Linking Anopheles bionomics and human behaviour in the Lao PDR

Laos aims to eliminate malaria by 2030, with residual transmission present in the south. Understanding how LLINs function in relation to vector and human behaviours, and linking this to epidemiological outcomes, is critical for improving vector control strategies. Integrating human behaviour observations (HBOs) with entomological and epidemiological data helps identify gaps in protection and inform national malaria programs.

Entomological surveillance of Anopheles mosquitoes was conducted alongside HBOs in both rainy (July-September 2022) and dry (February-April 2023) seasons. Nine villages with high malaria burden were surveyed across three ecotypes: villages, cultivation sites, and forested areas. Mosquitoes collected via indoor and outdoor human landing catches (HLCs) were identified morphologically. HBOs recorded spatial and temporal human presence and LLIN use. Human biting rates (HBR) were adjusted with HBO data to quantify spatial and temporal protection and identify gaps in protection. Anopheles indoor resting rates were also assessed.

Anopheles mosquitoes (n = 1012) belonging to 12 species were collected, with the highest numbers from cultivation sites (n = 511), followed by villages (n = 302) and forests (n = 198). During the dry season, more Anopheles were found in cultivation sites than villages, while this was reversed in the rainy season. Human behaviour adjusted biting rates, or exposure was highest outdoors, particularly between 18:00-22:00 and 03:00-06:00 in villages. LLIN use in villages prevented 42% of total Anopheles bites and 83% of indoor bites during sleeping hours. However, LLINs only reduced exposure by 26% at cultivation sites, where Anopheles were primarily biting outdoors.

This operational study demonstrates that peridomestic behaviours of both humans and Anopheles in Laos results in significant outdoor gaps in protection, especially in the early evening. High LLIN coverage remains essential for reducing indoor bites in villages. IRS may have limited impact due to the lack of indoor-resting vectors. Addressing these gaps requires innovative strategies. HBOs integrated with entomological data offer valuable insights into residual malaria transmission, particularly in outdoor and early evening settings, emphasizing the need for targeted interventions.