pable of infecting arthropod vectors have not been reported from these hosts. Birds, however, do experience viremias capable of infecting arthropod vectors.TRANSMISSION BY ARTHROPODS Mosquitoes and ticks serve as natural vectors of WN. Most virus isolates have been from mosquitoes, suggesting that they serve as primary vectors. Culex univittatus appears to be the major WN vector in Africa. Culex pipiens is a secondary vector in South Africa and may be the primary vector in Israel. Members of the Cx. vishnui complex are the primary vectors in India and Pakistan. West Nile infected ticks in the genera Argas, Hyalomma, and Ornithodoros have been collected in northern Africa and eastern Europe. The vector(s) responsible for the 1999 NYC outbreak have not yet been identified. The most likely candidates, however, are members of the Cx. pipiens species complex. Members of this complex have been implicated in West Nile outbreaks elsewhere in the world, and they are among the most common mosquitoes in NYC during the summer.VERTEBRATE HOSTS As with other mosquito-borne viruses, many vertebrate species show evidence of natural infection with WN. Wild and domestic birds, however, show the most consistent evidence of infection. Hooded Crows and House Sparrows in Egypt showed high antibody prevalence, and WN has been isolated from naturally infected Hooded Crows. West Nile was also isolated from horses showing signs of clinical encephalitis infection during the 1962-64 outbreak in France and from camels in Sudan. A WN virus was isolated from a tick collected on a camel in Central Kara-Kum in Eastern Europe. West Nile has been isolated from pigeons in Egypt, turtledoves in Turkey and wild bird species in Borneo, Cyprus, and Nigeria. Crows experimentially infected with WN showed high mortality. In nature, some crows must survive infection since antibody-positive individuals are often captured. A domestic pigeon infected with WN and showing clinical sig...
