Anopheles Bionomics in a Malaria Endemic Area of Southern Thailand

Abstract Background: Ivermectin mass drug administration (MDA) could accelerate malaria elimination in the Greater Mekong Subregion. This study was performed to characterize the bionomics of Anopheles in Surat Thani province, Thailand.Methods: Mosquitoes were collected via human landing collections, February - October of 2019. Anopheles were morphologically identified to species. Primary Anopheles malaria vectors were dissected to assess parity status and a subset evaluated for molecular identification and Plasmodium detection.Results: A total of 17,348 mosquitoes were collected. Of 5,777 Anopheles mosquitoes, 15 species were identified morphologically. The most abundant Anopheles were Anopheles minimus s.l. (87.87%, n = 5,035), Anopheles dirus s.l. (7.05%, n = 407), and Anopheles barbirostris s.l. (2.86%, n = 165). Molecular identification confirmed that of An. minimus s.l., 99.80% were An. minimus s.s. (n=484) and 0.2% An. aconitus (n = 1), of An. dirus s.l., 100% were An. baimaii (n = 348), and of An. maculatus s.l., 93.62% were An. maculatus s.s. (n = 44) and 6.38% An. sawadwongporni (n = 3). No Anopheles were Plasmodium positive (n = 879). An average of 11.46 Anopheles were captured per collector per night. There were differences between species in hour of collection (Kruskal-Wallis = 80.89, P < 0.0001, n = 5,666), with more An. barbirostris s.l. and An. maculatus s.l. caught earlier compared to An. minimus s.l. (P = 0.0001, P < 0.0001, respectively) and An. dirus s.l. (P = 0.0082, P < 0.001, respectively). The proportion of parous An. minimus s.l. captured by hour increased throughout the night (Wald Chi-square = 17.31, P=0.000, odds ratio = 1.0535 [1.0279 – 1.0796] 95% CI (n = 3,400). Overall, An. minimus s.l. parity was 67.68% (2,375/3,509) with an intra cluster correlation of 0.0378. A power calculation determined that an An. minimus s.l. parity reduction treatment effect size = 34%, with four clusters per treatment arm, a minimum of 300 mosquitoes dissected per cluster at an α= 0.05 will provide 82% power to detect a significant difference following ivermectin MDA. Conclusions: The study area in Surat Thani province is an ideal location to evaluate the impact of ivermectin MDA on An. minimus parity.