Which of the following statements about West Nile virus is true?

• A. West Nile virus is the mosquito-borne virus that currently causes the greatest number of human illnesses in the United States
• B. It is transmitted by Aedes mosquitoes
• C. It is not present in the United States
• D. It does not affect birds

Answer: (A)

West Nile virus is the mosquito-borne virus that currently causes the greatest number of human illnesses in the United States because it has become established across the country since it first appeared here in 1999, with humans primarily becoming infected through bites from Culex mosquitoes that have fed on infected birds. Birds are the main reservoir and amplification hosts, so the virus cycles between birds and mosquitoes, while humans and horses are incidental, non-persistent hosts.

The statement that it is transmitted by Aedes mosquitoes is not correct because, in the United States, the primary vectors are Culex species, not Aedes. The assertion that it is not present in the United States is false because West Nile virus is well established nationwide. The claim that it does not affect birds is also false because birds play a key role in the virus’s transmission cycle and can suffer illness or death from infection.