West Nile encephalitis

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Abstract
Sources and selection criteria We examined new information from recent outbreaks in America, Israel, and Southern Europe cited on PubMed and the internet to 11 December 2002. We also examined literature on West Nile virus from before 1966. Box 1: Related viruses Japanese encephalitis virus Numerically this is the most important of the neurogenic flavivirus, with an estimated 35-50 000 cases and 10 000 deaths annually.7 The virus is found in South East Asia, China, the Pacific Rim, and India, and is spreading with recent cases in northern Australia and Nepal. It is transmitted by Culex tritaeniorhynchus mosquitoes, which breed in rice paddies. Most people are infected during childhood, but only a few (about 1 in 150) develop fever, and even fewer develop central nervous system disease. In endemic areas most cases of encephalitis occur in children, but travellers are at also risk. Clinically the virus causes a severe meningoencephalomyelitis, often associated with extrapyramidal movement disorders; it can also present as meningitis or occasionally a polio-like flaccid paralysis. Seizures and raised intracranial pressure are common in children. There is no antiviral treatment. A formalin inactivated vaccine (too expensive for most residents of endemic areas) is recommended for travellers, though occasionally it causes side effects.8 St Louis encephalitis virus First identified in the 1930s after encephalitis outbreaks around St Louis, Missouri, this virus was, until recently, the most important flavivirus in the United States. The virus occurs naturally in many birds and is transmitted by Culex mosquitoes. Sporadic cases occur every year, particularly in the southern states,9 but in 1990 there were more than 200 cases, and in 1975 there was a major outbreak, with 3000 cases. Murray Valley encephalitis virus This Australian flavivirus cousin causes sporadic cases of encephalitis most years in the north and west of Australia and in New Guinea, with occasional small outbreaks (up to 20 cases).10 The virus is transmitted between wild birds by Culex mosquitoes. RETURN TO TEXT