Genetic and Epidemiological Study of Wild Poliovirus Type 1 Strains Circulating in Pakistan During 2015-2017

Abstract

Pakistan is among three countri es endemic for wild poliovirus (WPYI) circul ation, still struggling for eradication of poli omyelitis. Poliovirus causes a highl y infectious and communicable viral disease in humans resulting in irreversibl e para lysis. Poliomyelitis is incurabl e and th e only way is to eradicate the poliovirus itself. The obj ecti ves of this research study were to in vesti gate the epidemiology and genetic diversity of WPY} in Pakistan (2015-2017). Studying the geneti c links between w ild poliovirus type I infected paralytic cases and environme ntal isolates were helpful to reveal poliovirus transmission patterns. In Pakistan, high population mobility is a source of complex viral transmission patterns. Stud ying the poliovirus transmission patterns was valuabl e in understanding the hi gh-risk areas, defining the 'at risk' populations and finding the unimmuni zed cohorts. The study revealed three chroni c reservo ir areas with recurrent population mobility chall enging poliovirus eradication. Environmenta l surveillance unveil ed persistent circul ation of WPVl 111 areas with no reported poliomyelitis case. Environmental surveillance unveiled persistent circu lation of wi ld poliovirus type 1 in areas with no reported poliomyelitis case. After screening of 23606 stoo l specimens, 84 were positive for WPV I. 1726 environmental samples were collected from 60 active environmental sites and 252 were recorded as WPY I isolates. The phylogenetic analysis was done on these wild type I isolates using BEAST v2.3.0 software. As a result, poliovirus isolates were grouped into eleven distinct clusters which had 2:95% nucleotide homology in YP 1 coding region. Although the number of genctic clusters has reduced over study period (20 15 -20 17) w ith a sharp decl ine in incidence of clinical poliomyelitis cases (54 in 20 15 vs 08 in 20 17), the enormously frequent detection of WrY1 in sequentiall y collected sewage water sampl es f1'om multiple catchment areas highli ghts an alarming situation. Our data recognizes multiple reservoirs of WPV I within Pakistan in distant areas across all four provinces. This is evident thro ugh detection of WPYl strains with signifi cant geneti c diversity, representing not just a single or clonal transmission chain and provides strong evidence of sustained poliovirus infection in such areas over the years. Most burden of poliovirus was shared by three major reservoirs i. e. Karachi, Peshawar and Quetta block (64.2% in