Date Published: July 25, 2017
Publisher: Public Library of Science
Author(s): Tori L. Cowger, Cara C. Burns, Salmaan Sharif, Howard E. Gary, Jane Iber, Elizabeth Henderson, Farzana Malik, Syed Sohail Zahoor Zaidi, Shahzad Shaukat, Lubna Rehman, Mark A. Pallansch, Walter A. Orenstein, Juan C. de la Torre.
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.
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.
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.
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.
In 1988, the World Health Assembly resolved to eradicate poliovirus. Since then, polio incidence has decreased from an estimated 350,000 cases to 37 in 2016 [1, 2]. A cornerstone of global eradication efforts and the primary means for detecting poliovirus transmission is acute flaccid paralysis (AFP) surveillance–clinical surveillance for persons with AFP, and subsequent testing for polioviruses in stool specimens [3, 4].
ES provides a system for routine surveillance of poliovirus circulation independent of clinical disease (AFP). This study examines the contribution of ES as a supplement to AFP surveillance in Pakistan during a period of extensive WPV circulation. Because the two surveillance systems use completely different approaches to finding poliovirus, it is difficult to directly compare performance features. By using virologic results, it is possible to infer qualitative and quantitative benefits from supplementing AFP surveillance with ES. Throughout our study period, standard indicators for quality of AFP surveillance met or exceeded global standards in Pakistan (sensitivity of surveillance >2.0 cases of AFP detected per 100,000, and more than 80% of AFP cases with two adequate stool specimens) [23, 29, 32–37]. Even in the presence of high quality AFP surveillance, through several approaches, the analysis demonstrates the average net benefits of ES to detect circulation earlier than or missed by AFP surveillance.
Overall, targeted ES through strategic selection of sites has proven useful for detecting WPV circulation in Pakistan. This demonstrated effectiveness for public health response to WPV circulation supports the proposed plan to scale up ES and suggests that ES be explored for other important applications in the eradication endgame strategy, including detecting cVDPVs, monitoring the switch from OPV to IPV, and certification of a polio-free world. Environmental surveillance should be considered in areas at high risk of sustained poliovirus transmission such as localities in endemic countries and those with a history of frequent importations and sustained transmission.