveillance (ES) can detect circulating polioviruses from sewage without relying on clinical presentation. With extensive ES and continued circulation of polioviruses, Pakistan presents a unique opportunity to quantify the impact of ES as a supplement to AFP surveillance on overall completeness and timeliness of poliovirus detection.

Methods: Genetic, geographic and temporal data were obtained for all wild poliovirus (WPV) isolates detected in Pakistan from January 2011 through December 2013. We used viral genetics to assess gaps in AFP surveillance and ES as measured by detection of 'orphan viruses' (≥1.5% different in VP1 capsid nucleotide sequence). We compared preceding detection of closely related circulating isolates (≥99% identity) detected by AFP surveillance or ES to determine which surveillance system first detected circulation before the presentation of each polio case.

Findings: A total of 1,127 WPV isolates were detected by AFP surveillance and ES in Pakistan from 2011-2013. AFP surveillance and ES combined exhibited fewer gaps (i.e., % orphan viruses) in detection than AFP surveillance alone (3.3% vs. 7.7%, respectively). ES detected circulation before AFP surveillance in nearly 60% of polio cases (200 of 346). For polio cases reported from provinces conducting ES, ES detected circulation nearly four months sooner on average (117.6 days) than did AFP surveillance.

Interpretation: Our findings suggest ES in Pakistan is providing earlier, more sensitive detection of wild polioviruses than AFP surveillance alone. Overall, targeted ES through strategic selection of sites has important implications in the eradication endgame strategy.

Fig 1. Number of polio AFP cases reported and detection of wild poliovirus (WPV) by environmental surveillance by province in Pakistan by month and year–January 2011 through December 2013*.

*Three additional polio AFP cases were reported from Gilgit-Baltistan; One in May 2011 and two in Sept 2012. **From 2011–2013, ES was conducted at sites in Khyber Pakhtunkwa, Punjab, Sindh and Balochistan. There were no ES sites in Gilgit-Baltistan or Federally Administered Tribal Areas. Abbreviations: AFP: Acute Flaccid Paralysis; ES: Environmental Surveillance; BAL: Balochistan; FATA: Federally administered tribal areas; GB: Gilgit- Baltistan; KP: Khyber Pakhtunkhwa; PUN: Punjab; SIN: Sindh; WPV: Wild poliovirus.

Fig 2. Surveillance system circulation detection time of genetically-similar isolates* before polio case onset and difference in detection time between AFP and environmental surveillance overall, by year and by province environmental surveillance for Pakistan polio cases**– 2011–2013.

*Genetically-similar isolates were defined as isolates with ≥99.0% identity in VP1 capsid nucleotide sequence to the polio case isolate. **28 of 361 total polio cases (11 pairs and 2 triplets) had 100% identity in VP1 capsid nucleotide sequence with one or two other isolates. These pairs and triplets were considered together, resulting in 346 genetically-unique polio case isolate sequences for analysis. Abbreviations: ES: Environmental Surveillance; AFP: Acute Flaccid Paralysis; AFP-S: Acute Flaccid Paralysis surveillance. ¹ Time (days) from first isolate within 99% of VP1 capsid detected by environmental surveillance to symptom onset date for polio case. ² Time (days) from first isolate within 99% of VP1 capsid detected by AFP surveillance to symptom onset date for polio case. ³ Number of days sooner that environmental surveillance detected circulation before AFP surveillance; Negative numbers indicate AFP detection preceding ES detection and positive numbers indicate ES detection preceding AFP detection. ⁴ Polio case was reported from a province conducing environmental surveillance (i.e. Balochistan, Sindh, Punjab, Khyber Pakhtunkhwa). ⁵ Polio case was reported from a province not conducting ES (i.e. Federally Administered Tribal Areas, Gilgit Baltistan).