Indoor residual spraying — DTM&H MCQ

Correct answer: D — Indoor residual spraying

Explanation lettering: B = shown as A · D = shown as B · A = shown as C · C = shown as D

C is correct: Indoor residual spraying targets the specific vector behaviour described in the stem. Indoor-resting Anopheles mosquitoes shelter on walls and ceilings after feeding, and applying residual insecticide to these interior surfaces kills the vectors during this resting phase, breaking transmission even when bed-net coverage is high. This matters because insecticide-treated nets only protect against bites during sleep and do not address mosquitoes that rest indoors but feed or bite outside typical net-use hours, which explains why cases are rising despite net use. Indoor residual spraying is the WHO-endorsed complementary vector control tool specifically indicated when local Anopheles behaviour is endophilic (indoor-resting), as in this highland district. Why the other options are wrong: A. Mass dog vaccination: this targets rabies transmission via canine reservoirs, not malaria, which has no animal reservoir relevant to control in this context. B. Snail control: this is a schistosomiasis control measure aimed at the intermediate freshwater snail host, irrelevant to a mosquito-borne disease. D. Chlorination of wells: this prevents waterborne diseases such as cholera or typhoid by disinfecting drinking water, not vector-borne malaria transmission. E. Container source reduction: this reduces breeding sites for container-breeding, day-biting Aedes vectors (dengue, Zika), not indoor-resting nocturnal Anopheles. Key point: choose the vector control method that matches vector behaviour, indoor residual spraying for endophilic (indoor-resting) Anopheles when net use alone fails to control transmission.

Reference: WHO, Operational manual on indoor residual spraying: control of vectors of malaria and other vector-borne diseases (2024), https://www.who.int/publications/i/item/9789240083998