Substantial Population Structure of Plasmodium vivax in Thailand Facilitates Identification of the Sources of Residual Transmission

Background: Plasmodium vivax transmission in Thailand has been substantially reduced over the past 10 years, yet it remains highly endemic along international borders. Understanding the genetic relationship of residual parasite populations will help assess the risk of parasite re-introduction into malaria-free regions and track the origins of the introduced parasites. Materials and Methods: A total of 127 P. vivax isolates were genotyped from two western provinces (Tak and Kanchanaburi) and one eastern province (Ubon Ratchathani) of Thailand using 10 microsatellite markers. Genetic diversity, linkage disequilibrium, population structure and differentiation were investigated. Results and Conclusions: Genetic diversity was high in all provinces, but recent clonal expansion in Tak and bottleneck event in Kanchanaburi and Ubon Ratchathani were identified. Substantial population structure and genetic differentiation of parasites among provinces suggest limited gene flow among these sites. There was no haplotype sharing among the three sites, and a reduced panel of four microsatellite markers was sufficient to assign the parasites to their provincial origins. The results of this should be useful for monitoring parasite reintroduction after malaria elimination.