Low genetic polymorphism of the PvMSP1-19 vaccine candidate among Plasmodium vivax isolates along the Thailand-Myanmar border.

Plasmodium vivax remains the predominant malaria species in Thailand, accounting for approximately 97% of all reported cases, with transmission persisting in endemic border regions such as Kanchanaburi Province along the Thailand-Myanmar border. The C-terminal 19-kDa fragment of merozoite surface protein 1 (PvMSP1-19) has been proposed as a promising blood-stage vaccine candidate owing to its essential role in erythrocyte invasion and EGF-like domains. However, population-level genetic evidence from localized transmission foci in Southeast Asia remains limited. I this study, we investigated the genetic diversity and structural integrity of PvMSP1-19 in 109 P. vivax clinical isolates from Kanchanaburi, Thailand, between 2023 and 2025. Genomic DNA was extracted and the PvMSP1-19 region was amplified by PCR, sequenced by Sanger sequencing, and population genetic parameters were analyzed using DnaSP. Protein structure was predicted using AlphaFold3 and validated by MolProbity, PROCHECK, ProSA, and ERRAT2. Sequence analysis across 276 nucleotide positions identified only one segregating site, yielding two haplotypes with extremely low nucleotide and haplotype diversity. The dominant haplotype, corresponding to the Sal-1 reference strain, was present in 99.1% of isolates, and the single nucleotide substitution identified was synonymous, resulting in no amino acid substitution. A negative Tajima's D value suggested a trend consistent with purifying selection, although this was not statistically significant. Structural modelling of PvMSP1-19 confirmed a stable, high-quality conformation. These findings demonstrate that PvMSP1-19 is highly conserved in P. vivax populations circulating along the Thailand-Myanmar border and support its potential as a promising antigen for blood-stage vaccine development targeting vivax malaria in Southeast Asia.