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Virus / Flaviviridae / Type:

Flaviviridae: West Nile Virus







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West Nile Virus Disease:

  • Epidemiology, Disease Characteristics & Diagnosis
  • Treatment & Control

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General and References

Virus Summary

West Nile virus is classified within the family Flaviviridae (Virus Family), genus Flavivirus (arbovirus group B). Within the Flaviviridae (Virus Family) it is found in the antigenically-related group of viruses known as the Japanese Encephalitis virus serocomplex, which includes Japanese Encephalitis (JE) virus, Murray Valley virus and St Louis Encephalitis virus among others.

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Alternative Names (Synonyms)

(Classification of virus types is an evolving discipline. The information in Wildpro has been carefully referenced to the source material, as far as possible. Readers requiring further clarification should consult the source materials and more recent publications. Classification information in Wildpro will be altered when clear and scientifically endorsed new information regarding taxonomic divisions becomes available to us.)

  • WNV
  • WN virus

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Associated Diseases

West Nile Virus Disease.
Linked Diseases

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TAXA Group (where information has been collated for an entire group on a modular basis)

Parent Group

Flaviviridae (Virus Family) (group B arboviruses).

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Species Author

Debra Bourne (V.w5)


Suzanne I. Boardman (V.w6); Becki Lawson (V.w26); Dr Robert G. McLean (V.w42); Charalambos Billinis DVM DrMedVet (V.w174), George Valikos DVM (V.w175)


Detailed references are provided attached to specific sections.

(USA Contacts for Managing WNV Disease)

(Further Reading)
Click image for full contents list of ELECTRONIC LIBRARY

Click here for further reading on "Managing for West Nile Virus"

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Structure & Identification

Virus Morphology

  • Flaviviridae (Virus Family): mean diameter 43nm (B240.14.w14); 35-55nm. (J97.30.w1)
  • West Nile virus: about 45nm diameter (B244.w1); about 500 Angstroms diameter, without prominent surface projections or spikes. (J22.302.w1)
  • Flaviviridae (Virus Family) 40-70nm diameter. (B81); flaviviruses are about 40-60nm in diameter, comprising an electron dense core of about 30 nm and a surrounding lipid bilayer. (B324.32.w32); nucleocapsid 25-30 nm diameter with a surrounding lipid bilayer derived from host membranes; total diameter 40nm with surface projections 5-10 nm. (J286.44.w1, B81)
No. of particle polypeptides Flaviviridae (Virus Family) - Flaviviruses:
  • At each end of the genomic RNA is a sequence not encoding viral proteins: the 5 non-coding region (NCR) and the 3'-NCR, about 100 nucleotides and 400-700 nucleotides long respectively. (B324.32.w32, J285.3.w1)
    • The 5' NCR appears to influence flavivirus genome translation. (B324.32.w32)
  • Between the non-coding regions is a single long open reading frame (over 10,000 bases long) coding for a polypeptide which is then translated and processed into, in order from the 5' end, three structural peptides: capsid (C), premembrane (PrM)/Membrane (M) and envelope (E) and seven non-structural proteins: NS1, NS2a, NS2b,NS3, NS4a, NS4b and NS5. (B324.32.w32, J285.3.w1, J286.44.w1)
  • The nucleocapsid or core protein C is about 12 kd, the nonglycosylated membrane protein M about 8 kd and the envelope protein E 53 kd. (B243.31.w1, B324.33.w33)
  • The nonstructural proteins include three large, highly conserved proteins (NS1, NS3, NS5) and four small hydrophobic proteins (NS2A, NS2B, NS4A, NS4B). (J286.44.w1)
  • The role of the C protein is thought to be in formation of a ribonucleic complex with packaged genomic RNA. The C protein is highly basic; basic residues are found particularly at the N- and C- termini; they probably act cooperatively in specific binding of genomic RNA. (B324.32.w32)
  • Both the M and E proteins are associated with the lipid envelope by hydrophobic membrane anchors. (B324.33.w33)
  • The E protein is the major component of the virion surface; this containing the important antigenic determinants relating to haemagglutination inhibition and neutralization; it is this protein which induces immunological responses in the infected host. (B324.33.w33)
    • This protein probably interacts with cell viral receptors and mediates fusion between the virus and the cell membrane. (B324.32.w32)

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Virus Genome

Nucleic acid type/No. of strands
No. of Molecules / Strandedness
Molecular weight
  • Flaviviridae (Virus Family): No poly (A) at 3' end
    No virion transcriptase. (B81)
  • Flaviviruses: 5' end has a type I cap (m7G'ppp5'A) but there is no polyadenylated tail. (J285.3.w1, J286.44.w1)
  • Following the 5' cap is a conserved dinucleotide sequence AG. (J286.44.w1)
  • 3' end terminates with the dinucleotide AC (mosquito-borne flaviviruses). (J286.44.w1)
  • In the 5' noncoding region of WN virus are three short sequences, six to seven bases long, repeated once. (J286.44.w1)

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Viral Type Diversity (Sub-type/Subspecies)

Recognised Sub-types The following editorial comment summarises detailed information given within the LITERATURE REPORTS. Links to the LITERATURE REPORTS are provided at the bottom of this box.
  • There are significant differences between isolates of WN virus from different parts of the world, and different isolates can be found within a given geographical region.
  • Two major lineages of WN virus have been recognised. Lineage 1 contains isolates from Africa, including isolates from north, west and central Africa, Europe, the Middle East, India and North America. Kunjin virus is now recognised to be a subtype of WN virus within Lineage 1, forming a monophyletic clade (1b). Isolates from India also form a monophyletic clade (1c) within Lineage 1. All other Lineage 1 isolates are within the clade 1a. A Kunjin virus isolate from Malaysia has been shown not to fit within lineage I or lineage II.
  • Lineage 2 isolates had until recently been found only within sub-Saharan Africa, Madagascar and the Middle East. However, lineage 2 isolates have recently been detected in Europe.
  • Additional lineages have now been detected, including Rabensburg virus, isolated from Culex pipiens mosquitoes from the Czech Republic near the Austrian border (lineage 3), LEIVKrnd88190, isolated from Dermacentor marginatus ticks in the northwestern Caucasus Mountains in 1998 (lineage 4) and lineage 5 from India. Strain HU2925/06 from mosquitoes in southern Spain in 2006 has been suggested to belong to yet another new lineage.
  • Phylogenetic studies have shown that a single strain, most closely related to a strain from a goose in Israel, reached North America in 1999; changes in the virus since 1999 can be used to map the spread of WN virus across North America.
  • More than one strain may be circulating in a given region at a given time: in Israel in 2000, some isolates from humans with encephalitis were closely related to the New York 1999 strain while others were more closely related to strains isolated in Russia in 1999 and in Romania in 1997.
  • Findings of different subtypes of WN Virus in the same location and the same subtypes in different locations are consistent with spread of the virus by migrating birds and by local movements of birds and mosquitoes.
  • A study found that there were highly neuroinvasive lineage 2 strains as well as lineage 1 strains.

(J20.298.w1, J20.309.w1, J22.286.w1, J22.286.w2, J84.8.w6, J84.7.w18, J84.7.w28, J84.7.w33, J84.9.w22, J84.9.w23, J84.11.w6, J84.12.w10J91.61.w2, J133.951.w34, J135.98.w1, J214.267.w18, J223.78.w1, J275.25.w1, J279.7.w4, J486.16.w5, J486.16.w4, B241.49.w4, B244.w1, P39.3.w11)

In vitro differences (Laboratory test: differentiation)
  • Differences were noted between seven strains isolated in Nigeria for whether or not plaque formation occurred in a given cell type (secondary monkey kidney cells), the time to plaque formation and the size of plaques formed. (J88.19.w3)
  • Differences have been shown in infectivity for VERO cells and the size of plaques produced in those cells between different isolates of WN virus. (B241.49.w49)
In vivo differences (Affected animal: variation in infectivity and target species)
  • There are recognised differences in infectivity of strains of WN virus for a given host. Differences between strains have been observed in terms of pathogenicity for adult mice and in the results of experimental infections in humans; however these may be related not only to differences in the original isolates but also to the level of passage and the system used for virus passage. (B241.49.w49)
  • Different strains of WN virus vary in their neuroinvasiveness in mouse and hamster models. Neuroinvasiveness is associated with particular subtypes of the virus within both lineage 1 and lineage 2, containing a glycosylated envelope protein. (P39.3.w9, J214.267.w18)
  • There appears to have been a recent change in the virus which began in 1998 and resulted in morbidity and mortality of infected birds; prior to that time there had been only one report of clinical illness in a bird naturally infected with WN virus. (J64.19.w1)
  • Recently, neuroinvasive strains of lineage 2 virus have been found. (J20.296.w1)
CLICK THE LINKS FOR Literature Reports

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Virus Detection and Identification

Editorial Comment The following editorial comment summarises detailed information given within the LITERATURE REPORTS. Links to the LITERATURE REPORTS are provided at the bottom of this box.

See also:


  • The appropriate samples to be collected vary depending on whether diagnosis or virus detection is required from mosquitoes, dead wild birds, live sentinels, equines or humans.
  • Accurate labelling of samples is important.
  • Prior to collecting and shipping samples the laboratory to which the samples are to be sent should be consulted regarding the details of what samples are required, preservation required, shipping and labelling instructions. Detailed instructions are available from CDC.

(D67, D72, D147, P39.4.w9, P39.3.w15)


  • Serological assays for arboviruses include IgM ELISA, IgG ELISA, PRNT, CF, HI, IFA (immunofluorescent antibody) and dipsticks. (P39.4.w12)
  • Enzyme-linked Immunosorbent Assay (ELISA) techniques are in general used to detect antibodies and frequently replace the Haemagglutination Inhibition, Complement Fixation and Neutralisation Tests in many situations. The IgM capture ELISA (MAC-ELISA) is in current use for WN virus antibody confirmation. (B244.w1, B245.29.w29, D72)
    • An ELISA has been evaluated to detect antibody in wild birds, for use as a rapid screening test (the PRNT would be required for confirmation of results). (J84.8.w9)
    • Use of certain monoclonal antibodies can increase the sensitivity of the IgM ELISA in horses. (J485.122.w1)
  • Haemagglutination Inhibition (HI) is a serological test for WN virus antibody and, although it is still in use in many laboratories, there is an increasing tendency with time for sera to cross-react with other virus antigens within a given virus family; this test is gradually being replaced by ELISA techniques. (B245.29.w29)
    • HI may still be used for screening as it provides results within 24-48 hours. (J91.32.w1, V.w42, W181.28Jan04.WNV3)
    • This test has the advantage for surveillance of domestic and wild animals that a species-specific conjugate is not required. (P32.1.w1)
    • It has been noted that for flaviviruses HI tests on chicken sera are valid following an acetone extraction method. (J93.41.w2)
  • The Plaque Reduction Neutralization Test (PRNT) is a serological test for WN virus antibody and, although expensive and problematic, is useful to differentiate between infection of two viruses which are closely related antigenically. (B245.29.w29).
    • The PRNT is the confirmatory test for identification of WN virus specific antibody.
  • The Indirect Immunofluorescent Antibody (Indirect IFA) is a serological test for WN virus antibody. It uses an indirect format, therefore there is a problem with IgG competition for IgM binding. It is not a preferred test for serological diagnosis. (B245.29.w29). However it may be useful in laboratories without the time, funds or technical expertise for utilising ELISA. (J133.951.w43)
  • The Complement Fixation Test (CF) can be used to detect viral antibodies. Whilst it is more difficult to maintain quality control for this test compared with the haemagglutination inhibition test, it may still be used to detect complement fixing antibodies that may be present after the IgM has waned. (B244.w1, B245.29.w29)
  • An Immunoperoxidase Monolayer Assay (IPMA) has been described for testing human sera against several arboviruses, including WN virus, using a reaction readable visually; this may be of use for screening human sera in field situations. (J217.65.w1)

Comments on serological detection of antibody:

  • "For most arboviruses, serological cross-reactivity with related viruses increases as the infection progresses." (B245.29.w29)
  • Cross-reactions with related Flaviviruses occur in serological tests. (B244.w1)
  • It is important to remember, in areas in which more than one flavivirus occurs, that in the event of infection by one flavivirus and later a second flavivirus, the antibody titre in serum taken at the time of the second infection may be higher for the virus which caused the first infection than for that causing the current infection. (B244.w1)
  • Paired serum samples (acute-phase and convalescent phase) are useful, allowing the detection of a rise in antibody titre and, for patients in whom the acute-phase sample was taken too early for IgM antibodies to be detected, both IgM and IgG antibodies are likely to be found in the convalescent-phase serum sample. (B245.29.w29)
  • It is possible that if WNV Infection follows infection with another Flavivirus, IgM levels  measured by MAC-ELISA may give equivocal results. (J84.8.w1)
  • In the USA cross-reaction may occur with St Louis Encephalitis virus (SLE) in particular, both being members of the Japanese Encephalitis group of flaviviruses. (J214.267.w11)


  • Virus detection assays for arboviruses include virus isolation (in cell culture or mice), IFA, TaqMan RT-PCR, antigen-capture ELISA, RT-PCR/sequencing, dipsticks and NASBA. (P39.4.w12)
  • Using Cell Culture techniques, WN virus can be grown on a variety of mosquito (Culicidae - Mosquitoes (Family)) cell lines, mammalian cell lines and in suckling mice. In mosquito cell lines a cytopathic effect may not be seen and the culture needs to be screened by immunofluorescence [see section below] (B241.49.w49, B243.31.w1, B245.29.w29, J64.19.w1, J73.57.w1, J84.8.w7, J110.39.w1, D147)
  • A micro-virus neutralisation test (micro-VNT) has been used. (J6.36.w1)
  • A variety of Reverse Transcriptase - Polymerase Chain Reaction (RT-PCR) protocols, both standard and nested, have been described for detection of WN virus RNA. (J84.7.w25, J110.39.w1, J217.94.w1, B245.29.w29, D72)
    • PCR protocols can be used on a variety of sample types, including clinical samples such as CSF, post mortem tissue samples, and mosquito pools. 
    • PCR assays are highly sensitive but are susceptible to false positive results due to inadvertent contamination.
    • The TaqMan RT-PCR is a highly sensitive test for detection of virus RNA.
  • A Nucleic Acid Sequence-Based Amplification (NASBA) assay has been developed with very high sensitivity and specificity, and allowing a rapid test result. It may be useful to complement virus isolation and TaqMan PCR testing or even as an alternative to TaqMan. (J93.40.w3)
  • The Indirect Haemagglutination test was developed for rapid detection of WN virus antigen in a viraemic host, although the sensitivity is probably lower than virus isolation techniques. (J88.22.w1, B241.49.w49)
  • An indirect IFA using IgM MAb specific for WN virus envelope protein followed by goat anti-mouse IgM fluorescein conjugate has been used for detection of virus in cell culture and tissues. It was found by the Arbovirus Laboratory, Wadsworth center, NYSDH, that the indirect IFA was a valuable confirmatory assay for use on bird tissues but that non-specific binding made it problematic for use on mammalian tissues. (J93.41.w4)
  • Direct Immunofluorescence techniques are used for detecting WN virus antigen in cells, impression smears and cell culture. (J26.37.w1, J71.75.w1, J73.57.w1, J84.7.w29)
    • Immunohistochemistry (IHC) is extremely useful for histopathological studies in vertebrates and is particularly useful for use as a screening test in diagnostic laboratories which do not have BSL-3 facilities. (V.w42)
  • An ELISA based on a monoclonal antibody has been developed for detection of WN virus antigen in samples of mosquitoes and bird tissues. 
    • The VecTestTM dip stick test is an antigen-capture ELISA which may be used on mosquito homogenate, avian tissue, and oral or cloacal swabs from avian carcasses The test was shown to have a high specificity but was less sensitive than TaqMan or plaque assay. (P39.4.w10)

(J26.37.w1,J64.19.w1, J71.75.w1, J73.57.w1, J84.8.w1, J84.7.w29, J84.8.w9, J84.9.w18, J84.9.w21, J88.22.w1, J91.32.w1, J93.38.w4, J93.40.w3, J93.41.w4, J217.65.w1, B244.w1, B241.49.w49, B243.31.w1, B245.29.w29, D72, P39.4.w10, P39.4.w12,  P51.49.w2,  V.w42)

Further details of various tests, and their references, are provided in the literature reports below.

CDC recommended tests for various samples are:

  • Human serum or CSF: In 2003 first choice tests ELISA and PRNT, other possibilities TaqMan, NASBA and virus isolation; TaqMan may detect in 57% of acute CSF but less than 10% of serum samples. (P39.4.w9)
    • In 2002 first choice tests were ELISA and PRNT, second choice tests HI and IFA; TaqMan may be useful in acute CSF samples.(P39.3.w15)
  • Chicken serum: first choice tests ELISA and PRNT, second choice tests HI and IFA.[2002](P39.3.w15)
  • Equine serum: first choice tests ELISA and PRNT, second choice tests HI and IFA.[2002](P39.3.w15)
  • Human tissue: In 2003 first choice tests TaqMan and NASBA, other possibilities virus isolation and immunohistochemistry. In fatal cases approximately 100% of cases are positive by TaqMan and NASBA. (P39.4.w9)
    • In 2002 first choice tests were TaqMan, NASBA and virus isolation, second choice tests immunohistochemistry and standard RT-PCR (N.B. TaqMan and NASAB are more sensitive than virus isolation). 
  • Avian tissue: In 2003 first choice tests TaqMan, NASBA and virus isolation, second choice tests VecTest, Antigen capture ELISA RT-PCR; note that antigen-based tests require 100 plaque forming units. (P39.4.w9)
    • In 2002 first choice tests were TaqMan, NASBA and virus isolation, second choice tests antigen-capture ELISA and RT-PCR.(P39.3.w15)
  • Equine/other species tissues: first choice tests TaqMan, NASBA, nested RT-PCR and virus isolation, second choice tests standard RT-PCR.[2002](P39.3.w15)
  • Mosquito pool: In 2003 first choice tests TaqMan, NASBA or virus isolation, second choice tests VecTest, antigen-capture ELISA or RT-PCR. (P39.4.w9)
    • In 2002 first choice tests were TaqMan, NASBA and virus isolation, second choice tests antigen-capture ELISA and RT-PCR. (P39.3.w15)
  • N.B. for serological assays sera (or human CSF) tested initially by IgG-ELISA or IgM-ELISA should be confirmed by PRNT against WN virus, St. Louis encephalitis virus and dengue. (P39.3.w15)
CLICK THE LINKS FOR Literature Reports
Types of Techniques recorded as useful for viral identification
  • Enzyme-linked Immunosorbent Assay (ELISA)
  • Hemagglutination Inhibition (HI)
  • Plaque Reduction Neutralization Test (PRNT
  • Indirect Immunofluorescent Antibody (Indirect IFA)
  • Complement Fixation Test (CF
  • Immunoperoxidase Monolayer Assay ( IPMA
  • Cell Culture
  • Reverse Transcriptase - Polymerase Chain Reaction (RT-PCR)
  • Nucleic Acid Sequence-Based Amplification (NASBA)
  • Indirect Hemagglutination
  • Direct Immunofluorescence / Immunohistochemistry

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Associated Host Species and Hazard / Risk

Definitive Host Species (Agent undergoes final stage of replication for transmission)

Editorial Summary for Degree of Infectivity for other Species The following editorial comment summarises detailed information given within the LITERATURE REPORTS. Links to the LITERATURE REPORTS are provided at the bottom of this box.

A wide variety of vertebrate species may be infected with West Nile virus. However, the length of time for which the infected individual is viraemic (during which transmission to the intermediate host could occur) and the level of viraemia (affecting the likelihood of such transmission) is highly variable between species. Viraemia sufficient for transmission of infection may occur in a variety of bird species, whereas it appears that many mammal species, although becoming infected and potentially acting as definitive hosts, may be seen more specifically as "dead-end" hosts, in which viraemia is of too low a level and too short a duration to be likely to result in further transmission of the virus.


  • A wide variety of mammals have been recorded to be infected with West Nile virus, either by virus isolation or more frequently by detection of antibodies. With the exceptions of equines (Equidae - Horses (Family)) and humans (Homo sapiens - Human), natural infection appears to be associated with clinical disease only rarely. Levels of viraemia in mammals are generally low and in most species probably are only rarely sufficient to infect mosquitoes.


  • Virus has been detected in a very wide range of bird species representing 17 of the 23 orders of birds. However until 1997 clinical disease from natural infection had only been observed in one pigeon. After this date bird morbidity and mortality was observed initially in WNV disease outbreaks in domestic geese (Anser anser domesticus - Domestic goose) in Israel. Widespread mortality of birds occurred following the introduction of WN virus to the New York area, with the first fatalities observed in 1999, particularly in Corvus brachyrhynchos - American Crow and other corvids (Corvidae - Crows, Birds-of-Paradise etc. (Family)), and the list of species (many native species, but also non-native captive birds) clinically affected has increased as the virus has spread across North America.

Amphibians and Reptiles:

(References available in the detailed literature reports below)

CLICK THE LINKS FOR Literature Reports of Species Infected
ORDERS recorded overall as containing Definitive Host Species (incl. Experimental, captive and free-ranging) (Not including infection unconfirmed by Laboratory diagnosis)




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Intermediate Host and Vector Species (Agent uses an intermediate species for development and/or specific indirect transmission)

Editorial Summary for Degree of Infectivity for other Species The following editorial comment summarises detailed information given within the LITERATURE REPORTS. Links to the LITERATURE REPORTS are provided at the bottom of this box.

A large number of mosquito species (Culicidae - Mosquitoes (Family)) have been found infected with West Nile virus and/or have been shown capable of transmitting the disease in the laboratory. Several species of ticks (Argasidae - Soft ticks (Family) and Ixodida - Hard ticks (Family)) have also been shown to be potential vectors of the virus. The virus has been detected in blooded louse flies (Hippoboscidae - Keds, Louse-flies etc.(Family)) taken from a WN virus-positive bird and it is possible that there is a role for bird-feeding hemipteran bugs, mites, and ticks in transmission of WNV between birds. Other species of arthropods have been infected experimentally.

(References are available in detailed literature reports below)

CLICK THE LINKS FOR Literature Reports of Species Infected
Species ORDERS Reported (Not including infection unconfirmed by Laboratory diagnosis)

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Paratenic Species (Agent can survive on or in the species, but there is no replication or further development)

Editorial Summary for Degree of Infectivity for other Species The following editorial comment summarises detailed information given within the LITERATURE REPORTS. Links to the LITERATURE REPORTS are provided at the bottom of this box.

Paratenic hosts are not considered relevant to the epidemiology of WNV Infection.

CLICK THE LINKS FOR Literature Reports of Species Infected
Species ORDERS Reported (Not including infection unconfirmed by Laboratory diagnosis)
  • Not applicable.

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Degree of Hazard (Risk to Humans / other Species)

  • Biosafety level 3 (BSL-3). (J89.16.w1, P32.1.w1, D67)
  • "WN virus is classified as a BSL3 agent by the Subcommittee on Arbovirus Laboratory Safety (SALS) of the American Committee on Arthropod-Borne Viruses, and CDC. Therefore, it is recommended that laboratory investigations that involve handling of infectious virus require BSL3 containment." (D67)
  • Further information regarding the potential use of BSL2 facilities, with modifications, and the types of materials which might be processed and tests which might be conducted in such facilities, are given in: Epidemic/Epizootic West Nile Virus in the United States: Revised Guidelines for Surveillance, Prevention, and Control. (D67)
  • More than 20 cases of laboratory-acquired infection were reported up to 2002. (W181.28Jan04.WNV6)
    • Potential sources of infection in the laboratory include serum, CSF, other tissues and infected arthropods. (W181.28Jan04.WNV6)
    • Primary hazards are from procedures producing aerosols and from percutaneous inoculation; there are special hazards from animals (including mosquitoes) which are either naturally or experimentally infected. (W181.28Jan04.WNV6)
  • All handling of human and animal clinical specimens should take place in (minimum) Containment Level 2 facilities using Containment Level 3 operational practices. (W181.28Jan04.WNV6)
  • For virus isolation and for laboratory manipulation of WN virus Containment Level 3 facilities are recommended. (W181.28Jan04.WNV6)
Biological Containment Level - USA
  • Biosafety level 3 (BSL-3). (J89.16.w1, P32.1.w1, D67)

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Virus Life Cycle, Transmission, Physical/Chemical Factors and Biogeographical - Climatic Range

Life Cycle and Transmission (General cycle of replication and mechanisms of moving between hosts and habitats)

Editorial Comment The following editorial comment summarises detailed information given within the LITERATURE REPORTS. Links to the LITERATURE REPORTS are provided at the bottom of this box.


  • Following inoculation of WN virus by a mosquito (Culicidae - Mosquitoes (Family)) (normal route of infection) into a vertebrate host, and an incubation period, a period of viraemia develops. Many bird species develop a level of viraemia sufficient to infect mosquitoes feeding on them during the period of viraemia (usually one to four days), although different bird species vary considerably in the level and length of viraemia and therefore their suitability as reservoir hosts for infection of mosquitoes. Most mammal species (Mammalia - Mammals (Class)) generally do not develop viraemia of a sufficient titre for effective transmission to mosquitoes (i.e. they are 'dead-end' hosts) however some species may develop viraemia sufficient to transmit infection to mosquitoes. Birds (Aves - Birds (Class)) are therefore recognised as the major source of WN virus for infection of mosquitoes and thereby of other vertebrate hosts, including humans (Homo sapiens - Human) and horses (Equus caballus - Domestic horse). At least some species of amphibians and reptiles may also be suitable hosts for infection of mosquitoes which feed on them.
  • As virus may be found in skin and other organs of birds after the viraemic stage of infection it is possible that infected birds may continue to act as a source of virus for mosquitoes/ticks or for other animals (by ingestion) for a longer time.
  • A variety of bird species have been shown to shed virus orally and/or cloacally following infection.
  • It has been shown that infected mice and sparrows contain sufficient virus to infect predators which eat them.


  • The principle vectors of WN virus are mosquitoes which feed on an infected host and after a period of time, during which the mosquito can travel some distance, transfer the virus to a new host when they next feed.
  • Findings of different subtypes of WN Virus in the same location and the same subtypes in different locations are consistent with spread by migrating birds.
  • In Europe, the principle cycle of WN virus circulation is a rural, sylvatic, cycle, with transmission between wild, usually wetland birds and ornithophilic (bird-loving) mosquitoes. A second, urban, cycle may also occur (as in the outbreak in Bucharest, 1996-97), between synanthropic or domestic birds and mosquitoes (mainly Culex pipiens/molestus) (Culex pipiens complex - Northern and Southern house mosquitoes) feeding on birds and on humans.
  • A bird-tick (Argasidae - Soft ticks (Family) or Ixodidae - Hard ticks (Family)) cycle may occur in some dry warm habitats which do not contain mosquitoes.
  • It has been suggested that the rate and pattern of spread in North America can be explained by dispersive movements of Passer domesticus - House sparrow.
  • Vertical (transovarial and transtadial) transmission may occur from one generation of mosquitoes to the next generation.
  • Transmission in humans by means of organ transplants and blood transfusions has been documented.
  • Intrauterine infection (transplacental transmission) and transmission via breast milk have been documented in humans.
  • Transmission in the laboratory due to accidental inoculation of humans by contaminated sharps has been documented. 
  • Transmission to birds via ingestion of virus solution, infected mosquito or infected vertebrate host has been demonstrated experimentally.
  • Transmission to cats by ingestion of infected mice has been demonstrated.
  • Bird-to-bird transmission has been demonstrated in the absence of mosquitoes to act as vectors.
  • Virus has been found in the ovaries of birds which have survived infection, presenting the possibility of transovarial transmission in birds.


  • The virus usually enters the definitive host in a low dose through peripheral inoculation as the mosquito (or tick) feeds.
  • Bird to bird transmission has also been recorded where the virus may enter the definitive host by the oral or intra-tracheal route or possibly through skin damage. 
    • The precise route of bird to bird transmission is unclear although virus has been found in oral / cloacal swabs experimentally.

(J64.19.w1, J71.54.w1, J84.5.w2, J84.7.w22, J84.7.w22, J84.9.w2, J91.62.w1, J91.65S3.w1, J110.39.w4, J110.39.w5, J127.46.w1, J133.951.w9, J133.951.w37, J214.267.w9, J223.78.w1, J269.94S.w1, B240.14.w14, B241.49.w49, B243.31.w1, B244.w1, B324.33.w33, P38.2001.w1, W8.Nov01.WNV8, W27.29Sept02.wnv1, W27.04Oct02.wnv1, W27.19Dec02.wnv1, W181.28Jan04.WNV6, W380.Dec02.wnv1, N7.51.w2, N7.51.w3, N7.51.w4, N7.51.w5, N7.51.w7, N7.51.w8) 


  • Initial replication is thought to occur in dendritic cells at the site of inoculation, followed by movement to lymph nodes and then to the vascular system. In the vascular system the virus spreads to peripheral organs (e.g. spleen, live, heart, kidneys, lungs). The exact method by which virus enters the CNS is not clear.


  • The virus attaches to the cell via a receptor, enters the cell and the nucleocapsid is released.
  • The viral RNA is translated into three structural and seven non-structural proteins.
  • The positive sense viral RNA is translated into negative sense RNA and this in turn acts as a template for synthesis of more viral positive sense RNA.
  • Viral RNA and the C protein interact to form the nucleocapsid precursor; the envelope is thought to be acquired by budding into the endoplasmic reticulum.
  • Immature virions are thought to be transported via the secretory pathway to the cell surface where release of mature virions occurs.

(J214.267.w9, J285.3.w1, J286.44.w1, B240.14.w14, B324.32.w32, B324.33.w33, P48.1.w8) 

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Editorial Overviews Available

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Chemical  Toxicities / Disinfectants

Editorial Comment Flaviviridae (Virus Family) in general are inactivated by:
  • Lipid solvents (e.g. ether, chloroform, betapropiolactone, aldehydes, SDC (sodium deoxycholate), CHCl3)
  • Urea
  • Disinfectants (3% to 8% formaldehyde, 2% glutaraldehyde, 2% to 3% hydrogen peroxide, 500-5,000 ppm available chlorine, alcohol, 1% iodine, and phenol iodophors)
  • Enzymatic digestion (lipases and proteases; including caseinase, trypsin, chymotrypsin, and papain - this process can preserve certain antigenic reactivities, depending on the degree of breakdown of the virus particles)

(J97.30.w1, J116.5.w1, B240.14.w14, B243.31.w1)

West Nile Virus has been shown to be inactivated by:

  • Detergent-containing buffer after a period of 30 minutes at 37C (but not after 15 minutes). (J93.40.w2)
  • Susceptible to disinfectants including 3-8% formaldehyde, 2% gluteraldehyde, 2-3% hydrogen peroxide, 500 to 5000 ppm available chlorine, alcohol, 1% iodine, phenol iodophores, other organic solvents/detergents. (W181.28Jan04.WNV6)
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Physical Susceptibility (Inactivation)

Editorial Comment Flaviviridae (Virus Family) are readily inactivated by:
  • Ultraviolet light
  • Temperatures over 56C
  • pH 6.0 and below

Optimum conditions of Flaviviridae (Virus Family) stability include:

  • Storage at -60C
  • pH 8.4 to 8.8

(B240.14.w14, B243.31.w1, B245.29.w29, J93.40.w2, J97.30.w1, J120.20.w1)

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Environments - External Habitats (Biogeographical / Climate Type)

Editorial Overview
  • The geographical range of WN virus as indicated by virus isolation includes temperate, subtropical and tropical zones. High endemic or epidemic activity has been reported only from temperate and subtropical areas. In temperate and subtropical regions, peak virus activity usually has occurred during the season of high temperature (i.e., the summer) or of high rainfall. High temperatures may increase mosquito abundance and also (as shown experimentally) increase their competence as vectors. High rainfall may contribute by increasing vector density.
  • Ecological habitats in which virus transmission occurs include coastal plains and river delta areas, forests, semi-arid areas, and highland plateaus.
  • The range of West Nile virus is determined by the habitat needs of the host and vector species, and the viral range has the potential to expand wherever suitable vectors and definitive hosts co-exist and where an infected vector or host can reach.
  • The ability of mosquitoes to transmit WN virus may be affected by environmental temperatures.
  • Virus viability outside the host has been shown to decrease rapidly in faecal material from chickens at ambient temperature and in minimal essential medium with 10% bovine fetal serum held at 28C but survival was increased at 4C and viable virus has been isolated from fresh frozen plasma from a human blood donor.

(J84.7.w22, J93.40.w2, J94.33.w1, J110.38.w2, J110.39.w2, J110.39.w4, B241.49.w49, B244.w1, P33.3.w1, N7.51.w3)

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Habitat Biomes where virus appears to be able replicate and transfer between species sufficiently well to become permanently established in Biome (Become Endemic)

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Distribution and Geographical Occurrence

Editorial Overview

In general: 

  • West Nile virus has been reported from the USA, Canada, Europe, the Middle East, Africa and Asia, while Kunjin virus which is closely related or a subtype of WN virus has been recorded in Australia and Southeast Asia.
  • Recent outbreaks of infection associated with clinical disease have occurred in various countries in Europe, the Middle East and Africa, since 1999 in North America and since 2002 in horses in Central America.
  • In South Africa infection in horses appears to be common but is not associated with neurological disease. 

Western Hemisphere - North, Central and South America: 

(J19.131.w1, J84.5.w2, J84.9.w9, J84.9.w10, J84.9.w16, J84.9.w20, J84.9.w23, J87.35.w1, J90.2001.w3, J90.16.w4, J90.16.w5, J214.267.w10, J260.33.w1, B240.14.w14, B244.w1, N7.48.w1, P5.41.w3, W27.04Oct02.wnv2, W27.15Mar03.wnv1, W27.15Mar03.wnv2,  W27.04May03.wnv2, W27.15Mar03.wnv1, W27.15May03.wnv2, W27.13Jun03.wnv1, W27.01Jul03.wnv1, W27.11Oct03.wnv1, W181.19Jan04.WNV1, W181.19Jan04.WNV2, W181.19Jan04.WNV3, W181.19Jan04.WNV4, N7.51.w6, N7.52.w6)

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General Regions with literature reports of virus in last three years (not including experimental)

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Authors & Referees

Authors Debra Bourne MA VetMB PhD MRCVS(V.w5)
Referee Suzanne I. Boardman (V.w6); Becki Lawson (V.w26); Dr Robert G. McLean (V.w42); Charalambos Billinis DVM DrMedVet (V.w174), George Valikos DVM (V.w175)

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