Genetic signatures of  Plasmodium vivax Circumsporozoite Surface Protein during malaria resurgence in Thailand

The Plasmodium circumsporozoite surface protein (CSP) is the best characterized pre-erythrocytic vaccine target for malaria. It is a multifunctional protein important for sporozoite mobility, mosquito salivary gland invasion, and hepatocyte invasion. We analyzed diversity of Plasmodium vivax CSP gene ( pvcsp ) during the 2022–2023 malaria resurgence in northwestern Thailand and assessed how pvcsp haplotypes may affect parasite development in the mosquitoes. Amplicon sequencing of 69 P. vivax isolates revealed both canonical pvcsp variants: VK210 ( n = 66) and VK247 ( n = 3). The VK210 type exhibited high polymorphism within the central repeat region, with 21 haplotypes (H1–H21) composed of 13–20 repeat motifs. Haplotype H2 was the most common, accounting for half of all VK210 sequences, and in membrane feeding assays with Anopheles dirus , appeared to produce more salivary-gland sporozoites per oocyst than other haplotypes, suggesting that repeat-region variation may modulate vector competence. Together, these findings report contemporary pvcsp diversity in Thailand’s highest transmission area, provide functional evidence that repeat-region polymorphisms shape vector-parasite interactions, and highlight three globally prevalent motifs (GDRADGQPA, GDRAAGQPA, ANGAGNQPG) as prime targets for future PvCSP vaccines.