West Nile Virus


About the Virus

Clinical Signs






Zoonotic Risk

Veterinarian's Role

Helpful Links


west nile

What is West Nile Virus (WNV)?

West Nile Virus is a mosquito-borne flavivirus that is commonly found in tropical and temperate regions like Africa, West Asia, and the Middle East. It is closely related to the viruses that cause Yellow Fever, St. Louis encephalitis, and Dengue fever. Wild birds act as the main reservoir, and the virus is spread by mosquitoes that have fed on blood of the infected birds. It can infect humans, reptiles, mammals, and many avian species.


Group: Group IV (+) ssRNA virus
Family: Flaviviridae
Genus: Flavivirus

West Nile Virus belongs to a viral family called Flaviviridae. There are three genera in this family including Flavivirus (type species Yellow Fever), Hepacivirus (type species Hepatitis C), and Pestivirus (type species Bovine Virual Diarrhea). Among these three genera WNV belongs to genus Flavivirus.



West Nile Virus is a small (approx, 50 nm diameter) spherical lipid enveloped virus. It contains a positive, single-stranded RNA genome that is packaged within the core protein C. The viral envelope protein is made of E proteins and M proteins which are embedded in a lipid bilayer. The E proteins play roles in virion assembly, cell receptor recognition, cell receptor recognition, fusion with cell membranes, agglutination of RBC and induction of immune mediated responses.

wnv structure


West Nile virus was first isolated from a feverish adult woman in the West Nile district of Uganda in 1937. The ecology was characterized in Egypt in the 1950s. The virus was recognized as a cause of severe human meningioencephalitis in elderly patients in Israel in 1957. Equine cases were first reported in Egypt and France in the 1960s. The disease has spread throughout the world including Africa, Europe, the Middle East, Central Asia and most recently, North America. It has been detected in humans, animals, and mosquitoes in all of these regions.

WNV first appeared in North America in 1999 in New York City. Since then it has been found in 47 States. The virus was first detected in Canada in dead birds found in Southern Ontario in 2001. It has spread from Ontario, to Quebec, Manitoba, Saskatchewan, and Alberta.

wvn map


Wild birds are the main reservoir for West Nile virus and the virus is spread by mosquitoes. There are about forty species of mosquitoes in North America that have known to be able to carry the virus. Some of the common carrier species are Culex Quinquefasciatus, Culex Pipiens, Culex Restuans, Aedes Albopictus, and Aedes Vexans.

                                       Aedes Albopictus                    Culex Quinqufasciatus
Mosquitoes contract WNV when they feed on blood of infected birds. The infected blood circulates within a mosquito's salivary glands for a few days. After an incubation period of 10 to 14 days, infected mosquitoes can then transmit WNV to humans, other birds, and other vertebrate animals while biting to take blood. During the feeding, the virus passes from the mosquito’s salivary glands into the bloodstream of the bitten animal where it then multiplies and may cause disease.


Species Affected

West Nile virus has been found more than 150 species of wild birds in North America including crows, blue jays, magpies, ravens, robins, hawks, falcons and owls. Some of these species will not show any symptoms once they are infected but others will often get sick and die.

The virus has been also reported to infect humans, horses, dogs, cats, squirrels, rabbits, bats, skunks, chipmunks, and reptiles. It has been suggested that humans and mammals are incidental or dead end hosts, because they do not carry much WNV in their blood and thus do not act as an effective source of infection for mosquitoes.



Wikipedia: The Free Encyclopedia. http://en.wikipedia.org/wiki/West_Nile_virus

Canadian Food Inspection Agency. http://www.inspection.gc.ca/english/anima/heasan/disemala/wnvvno/wnvfse.shtml

Wikipedia: The Free Encyclopedia. http://en.wikipedia.org/wiki/Flaviviridae

Public Health Agency of Canada. http://www.phac-aspc.gc.ca/msds-ftss/msds175e.html

University of Minnesota : Environment & Occupational Health.

Russel Knightley Media: Scientific Illustration & Science Animation.

Flavivirus Pictures from CDC. http://www.lib.uiowa.edu/hardin/md/cdc/2290.html

Center For Disease Control: Emerging Infectious Diseases.

West Nile Virus. http://www.west-nile-virus-prevention.com/index.html

Community Health Administration: West Nile Virus. http://edcp.org/html/wn.html

Image Referecnes

DuPage County Health Department Illinois WNV brochure: http://www.dupagehealth.org/ehs/wnv/brochure.asp

University of Wisconsin Introduction to The Microbial World: http://www.bact.wisc.edu/themicrobialworld/intro.html

West Nile Virus: http://www.west-nile-virus-prevention.com/history.html

University of Minnesota : Environment & Occupational Health. http://enhs.umn.edu/hazards/hazardssite/westnilevirus/molaction.html

Food and Envrionmental Hygiene Department: The Government of the Hong Kong Special Administrative Region

Digiscoped Images of British Birds: http://homepage.ntlworld.com/andy.holt/digiscope/magpie/magpie.htm

Birds of the Kaweah River Delta Region: http://kaweahoaks.com/html/crow.html

PCforum SK: http://baby.by.ru/Cliparts/rabbit.jpg

Soque River Watershed Association: http://www.soque.org/Photo_Album/photo_gallery.htm

Maryland Department of Health and Mental Hygiene: http://edcp.org/html/wn.html

River Valley Arabians: http://www.rivervalleyarabians.com/mares/arabian_horses.html