Which statement is true of West Nile virus infections in the United States?

• A. West Nile virus cycles between mosquitoes and birds and can be transmitted by mosquitoes to mammals and reptiles
• B. Because the vector is known, the disease is easy to control
• C. Quarantine of horses is important in control of WNV disease because horses contribute to the spread of the virus
• D. Infections in commercial poultry significantly contribute to WNV infections in geographical areas where most commercial poultry are raised

Answer: (A)

West Nile virus is maintained in nature by a cycle between mosquitoes and birds, with birds serving as the main amplifying hosts. When a mosquito bites an infected bird, the virus replicates in the mosquito and can then be transmitted to other birds. Humans and horses can become infected when bitten by an infected mosquito, but they generally do not contribute to the ongoing transmission cycle. This combination—mosquito-bird cycling and spillover to mammals via mosquitoes—is the key idea.

The statement captures that ecology: a vector-borne cycle centered on birds, with mosquitoes capable of transmitting the virus to mammals. It’s not primarily spread by poultry, and horses are not used to propagate the virus (they are typically dead-end hosts), so quarantine of horses isn’t a main control strategy. Knowing the vector doesn’t automatically make control easy, because many mosquito species and high-traffic bird populations create a persistent challenge. Occasional infections in other vertebrates, including some reptiles, can occur, but they do not drive the maintenance of the virus in most U.S. settings—the bird–mosquito cycle remains the foundation.