What is the most likely source of introduction for the West Nile virus in the U.S.?

The most likely source of introduction for the West Nile virus in the U.S. is indeed infected mosquitoes and birds. West Nile virus primarily cycles between birds and mosquitoes, with birds acting as the primary reservoir for the virus. Mosquitoes become infected when they feed on the blood of these infected birds, and subsequently, they can transmit the virus to other birds, mammals, and humans through their bites.

Infected birds are particularly important in the epidemiology of the virus because they can carry the virus without showing symptoms, thus facilitating the spread. The Culex species of mosquitoes, which are common in many areas of the United States, are the main vectors responsible for transmitting the virus from birds to people. This highlights the ecological relationship between these two groups in the transmission cycle.

While humans and other mammals can be infected with West Nile virus, they are generally considered "dead-end" hosts, meaning that the virus does not efficiently replicate in these organisms to allow for further transmission. Domesticated pets and imported exotic animals are not typical sources of the virus's introduction or spread like mosquitoes and birds are. Therefore, the emphasis on mosquitoes and birds as the primary vectors underscores their crucial role in the transmission dynamics of West Nile virus.