ISSN 2398-2969      

West Nile virus


Synonym(s): WNF, flavivirus




  • Phylum/division: viruses.
  • Class: RNA positive strand viruses.
  • Family:  Flaviviridae.
  • GenusFlavivirus.
  • Species:West Nile virus.


  • First isolated, in 1937, from a 37-year old woman from Omogo, in the West Nile district of Uganda.
  • Flavivirus: from the LatinFlavus, meaning yellow. 
  • Flaviviruses are named from the yellow fever virus, which causes jaundice.

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Clinical Effects



  • West Nile virus cycles between the host species and the mosquito vector.
  • Habitat therefore reflects the species infected.
  • In Europe and Africa, the principal vectors areCulex pipiens,Culex univittatusandCulex antennatus, and in India, species of theCulex vishnuicomplex.
  • In the US,Culex pipiensis the principal vector in the eastern US,Culex trasalisin the midwestern and western US, andCulex quinquefasciatusin the southeastern US.
  • In laboratory studies, species in the orders Passeriformes (song birds), Charadriiformes (shorebirds), Strigiformes (owls) and Falconiformes (hawks) developed viremia levels sufficient to infect most feeding mosquitoes, whereas species of Columbiformes (pigeons), Piciformes (woodpeckers) and Anseriformes (ducks), did not.


  • Following inoculation of the virus by an infected mosquito into a vertebrate host and a period of incubation, viremia can develop.
  • Initial replication is thought to take place in dendritic cells at the site of inoculation.
  • The virus then passes through the lymphatic system and into the vascular system, where it then spreads to peripheral organs.
  • Following attachment to a cell receptor, the virus enters cells, releases its nucleocapsid and commences RNA translation. Cell infection and viral replication lead to viremia.
  • Many bird species develop sufficient levels of viremia to infect mosquitoes feeding on them during the period of viremia.
  • Different bird species differ markedly in the length and level of viremia with significant impact on their suitability as reservoir hosts.
  • In Europe, WNV circulates through a rural, sylvatic, cycle, with transmission between wild, usually wetland birds and ornithophilic (bird-loving) mosquitoes.
  • Other cycles are also possible.


  • The principal vectors of WNV are mosquitoes.
  • After feeding on an infected host, the virus can be transmitted to another host at a subsequent feeding.
  • Other modes of transmission have also been identified.
  • In humans, these include blood transfusions, organ transplants, conjunctival exposure to infected blood, occupational exposure, breast feeding and intra-uterine exposure.
  • Some species of crows, jays, magpies, gulls and other birds can also shed WNV in oral and cloacal secretions transmitting the virus directly.
  • Vertical (transovarial and transtadial) transmission may occur from one generation of mosquito to the next.
  • Ticks can transmit the virus.

Pathological effects

  • When the large number of humans and animals demonstrating antibodies to WNV are considered together with the low number of clinical cases, it appears likely that the number of individuals developing clinical signs following infection is very low.
  • This is probably true for rabbits.
  • In mammals other than man and the horse, clinical signs may include various non-specific signs, including lethargy and reduced appetite. Overt neurological signs such as head tilt/torticollis   Head tilt  , ataxia and incoordination, tremors, hind limb paralysis   Paresis / paralysis: limb  , tetraplegia, loss of righting ability and recumbency may be seen; pyrexia may also be present.


Control via animal

  • None required where WNV is not circulating in local mosquito populations.
  • Monitoring sentinel species will provide information on changing patterns of distribution.
  • The use of insect repellents is indicated in the face of an outbreak.

Control via environment

  • West Nile virus control is primarily achieved through targeting the mosquito vectors.
  • Elimination of mosquito breeding sites.
  • Factors promoting the reproduction of mosquito vectors including flooded basements and all other pools of stagnant water should be identified and eliminated.
  • The larvicidal treatment of breeding areas has been recommended.
  • The environmental impact of spraying highly toxic compounds must be weighed against the potential benefits. Currently this approach may play a role in eliminating the introduction of infected mosquitoes on airplanes arriving from endemically affected areas.
  • Ecological measures should not be overlooked; these include the use of larval predators.


  • No commercial vaccine available for use in rabbits, although vaccines have been produced to protect vulnerable equines.

Other countermeasures

  • The use of insect repellents   Therapeutics: parasiticides  such as permethrin, together with mosquito nets may be indicated in the face of an outbreak.

Permethrin and pyrethrin can be toxic to rabbits.


  • WNV can be destroyed by many disinfectants including sodium hypochlorite solutions, 2-3% hydrogen peroxide, 2% glutaraldehyde, 3-8% formaldehyde, 1% iodine.
  • WNV is inactivated by UV light and heat (30 min at 56°C/132.8°F).


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Reisen W K, Fang Y & Martinez V M (2006) Effects of temperature on the transmission of West Nile virus by Culex tarsalis (Diptera: Culicidae). J Med Entomol 43 (2), 309-317 PubMed.
  • Hayes E B, Komar N, Nasci R S et al (2005) Epidemiology and transmission dynamics of West Nile virus disease. Emerg  Infect Dis 11 (8), 1167-1173 PubMed.
  • Tiawsirisup S, Platt K B, Tucker B J et al (2005) Eastern Cottontail rabbits (Sylvilagus floridanus) develop West Nile virus viraemias sufficient for infecting select mosquito species. Vector Borne & Zoonotic Dis (4), 342-350 PubMed.
  • Spielman A, Andreadis T G, Apperson C S et al (2004) Outbreak of West Nile virus in North America. Science 306 (5701), 1473-1475 PubMed.
  • Service M W (1977) Ecological and biological studies on Aedes cantans in southern England. J Appl Ecol 14 (1), 159-196 VetMedResource.
  • Service M W (1971) Feeding behaviour and host preferences of British mosquitoes. Bulletin Entomol Res 60 (4), 653-661 PubMed.

Other sources of information

  • Lindenbach B D & Rice C M (2001) Flaviviridae: the Viruses and their Replication. In:Fields Virology. 4th edn. Eds: Knipe D M & Howley P M. Lippincott, Williams & Wilkins, Philadelphia.
  • Snow K R (1990) Mosquitoes. Naturalists Handbook Series. Richmond Publishers, London.

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