It is unknown how West Nile virus entered the U.S. in 1999. Based on what you know about the virus, what would be the most likely source(s) for introduction:

• A. Infected humans
• B. Infected horses
• C. Any infected mammal
• D. Infected mosquitoes and birds

Answer: (D)

West Nile virus is transmitted through mosquitoes and has birds as the main reservoir that amplifies the virus. Introducing the virus to a new area would most likely involve hosts that can sustain the virus long enough to infect feeding mosquitoes. Infected birds can travel long distances, spreading the virus to new regions where local mosquitoes can then continue transmission. Infected mosquitoes can also seed a new area if they bring the virus with them or establish local transmission. Humans and horses are typically dead-end hosts; their viremia isn’t high enough to reliably infect mosquitoes, so they don’t kick off new transmission cycles. So the most likely sources are infected mosquitoes and birds.