Culex and Japanese encephalitis — DTM&H MCQ

Correct answer: D — Culex mosquito

The correct answer is D, Culex mosquito. Japanese encephalitis virus is transmitted through an enzootic cycle in which Culex species mosquitoes become infected after biting amplifying hosts, chiefly pigs and wading birds, and then transmit the virus to humans as incidental hosts. The stem's clues (rice paddies, rainy season, pig farms) are classic ecological determinants because flooded rice fields provide breeding sites and pigs act as viral amplifiers, sustaining high mosquito infection rates. This matches the epidemiology of paediatric encephalitis across rural South and South East Asia, including Cambodia, where JE remains a leading cause of viral encephalitis in children. No other vector sustains the pig-bird-mosquito cycle required for JE transmission. Why the other options are wrong: E. Reduviid bug: This is the vector for Trypanosoma cruzi (Chagas disease) in the Americas, an entirely different parasite with no mosquito-borne flavivirus link and no relevance to South East Asian rice-farming ecology. C. Anopheles mosquito: Anopheles transmits malaria (Plasmodium species) via a very different life cycle involving sporozoite injection from salivary glands, not flavivirus amplification through pigs and birds. B. Aedes aegypti mosquito: Aedes aegypti is the principal vector for dengue, chikungunya, Zika and yellow fever; it is a day-biting, urban container-breeder, not associated with rice-field or pig-farm ecology. A. Phlebotomine sandfly: Sandflies transmit Leishmania species and sandfly fever viruses, unrelated to JE and not linked to pig reservoirs or rice paddy breeding sites. Key point: Japanese encephalitis is transmitted by Culex mosquitoes within a pig and wading bird amplification cycle, explaining its strong association with rice-growing, pig-farming rural Asia.

Reference: NaTHNaC (TravelHealthPro), Japanese Encephalitis factsheet, https://travelhealthpro.org.uk/factsheet/55/japanese-encephalitis