How can residual transmission occur despite successful mosquito control?

• A. All mosquitoes are eliminated everywhere.
• B. Transmission stops completely after control.
• C. It occurs only when vaccines fail.
• D. Incomplete coverage, refugia in outdoor populations, insecticide resistance, and human behavior leading to exposure.

Answer: (D)

Residual transmission happens because control measures rarely remove every mosquito or all opportunities for transmission. Even when overall vector levels drop, pockets of mosquitoes can persist where interventions don’t reach them fully—areas outside sprayed zones, outdoor breeding sites, or places without screens—creating refugia that keep biting and transmitting. Insecticide resistance means some mosquitoes survive standard interventions and can still acquire and pass on the pathogen. Human behavior also plays a role: people may be outdoors during times when mosquitoes are active, or may not consistently use nets or repellents, increasing exposure despite overall control success. Together, these factors allow transmission to continue even after mosquito control appears successful. The other options imply complete elimination or a single cause, which don’t reflect the real, multifaceted nature of residual transmission.