Diversity and population structure of Plasmodium falciparum in Thailand based on the spatial and temporal haplotype patterns of the C-terminal 19-kDa domain of merozoite surface protein-1

BACKGROUND: The 19-kDa C-terminal region of the merozoite surface protein-1 of the human malaria parasite Plasmodium falciparum (PfMSP-1(19)) constitutes the major component on the surface of merozoites and is considered as one of the leading candidates for asexual blood stage vaccines. Because the protein exhibits a level of sequence variation that may compromise the effectiveness of a vaccine, the global sequence diversity of PfMSP-1(19) has been subjected to extensive research, especially in malaria endemic areas. In Thailand, PfMSP-1(19) sequences have been derived from a single parasite population in Tak province, located along the Thailand-Myanmar border, since 1995. However, the extent of sequence variation and the spatiotemporal patterns of the MSP-1(19) haplotypes along the Thai borders with Laos and Cambodia are unknown. METHODS: Sixty-three isolates of P. falciparum from five geographically isolated populations along the Thai borders with Myanmar, Laos and Cambodia in three transmission seasons between 2002 and 2008 were collected and culture-adapted. The msp-1 gene block 17 was sequenced and analysed for the allelic diversity, frequency and distribution patterns of PfMSP-1(19) haplotypes in individual populations. The PfMSP-1(19) haplotype patterns were then compared between parasite populations to infer the population structure and genetic differentiation of the malaria parasite. RESULTS: Five conserved polymorphic positions, which accounted for five distinct haplotypes, of PfMSP-1(19) were identified. Differences in the prevalence of PfMSP-1(19) haplotypes were detected in different geographical regions, with the highest levels of genetic diversity being found in the Kanchanaburi and Ranong provinces along the Thailand-Myanmar border and Trat province located at the Thailand-Cambodia border. Despite this variability, the distribution patterns of individual PfMSP-1(19) haplotypes seemed to be very similar across the country and over the three malarial transmission seasons, suggesting that gene flow may operate between parasite populations circulating in Thailand and the three neighboring countries. CONCLUSION: The major MSP-1(19) haplotypes of P. falciparum populations in all endemic populations during three transmission seasons in Thailand were identified, providing basic information on the common haplotypes of MSP-1(19) that is of use for malaria vaccine development and inferring the population structure of P. falciparum populations in Thailand.