West Nile virus (WNV) is a mosquito-borne virus that circulates among birds, but can also affect other species, particularly humans and horses. Many WNV strains are thought to be maintained in Africa; however, migrating birds carry these viruses to other continents each year, and some strains have become established outside Africa. At one time, the distribution of WNV was limited to the Eastern Hemisphere, and it was infrequently associated with serious illness. Clinical cases usually occurred sporadically in humans and horses, or as relatively small epidemics in rural areas. Most human infections were asymptomatic, and if symptoms occurred, they were typically mild and flu-like. Severe illnesses, characterized by neurological signs, seemed to be uncommon in most outbreaks. Birds appeared to be unaffected throughout the Eastern Hemisphere, possibly because they had become resistant to the virus through repeated exposure.
Iowa State University
Iowa State University Center for Food Security and Public Health, "West Nile Virus Infection" (2013). Center for Food Security and Public Health Technical Factsheets. 146.