St. Louis Encephalitis is primarily transmitted by which mosquito vectors?

• A. Main vectors: Culex pipiens (most U.S.), Culex nigripalpus, Culex salinarius
• B. Primary vector: Aedes aegypti
• C. Vector: Anopheles gambiae
• D. Birds only

Answer: (A)

St. Louis Encephalitis Virus is spread in a bird–mosquito cycle, with birds serving as the main reservoir and certain Culex mosquitoes acting as the primary vectors to humans. In the United States, the mosquitoes most frequently implicated are Culex species that feed on birds but will bite humans when the opportunity arises. The key vectors here are Culex pipiens (the commonly found one across much of the U.S.), along with Culex nigripalpus and Culex salinarius in other regions. This combination explains why these species are identified as the main transmitters.

Aedes aegypti is not the primary carrier of St. Louis Encephalitis; it specializes more in viruses like dengue, Zika, and yellow fever. Anopheles gambiae is associated with malaria, not SLEV. Saying “birds only” ignores how mosquitoes transmit the virus from birds to humans, so humans get infected through the bite of an infected Culex mosquito rather than directly from birds.