Taxonomic name: West Nile Virus (WNV)
Synonyms:
Common names: West Nile virus (English)
Organism type: micro-organism West Nile virus is a member of the flavivirus genus, a genus of arboviruses (arthropod borne viruses) spread by mosquitoes and known to cause disease in humans and animals. In the United States, birds appear to be the major carrier of the disease. When certain Culex spp. mosquitoes feed on the blood of infected birds, the insects ingest the virus. When these mosquitoes bite people, they can pass the virus into humans. Most people infected with West Nile virus have only mild illness. In the worst cases, however, the virus causes encephalitis, an inflammation of the brain. No specific medication exists to treat West Nile virus infection, and no vaccine is available to prevent it. Control measures focus on reducing mosquito breeding habitat: standing water in urban areas, agricultural areas, and wetlands. Description
According to Solomon et al. (2003), like other flaviviruses, West Nile virus is a small virus, with a single stranded, positive sense RNA genome comprising about 11,000 nucleotides wrapped in a nucleocapsid and surrounded by a lipid membrane. Under a microscope the virions appear roughly as spheres 40-65 nm in diameter. Similar Species
Japanese encephalitis virus, Kunjin Virus, Murray Valley encephalitis virus, St. Louis encephalitis virus
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Occurs in:
agricultural areas, urban areas, wetlands Habitat description
According to the CDC (2003), the virus can infect people, horses, many types of birds, and some other animals. The primary vectors for West Nile Virus in the United States are the Culex sp. of mosquito, which commonly breed in urban areas and prefer to feed on birds. Mosquitoes thrive wherever standing water exists, including wetland and agricultural areas. General impacts
According to the CDC (2003), most people infected with the West Nile virus will not have any type of illness. It is estimated that 20% of the people who become infected will develop West Nile fever and display mild flu-like symptoms, including fever, headache, body aches, and occasionally a skin rash on the trunk of the body and swollen lymph glands. More severe cases of this virus can be called West Nile encephalitis, West Nile meningitis, or West Nile meningoencephalitis. Encephalitis refers to an inflammation of the brain, meningitis is an inflammation of the membrane around the brain and the spinal cord, and meningoencephalitis refers to inflammation of the brain and the membrane surrounding it. Symptoms of West Nile encephalitis (or meningitis) include headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, and paralysis. More than 3500 cases and 200 deaths from West Nile virus were reported in the United States in 2002. NJDA (2001) states that this disease has had a devastating effect on the equine community. Between 1999 and 2002, many horses have been clinically affected with West Nile fever and many of those horses died. The economic and medical losses to the owners and the industry are significant. The CDC (2003) reports that over 110 species of birds are known to have been infected with West Nile virus. Infected birds, particularly crows and jays, can die or become ill, but most survive. Geographical range
West Nile Virus was first discovered in Uganda in 1937, and it commonly occurs in Africa, the Middle East, parts of Asia and Australia, and parts of Europe (CDC, 2003). It was discovered in the United States in 1999 (Solomon et al., 2003). The geographic range of the virus has since increased rapidly, and now includes 44 states, the District of Columbia, and a number of Canadian provinces (CDC, 2003). Introduction pathways to new locations
Other: West Nile virus is introduced to new locations through infected birds (CDC, 2003).
Local dispersal methods
Consumption/excretion: When Culex spp. feed on the blood of infected birds, the mosquitoes ingest the virus and can pass the virus into humans (CDC, 2003). Management information
Preventative measures: According to the CDC (2003), the most effective and economical way to control West Nile virus is to control mosquitoes by larval source reduction through locally funded abatement programs that monitor mosquito populations and initiate control before disease transmission occurs. No vaccine or specific treatment exists to prevent or combat the West Nile virus infection. Treatment of severe illness includes hospitalization, use of intravenous fluids and nutrition, respiratory support, prevention of secondary infections, and good nursing care. Medical care should be sought as soon as possible for persons who have symptoms suggesting severe illness. Individuals should reduce their contacts with mosquitoes by taking the following actions: 1) when outdoors, clothing should be worn that covers the skin, such as long sleeve shirts and pants, 2) effective insect repellent containing DEET (N,N-diethyl-meta-toluamide) should be applied to clothing and exposed skin, and 3) outside activity should be curbed during the hours that mosquitoes are feeding, which often includes dawn and dusk. An effort should also be taken to reduce the number of mosquitoes in outdoor work or play areas by draining sources of standing water, which will reduce mosquito breeding habitat. In addition, screens should be applied to doors and windows and regularly maintained to keep mosquitoes from entering buildings. Reproduction
As described by the Solomon et al. (2003), replication occurs in the cytoplasm of all host cells and takes 20-30 hours for the assembly and release of flaviviruses. Assembly occurs during budding of the host cell, characteristically into cytoplasmic vacuoles rather than the cell surface as occurs with its arbovirus cousin, the togavirus. Release occurs when the cell lyses. Reviewed by: Michael Holbrook. Department of Pathology, Center for Biodefense and Emerging Infectious Diseases, and Sealy Center for Vaccine Development, University of Texas Medical Branch, Galveston. USA
Compiled by: National Biological Information Infrastructure (NBII) & IUCN/SSC Invasive Species Specialist Group (ISSG)
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Last Modified: Friday, 31 March 2006
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