Which sources are most likely responsible for the introduction of West Nile virus into the U.S.?

The introduction of West Nile virus into the United States is primarily attributed to infected mosquitoes and birds. The virus is maintained in a cycle primarily involving birds as the main hosts, which can then be transmitted to mosquitoes that feed on them. Mosquitoes act as vectors, spreading the virus to other birds and occasionally to humans, leading to outbreaks. This dynamic highlights the critical role of wildlife and insect interactions in the epidemiology of arboviruses like West Nile.

While other sources such as infected human travelers or livestock may contribute to the disease's spread, they are not the primary vehicles for its introduction into new regions. Imported plants and seeds have no known association with the transmission of West Nile virus, making them irrelevant in this context. The combination of birds and mosquitoes is essential to the life cycle of the virus, thereby establishing their significance as the main sources responsible for its introduction in the U.S.