West Nile Virus (WNV)
Basic Information 
Definition
• A positive sense, single stranded, enveloped RNA virus, is a flavivirus of the family Flaviviridae.
• WNV is subcategorized as a member of the Japanese encephalitis virus group, which also includes St. Louis encephalitis virus.
• It is a vector borne virus, which is cycled between birds and ornithophilic mosquitos.
• WNV was first identified in 1937 from the blood of a woman in Uganda and since then has been the cause of several human epidemic and veterinary epizootics in northern Africa, southern Europe, and the Middle East. In 1999, WNV was identified as the cause of encephalomyelitis in several horses, birds, and humans in the United States. Subsequent to this, the virus has spread across the continental United States as well as portions of northern Mexico and southern Canada.
• House sparrows, crows, and birds of prey are the most common avian species affected.
• Horses and humans are aberrant hosts and appear to not be capable of amplification of the virus.
Typical Normal Range (US Units; SI Units)
• Normal IgM titer should be negative or <1 : 4.
• There are no published reference intervals for serology. A fourfold increase in titers between acute and convalescent blood samples is required for definitive diagnosis.
