[West Nile virus, an emerging arbovirus].

The West Nile virus (family Flaviviridae, genus Flavivirus), first isolated in 1937 in Africa, is an arthropod-borne virus transmitted through Culex mosquitoes bite. Its natural cycle includes a vector (Culex mosquitoes), a reservoir (birds of various species) and humans and large mammals are epidemiological dead-ends. PROGRESSIVE DISSEMINATION: This virus recently drew attention after several epidemics and epizooties were identified in France, Russia, Romania, and Israel. However, the most striking fact is its introduction in 1999 into the north-eastern United States, a country previously West Nile virus-free, and its progressive spread according to North-South and West-East gradients to finally cover the near-totality of the north American territory (USA, Canada and Mexico), in 2002. The year 2002 has been particularly prone to disseminating West Nile virus in the Unites States: more than 4,000 human cases have been recorded and resulted in the death of almost 300. In 2003, around 10,000 cases were recorded. From January to October 2004, around 2,000 human cases have been recorded in the USA. Among infected patients, few develop a symptomatic form, and only a small proportion of the latter will develop a neurological form. REGARDING DIAGNOSIS AND SURVEILLANCE: Although virus isolation remains the technique of reference, routine diagnosis basically relies on serology (detection of increased levels of specific antibodies on two blood tests at 21 days interval, detection of IgM on a single test) and on molecular techniques (detection of the viral genome); the latter have been developed in quantitative and real-time formats that provide results within 24 hours. We describe the situation in France following the equine epizooty that occurred in 2000, and the surveillance modalities initiated at that time.