Identifying villages and breeding habitats for dengue transmission in Thailand: insights from long-term larval surveys

Abstract

Background: The Anopheles dirus complex plays a significant role as a malaria vector in the Greater Mekong Subregion (GMS), with varying degrees of vector competence among species. Accurate identification of sibling species in this complex is essential for understanding malaria transmission dynamics and deploying effective vector control measures. However, the original molecular identification assay, Dirus allele-specific polymerase chain reaction (AS-PCR), targeting the ITS2 region, has pronounced nonspecific amplifications leading to ambiguous results and misidentification of the sibling species. This study investigates the underlying causes of these inconsistencies and develops new primers to accurately identify species within the Anopheles dirus complex. Methods: The AS-PCR reaction and thermal cycling conditions were modified to improve specificity for An. dirus member species identification. In silico analyses with Benchling and Primer-BLAST were conducted to identify problematic primers and design a new set for Dirus complex species identification PCR (DiCSIP). DiCSIP was then validated with laboratory and field samples of the An. dirus complex. Results: Despite several optimizations by reducing primer concentration, decreasing thermal cycling time, and increasing annealing temperature, the Dirus AS-PCR continued to produce inaccurate identifications for Anopheles dirus, Anopheles scanloni, and Anopheles nemophilous. Subsequently, in silico analyses pinpointed problematic primers with high Guanine-Cytosine (GC) content and multiple off-target binding sites. Through a series of in silico analyses and laboratory validation, a new set of primers for Dirus complex species identification PCR (DiCSIP) has been developed. DiCSIP primers improve specificity, operational range, and sensitivity to identify five complex member species in the GMS accurately. Validation with laboratory and field An. dirus complex specimens demonstrated that DiCSIP could correctly identify all samples while the original Dirus AS-PCR misidentified An. dirus as other species when used with different thermocyclers. Conclusions: The DiCSIP assay offers a significant improvement in An. dirus complex identification, addressing challenges in specificity and efficiency of the previous ITS2-based assay. This new primer set provides a valuable tool for accurate entomological surveys, supporting effective vector control strategies to reduce transmission and prevent malaria re-introducing in the GMS. Graphical Abstract: (Figure presented.) ? The Author(s) 2024.