What is one potential source for the introduction of West Nile virus into the United States?

The introduction of the West Nile virus into the United States is primarily attributed to infected birds or mosquitoes that originate from endemic regions where the virus is present. These birds, particularly certain species, serve as amplifying hosts for the virus. When mosquitoes bite these infected birds, they can subsequently transmit the virus to other hosts, including humans and other animals.

This route aligns with the patterns of transmission observed for West Nile virus, which was first identified in the U.S. in 1999, likely stemming from a viral strain that had spread through migratory birds or other carriers.

Local mosquito populations and their role in transmission become relevant only after the initial introduction of the virus. Contaminated livestock feed is not a recognized vector for the West Nile virus. Direct human transmission does not occur; instead, humans are considered incidental hosts, meaning they can contract the virus primarily through mosquito bites rather than from other humans.