Monitoring of Plasmodium falciparum and Plasmodium vivax using microsatellite markers indicates limited changes in population structure after substantial transmission decline in Papua New Guinea

Monitoring the genetic structure of malaria parasite populations has been proposed as a novel and sensitive approach to quantify the impact of malaria control and elimination efforts. Here we describe the first population genetic analysis of sympatric Plasmodium falciparum (Pf) and Plasmodium vivax (Pv) populations following nationwide distribution of long-lasting insecticide treated nets (LLIN) in Papua New Guinea (PNG). Parasite isolates from serial cross-sectional studies pre-(2005-6) and post-LLIN (2010-2014) were genotyped using microsatellite markers. Despite parasite prevalence declining substantially in these communities (East Sepik: Pf=54.9-8.5%, Pv=35.7-5.6%, Madang: Pf=38.0-9.0%, Pv: 31.8-19.7%), genetically diverse and intermixing parasite populations remained. P. falciparum diversity declined modestly post-LLIN relative to pre-LLIN (East Sepik: Rs = 7.1-6.4, He = 0.77-0.71; Madang: Rs= 8.2-6.1, He = 0.79-0.71). Unexpectedly, population structure present in pre-LLIN populations was lost post-LLIN, suggesting that more frequent human movement between provinces may have contributed to higher gene flow between provinces. P. vivax prevalence initially declined but increased again in one province, yet diversity remained high throughout the study period (East Sepik: Rs=11.4-9.3, He=0.83-0.80; Madang: Rs=12.2-14.5, He=0.85-0.88). Although genetic differentiation values increased between provinces over time, no significant population structure was observed at any time point. For both species, the emergence of clonal transmission and significant multilocus linkage disequilibrium (mLD) due to increased focal inbreeding post-LLIN was a strong indicator of impact on the parasite population using these markers. After eight years of intensive malaria control in PNG and substantial prevalence decline the impact on parasite population diversity and structure detectable by microsatellite genotyping was limited.