Which of the following measures is likely to limit outbreaks of West Nile virus in the United States?

• A. Test and slaughter of infected herds and flocks
• B. A quarantine on the movement of horses
• C. A ban on the importation of psittacine birds
• D. Mosquito control

Answer: (D)

The key idea is that West Nile virus spreads mainly through mosquitoes that bite infected birds and then transmit the virus to humans and horses. By reducing the mosquito population or preventing bites, you directly cut the main pathway of transmission, lowering the chance that people and horses become infected. This makes mosquito control the most effective way to limit outbreaks.

Other options target different scenarios or hosts and don’t address the primary transmission route. Testing and culling infected herds is for diseases confined to livestock and their spread, not a virus that cycles between birds and mosquitoes. Quarantining horses won’t stop the mosquito–bird–mosquito cycle, since mosquitoes bite many hosts and can still spread the virus regardless of horse movements. A ban on importing psittacine birds addresses other avian diseases, not West Nile virus in the United States.