In some cases, animals other than the primary hosts may become infected with the virus. The West Nile virus, for example, can also infect humans. These alternative hosts are often referred to as dead end hosts, as they are not part of the virus normal transmission cycle. These organisms do not usually produce enough virus particles to allow them to become a reservoir for the virus. The infection thus reaches a dead end in this host.
Mosquitoes are usually the primary vectors for transmission of the West Nile virus.
In Africa and Asia, however, ticks have been found carrying the virus. In warmer climates, the virus can be found year-round. West Nile virus causes an infection similar to the St. Louis encephalitis, but with milder symptoms. In most cases, the infection is asymptomatic or causes mild flu-like symptoms. In general, wild birds are the primary hosts of the West Nile virus.