Which mosquito-borne virus currently causes the greatest number of human illnesses in the United States?

• A. West Nile virus
• B. Zika virus
• C. Dengue virus
• D. Yellow fever virus

Answer: (A)

West Nile virus is the mosquito-borne illness that causes the most human cases in the United States right now because it is established nationwide in a bird–mosquito cycle and is transmitted by Culex mosquitoes year after year. Since its introduction in the U.S. in the late 1990s, it has become enzootic across many states, so humans are repeatedly exposed. Most infections are mild or asymptomatic, but a meaningful portion can progress to severe neuroinvasive disease, which brings people into care and is counted in surveillance data, contributing to higher overall illness numbers than the other viruses listed.

The other viruses do cause illnesses in the U.S., but not at the same sustained, nationwide level. Zika virus caused a large, well-publicized outbreak a few years ago, mainly tied to travel and local transmission in limited areas, and its numbers have since declined. Dengue occurs primarily in the regions the U.S. touches (notably some areas of the Caribbean, Central America, and U.S. territories) and only occasionally produces locally transmitted cases in the continental United States. Yellow fever is not circulating locally in the United States today and is largely a travel-associated risk. So, West Nile remains the most consistently reported mosquito-borne illness in the country.

In terms of control, reducing Culex breeding habitats and applying appropriate mosquito-control measures help limit West Nile transmission, while personal protection and barriers remain important to reduce individual risk.