About West Nile Virus

West Nile fever is a mosquito borne virus belonging to the Japanese encephalitis serogroup of flaviviruses (St. Louis encephalitis, Kunjin, and Murray Valley encephalitis viruses), transmitted among vertebrates and various Culex mosquito vectors, with humans and horses serving as incidental hosts, without further viral amplification. For many years, WNV was considered to be endemic, especially in Africa; however, sizeable epidemics, numbering in hundreds or thousands of cases have occurred in Africa and the Middle East. Most West Nile virus infections in humans are subclinical. Symptomatic disease occurs in approximately 1 of every 100 infections and is characterized by a mild febrile illness with petechial rash and headache. Severe neurologic disease, mostly meningitis or encephalitis, occurs in and 1 in 150 cases.

West Nile virus - an emerging pathogen

A series of outbreaks of neurologic WNV infection in humans have been reported recently: Romania 1996, Czech Republic in 1997, Russia in 1999, Israel in 2000. Smaller outbreaks of equine cases have been reported in Morocco in 1996, Italy in 1998, France and Israel in 2000. During 1999, WNV first established itself in the USA, initially in the Northeast, then spreading rapidly throughout North America and Canada. The 1999 outbreak in the NY metropolitan area resulted in 61 human cases including seven deaths. Exotic zoo birds and horses also were affected and had high death rate, in fact mortality in a wide variety of bird species has been a hallmark of WNV activity in the United States.

In addition, new modes of transmission were reported: through blood donations, organ transplant, transplacental transfer, and breast-feeding.[5,7,8,31-33]. The first confirmed West Nile infection death resulting from transmission through donated blood was reported in 2002 in US. In the fall of 2002, the U.S. Food and Drug Administration challenged the industry to develop a test for direct detection of WN in donated blood by the summer of 2003. Within nine months, Gen-Probe and Chiron had developed the Procleix WNV Assay and brought it to market under an IND.

WN virus can be divided genetically into two lineages (26-29). Only members of lineage 1 WN viruses (the lineage of the WN virus causing the human outbreak in South Africa in 1974 is under contention). Lineage 1 WN viruses have been associated with clinical human encephalitis and have been isolated from Africa, India, Europe, Asia, and North America. This lineage includes the first isolate from Romania in 1996 (ROM96), as well as the genetically distinguishable isolate responsible for the U.S. outbreak (NY99). Lineage 2 WN viruses are maintained in enzootic foci in Africa and have not been associated with clinical human encephalitis.

Currently, a licensed human vaccine is not available to prevent WN virus disease. Mosquito control is the only practical strategy to combat the spread of disease; however, effective spraying in an attempt to eliminate mosquitoes is difficult to perform in urban areas.