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West Nile virus (family Flaviviridae , genus Flavivirus ) was first isolated from a febrile woman in Uganda in 1937 and subsequently identified as the etiologic agent of sporadic cases of WN disease as well as of major outbreaks in Africa, Europe, Asia, Australia, and the Middle East. In 1996, more frequent and severe outbreaks were noted, especially in the Mediterranean Basin. Following a single introduction into the United States in 1999, West Nile virus (WNV) rapidly expanded its range in a stepwise progression and within 5 years was well entrenched in North America and parts of Canada, and the Caribbean, Mexico and Central America, and South America. WNV is a zoonotic pathogen, maintained in an enzootic cycle transmitted between ornithophilic mosquitoes and avian hosts, and infects humans, equids, and many other vertebrates. Culex species mosquitoes are the predominant vectors worldwide with the particular species varying with location, but the virus has infected approximately 75 mosquito species worldwide and birds of more than 300 species. Infection leads to a broad spectrum of disease, ranging from fever to severe neurologic disease, including meningitis, encephalitis, and acute flaccid paralysis. Less common alternative modes of transmission have been noted, such as transfusion and transplantation transmission in humans, and fecal–oral and peroral transmission in experimental animals. Severe disease is more common in older individuals and immunocompromised hosts are at greatest risk. Intensive research has led to rapid advances in vaccine development, therapeutics, a better understanding of molecular biology of the virus and immunopathogenesis, as well as viral ecology. |
