infectious diseases
West Nile Virus
Introduction
West Nile virus is a flavivirus that is now the primary cause of Overview of Arbovirus, Arenavirus, and Filovirus Infections encephalitis in the United States. Most patients have mild or no symptoms. About 1 out of 150 patients develop a severe infection involving the central nervous system. Diagnosis is by serologic testing. Treatment is supportive with close monitoring for severe infection. West Nile virus was first introduced into the United States in 1999 in New York City. It is now present in all 48 contiguous states (not in Alaska), southern Canada, Mexico, Central and South America and Caribbean Islands. West Nile virus also is widely distributed in Africa, Middle East, southern Europe, the former Soviet Union, India, and Indonesia. West Nile virus is present in many species of birds. Many infected birds are asymptomatic but others, especially crows and jays, become sick and die and thus possibly are an indicator of disease in an area. Horses infected by West Nile virus may become ill and die. Occurrence of equine cases are a good indication of West Nile virus transmission in a locality. An equine vaccine is available. The virus is transmitted among birds and to humans mainly by the culex mosquito but also may be transmitted by blood transfusion, organ transplantation, or occasionally transplacentally to a fetus. 