Study on the Efficacy and Persistence of Microencapsulated Lambda-Cyhalothrin Applied on Outdoor Surfaces to Develop Malaria Vector-Control Approach in Thailand

Abstract

Malaria remains a significant public health problem in Thailand’s border areas, particularly along the western frontier, where the primary vectors are Anopheles minimus, Anopheles dirus, and Anopheles maculatus. Core interventions, including long-lasting insecticidal nets and indoor residual spraying, remain effective but are insufficient to control transmission occurring outdoors. This field experimental research aimed to assess the efficacy of outdoor residual spraying with a lambda-cyhalothrin microcapsule applied to the outer wall surfaces of houses made of wood, bamboo, and cement, to develop vector control strategies tailored to outdoor biting behavior. A combined field and laboratory study was conducted over nine months (August 2024–May 2025). Entomological data were collected, and the residual activity of the insecticide was evaluated using the WHO cone bioassay test. Data were analyzed using Generalized Linear Models.

The study found that Anopheles minimus was the most abundant species (53.1%), confirming the epidemiological risk in the area. At 80 mg/m², residual activity persisted for 5–6 months on wood, 4–5 months on bamboo, and 2–3 months on cement, while at 40 mg/m² it lasted 3–4 months, 2–3 months, and 1–2 months, respectively. Efficacy declined significantly over time (p-value<0.001). In conclusion, the use of a lambda-cyhalothrin microcapsule formulation at 80 mg/m² is appropriate for outdoor residual spraying, particularly on wooden surfaces, and should be considered as a complementary measure alongside indoor residual spraying and long-lasting insecticidal nets to enhance the effectiveness of outdoor malaria vector control and to strengthen the sustainability of malaria prevention and control strategies in Thailand.