Biodiversity-related aspects of West Nile virus and its cycle in nature

West Nile virus is an arthropod-transmitted virus maintained in a cycle involving mosquitoes and birds. It belongs to the Family Flaviviridae, a virus family that includes many viruses capable of causing serious disease in humans. Most West Nile virus infections do not result in disease but, if disease occurs, symptoms range from fever, the most common manifestation, to encephalitis. West Nile virus has a widespread geographic distribution documented in Africa, Europe, Asia, Australia and North America. It is an emerging pathogen with an increased number of epidemics observed over the past decade in Europe and North America. West Nile virus has evolved greatly over the years to include strains with varying genetic makeup and biological characteristics. The incursion of West Nile virus into North America, likely in 1999, followed by a continent wide spread of the virus has allowed us to fully document the successful introduction of an exotic viral species into a new ecosystem providing a vivid example of infectious disease history. The ability of the different strains of West Nile to amplify on different continents is directly influenced by the diversity of mosquito and bird species present as well as by climatic conditions.