Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide

Datasheet

West Nile fever

Toolbox

Datasheet

West Nile fever

Summary

  • Last modified
  • 14 July 2018
  • Datasheet Type(s)
  • Animal Disease
  • Preferred Scientific Name
  • West Nile fever
  • Overview
  • West Nile virus (WNV) is a mosquito-borne flavivirus that causes West Nile fever (WNF), which is...

  • There are no pictures available for this datasheet

    If you can supply pictures for this datasheet please contact:

    Compendia
    CAB International
    Wallingford
    Oxfordshire
    OX10 8DE
    UK
    compend@cabi.org
  • Distribution map More information

Don't need the entire report?

Generate a print friendly version containing only the sections you need.

Generate report

Pictures

Top of page
PictureTitleCaptionCopyright

Identity

Top of page

Preferred Scientific Name

  • West Nile fever

International Common Names

  • English: Near Eastern equine encephalitis; West Nile; West Nile encephalitis; West Nile infection; West Nile viral encephalitis; West Nile virus encephalomyelitis in horses and sheep - zoonosis; West Nile virus equine encephalomyelitis; West Nile virus in birds; WNV infection
  • French: lourdige

English acronym

  • WNF

Overview

Top of page

West Nile virus (WNV) is a mosquito-borne flavivirus that causes West Nile fever (WNF), which is endemic in Africa, the Middle East and south-western Asia. WNV has recently emerged in temperate regions of Europe and North America and presents a threat to public and animal health. The virus was first isolated in the West Nile district of Uganda in 1937 from a native adult human female suffering a mild febrile illness (Smithburn et al., 1940). Subsequently, it was isolated from birds and mosquitoes in Egypt (Taylor et al., 1956) and implicated as a cause of human meningoencephalitis in Israel during the 1950s (Weinberger et al., 2001). A major human epidemic occurred in South Africa during 1974 (McIntosh et al., 1976) and human epidemics have been reported  in Romania (Tsai et al., 1998), Southern Russia (L'vov et al., 2000), North-eastern USA (Lanciotti et al., 1999) and Israel. (Chowers et al., 2001).


 A major epidemic has occurred in at least 38 States in N America in 2002. There have been 3698 laboratory confirmed cases in humans with 198 deaths. Some 8710 equines have been infected, 3 canines and at least 10 other species. Two thousand six hundred and twelve crows have been killed, together with some 6060 other avian hosts. Canada has also been involved.

Disease among horses caused by WNV was reported in Egypt and France during the early 1960s (Schmidt and El Mansoury, 1963; Hannoun et al., 1969). Cases of WNV equine encephalitis have been reported more recently from countries in North Africa and Europe bordering the Mediterranean Sea and from the Northeastern USA (Ostlund et al., 2000; Murgue et al., 2001; Trock et al., 2001). Mortality among domestic flocks of geese has been reported in Israel (Weinberger et al., 2001).

WNV is a member of the Japanese encephalitis virus sero-complex in the genus Flavivirus of the family Flaviviridae (Heinz et al., 2000). Other members of the group include Japanese encephalitis virus (JEV), St. Louis encephalitis virus (SLEV), Murray Valley encephalitis virus (MVEV) and Kunjin virus (KUNV). They are closely related antigenically and can cross-react, often confusing interpretation when diagnostic serological tests are performed.

Mosquitoes are primarily responsible for vector transmission, with birds acting as the primary reservoir host (Hayes, 1989; Peiris and Amerasinghe, 1994; Hubalek and Halouzka, 1999). Virus has been isolated from many vertebrate species. Humans and equines are normally considered to be accidental or 'dead end' hosts.

Since the mid 1990s three distinct epidemiological trends have emerged, an increase in frequency of outbreaks among humans and equines, an increase in the severity of human disease and high avian death rates accompanying outbreaks (Peterson and Roehrig, 2001). There has been a major increase and extension of WNV activity across much of the United States and Canada in the year 2002, which has involved humans, horses and wild and domestic birds and ruminants.

Host Animals

Top of page
Animal nameContextLife stageSystem
Anser anser (geese)Domesticated host
Equus caballus (horses)Domesticated host
Gallus gallus domesticus (chickens)
Homo sapiens

Hosts/Species Affected

Top of page

WNV has been isolated most frequently from humans, equines and many different species of wild and domestic birds. Infection by West Nile virus causing acute onset of anorexia and fever has also been recorded in alpacas (Kutzler et al., 2004). The virus has been isolated sporadically from camel, dog and a variety of small mammals (Hayes, 1989; Hubalek and Halouzka, 1999).

Systems Affected

Top of page multisystemic diseases of large ruminants
nervous system diseases of poultry

Distribution

Top of page

WNV is the most widely distributed of the flaviviruses, having been isolated in Africa, Asia, Europe, Middle East, Oceania, North America and Russia (Hubalek and Halouzka, 1999).

Distribution Table

Top of page

The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Continent/Country/RegionDistributionLast ReportedOriginFirst ReportedInvasiveReferenceNotes

Asia

AfghanistanNo information availableOIE, 2009
ArmeniaDisease never reportedNULLHubalek and Halouzka, 1999; OIE, 2009
AzerbaijanDisease never reportedNULLHubalek and Halouzka, 1999; OIE, 2009
BahrainDisease never reportedOIE, 2009
BangladeshDisease never reportedOIE, 2009
BhutanDisease never reportedOIE, 2009
CambodiaNo information availableOIE, 2009
ChinaDisease never reportedNULLHubalek and Halouzka, 1999; OIE, 2009
-Hong KongNo information availableOIE, 2009
Georgia (Republic of)PresentHubalek and Halouzka, 1999
IndiaNo information availableNULLHayes, 1989; OIE, 2009
IndonesiaNo information availableOIE, 2009
IranDisease never reportedOIE, 2009
IraqNo information availableNULLHubalek and Halouzka, 1999; OIE, 2009
IsraelPresentNULLMarberg et al., 1956; OIE, 2009
JapanDisease never reportedOIE, 2009
JordanNo information availableNULLBatieha et al., 2000; OIE, 2009
KazakhstanNo information availableNULLHubalek and Halouzka, 1999; OIE, 2009
Korea, Republic ofDisease never reportedOIE, 2009
KuwaitDisease never reportedOIE, 2009
KyrgyzstanNo information availableOIE, 2009
LaosDisease never reportedOIE, 2009
LebanonDisease never reportedNULLHubalek and Halouzka, 1999; OIE, 2009
MalaysiaDisease never reportedNULLHubalek and Halouzka, 1999; OIE, 2009
-SarawakPresentChing et al., 1970
MongoliaNo information availableOIE, 2009
MyanmarDisease never reportedOIE, 2009
NepalDisease never reportedOIE, 2009
OmanNo information availableNULLOIE, 2003; OIE, 2009
PakistanDisease never reportedNULLHayes, 1989; OIE, 2009
PhilippinesDisease never reportedNULLHubalek and Halouzka, 1999; OIE, 2009
QatarNo information availableOIE, 2009
Saudi ArabiaNo information availableOIE, 2009
SingaporeDisease never reportedOIE, 2009
Sri LankaDisease never reportedNULLHubalek and Halouzka, 1999; OIE, 2009
SyriaDisease never reportedNULLHubalek and Halouzka, 1999; OIE, 2009
TajikistanDisease never reportedNULLHubalek and Halouzka, 1999; OIE, 2009
ThailandNo information availableNULLHubalek and Halouzka, 1999; OIE, 2009
TurkeyNo information availableNULLHubalek and Halouzka, 1999; OIE, 2009
TurkmenistanPresentHubalek and Halouzka, 1999
United Arab EmiratesDisease not reportedOIE, 2009
VietnamNo information availableOIE, 2009
YemenNo information availableOIE, 2009

Africa

AlgeriaDisease never reportedLe et al., 1996; OIE, 2012
AngolaNo information availableOIE, 2009
BeninNo information availableOIE, 2009
BotswanaDisease never reportedHubalek and Halouzka, 1999; OIE, 2012
Burkina FasoNo information availableOIE, 2009
Central African RepublicPresentHayes, 1989
ChadNo information availableOIE, 2009
CongoNo information availableHayes, 1989; OIE, 2009
Congo Democratic RepublicPresentHayes, 1989
Côte d'IvoirePresentHubalek and Halouzka, 1999
DjiboutiNo information availableOIE, 2009
EgyptNo information availableNULLHayes, 1989; OIE, 2009
EritreaNo information availableOIE, 2009
EthiopiaDisease never reportedHayes, 1989; OIE, 2012
GabonNo information availableOIE, 2009
GambiaNo information availableOIE, 2009
GhanaNo information availableOIE, 2009
GuineaNo information availableOIE, 2009
Guinea-BissauNo information availableOIE, 2009
KenyaDisease never reportedNULLHayes, 1989; OIE, 2009
LesothoDisease never reportedOIE, 2009
LibyaDisease never reportedOIE, 2012
MadagascarLast reported2010Hayes, 1989; OIE, 2012
MalawiNo information availableOIE, 2009
MaliNo information availableOIE, 2009
MauritiusDisease never reportedOIE, 2012
MoroccoLast reported2010Ostlund et al., 2000; OIE, 2012
MozambiqueNo information availableNULLHubalek and Halouzka, 1999; OIE, 2009
NamibiaDisease never reportedOIE, 2012
NigeriaDisease never reportedHayes, 1989; OIE, 2012
RwandaDisease never reportedOIE, 2009
SenegalNo information availableHubalek and Halouzka, 1999; OIE, 2009
SeychellesOIE, 2012Disease suspected
South AfricaLast reported2010Hayes, 1989; OIE, 2012
SudanDisease never reportedTaylor et al., 1956; OIE, 2012
SwazilandNo information availableOIE, 2009
TanzaniaNo information availableOIE, 2009
TogoNo information availableOIE, 2009
TunisiaLast reported2008Hubalek and Halouzka, 1999; OIE, 2012
UgandaNo information availableSmithburn et al., 1940; OIE, 2009First reported 1937
ZambiaDisease never reportedOIE, 2012
ZimbabweDisease never reportedOIE, 2012

North America

CanadaRestricted distributionOIE, 2009
-OntarioPresentPowell, 2002
GreenlandDisease never reportedOIE, 2009
MexicoDisease not reportedOIE, 2009
USAPresentNULLPowell, 2002; OIE, 2009
-AlabamaPresentPowell, 2002
-ArkansasPresentPowell, 2002
-ConnecticutPresentPowell, 2002
-DelawarePresentPowell, 2002
-FloridaPresentPowell, 2002
-GeorgiaPresentPowell, 2002
-IllinoisPresentPowell, 2002
-IndianaPresentPowell, 2002
-IowaPresentPowell, 2002
-KentuckyPresentPowell, 2002
-LouisianaPresentPowell, 2002
-MainePresentPowell, 2002
-MarylandPresentPowell, 2002
-MassachusettsPresentPowell, 2002
-MichiganPresentPowell, 2002
-MississippiPresentPowell, 2002
-MissouriPresentPowell, 2002
-New HampshirePresentPowell, 2002
-New JerseyPresentPowell, 2002
-New YorkPresentPowell, 2002
-North CarolinaPresentPowell, 2002
-OhioPresentPowell, 2002
-PennsylvaniaPresentPowell, 2002
-Rhode IslandPresentPowell, 2002
-TennesseePresentPowell, 2002
-VirginiaPresentPowell, 2002
-WisconsinPresentPowell, 2002

Central America and Caribbean

BelizeDisease not reported20050826OIE, 2004; OIE, 2009
Cayman IslandsPresentPowell, 2002
Costa RicaDisease never reportedOIE, 2009
CubaRestricted distributionNULLOIE, 2005; OIE, 2009
Dominican RepublicDisease not reportedOIE, 2009
El SalvadorNo information availableOIE, 2009
GuadeloupePresentOIE, 2009
GuatemalaAbsent, reported but not confirmedOIE, 2009
HaitiPresentOIE, 2009
HondurasDisease never reportedOIE, 2009
JamaicaDisease never reportedOIE, 2009
MartiniqueDisease never reportedOIE, 2009
NicaraguaDisease never reportedOIE, 2009
PanamaDisease never reportedOIE, 2009

South America

ArgentinaDisease not reportedOIE, 2009
BoliviaDisease never reportedOIE, 2009
BrazilDisease never reportedOIE, 2009
ChileDisease never reportedOIE, 2009
ColombiaDisease never reportedOIE, 2009
EcuadorDisease never reportedOIE, 2009
French GuianaDisease not reportedOIE, 2009
PeruDisease never reportedOIE, 2009
UruguayDisease never reportedOIE, 2009
VenezuelaDisease never reportedOIE, 2009

Europe

AlbaniaDisease never reportedNULLHubalek and Halouzka, 1999; OIE, 2009
AustriaPresentNULLHubalek and Halouzka, 1999; OIE, 2009
BelarusNo information availableNULLHubalek and Halouzka, 1999; OIE, 2009
BelgiumDisease not reportedOIE, 2009
Bosnia-HercegovinaPresentHubalek and Halouzka, 1999
BulgariaDisease never reportedNULLHubalek and Halouzka, 1999; OIE, 2009
CroatiaDisease never reportedNULLHubalek and Halouzka, 1999; OIE, 2009
CyprusDisease never reportedNULLHayes, 1989; OIE, 2009
Czech RepublicDisease never reportedNULLHubalek and Halouzka, 1999; OIE, 2009
Czechoslovakia (former)PresentHubalek and Halouzka, 1999
DenmarkDisease never reportedOIE, 2009
EstoniaNo information availableOIE, 2009
FinlandDisease never reportedOIE, 2009
FranceDisease not reported200704Murgue et al., 2001; OIE, 2009
-CorsicaWidespreadHubalek and Halouzka, 1999
GermanyDisease never reportedOIE, 2009
GreeceDisease never reportedNULLHubalek and Halouzka, 1999; OIE, 2009
HungaryRestricted distributionNULLHubalek and Halouzka, 1999; OIE, 2009
IcelandDisease never reportedOIE, 2009
IrelandDisease never reportedOIE, 2009
ItalyRestricted distributionNULLCantile et al., 2000; OIE, 2009
LatviaDisease never reportedOIE, 2009
LiechtensteinDisease not reportedOIE, 2009
LithuaniaDisease never reportedOIE, 2009
LuxembourgDisease never reportedOIE, 2009
MacedoniaDisease never reportedOIE, 2009
MaltaDisease never reportedOIE, 2009
MoldovaPresentHubalek and Halouzka, 1999
MontenegroDisease never reportedOIE, 2009
NetherlandsDisease never reportedOIE, 2009
NorwayDisease never reportedOIE, 2009
PolandNo information availableNULLHubalek and Halouzka, 1999; OIE, 2009
PortugalDisease never reportedNULLFilipe and de, 1990; OIE, 2009
RomaniaPresentNULLHubalek and Halouzka, 1999; OIE, 2009
Russian FederationAbsent, reported but not confirmedOIE, 2009
SerbiaDisease never reportedOIE, 2009
SlovakiaDisease never reportedNULLHubalek and Halouzka, 1999; OIE, 2009
SloveniaDisease never reportedOIE, 2009
SpainDisease never reportedNULLFilipe and de, 1990; OIE, 2009
SwedenDisease never reportedOIE, 2009
SwitzerlandDisease never reportedOIE, 2009
UKDisease never reportedOIE, 2009
UkraineDisease never reportedNULLHubalek and Halouzka, 1999; OIE, 2009
Yugoslavia (former)PresentHubalek and Halouzka, 1999

Oceania

AustraliaDisease never reportedNULLScherret et al., 2001; OIE, 2009
French PolynesiaDisease never reportedOIE, 2009
New CaledoniaDisease never reportedOIE, 2009
New ZealandDisease never reportedOIE, 2009

Pathology

Top of page

Cantile et al. (1999) and Ostlund et al. (2000) described gross and histological lesions in horses that have died from WNV infection. Postmortem examination revealed little or no gross lesions. External injuries were observed as a result of trauma during recumbency. Lesions if present were limited to the central nervous system. The dura was thickened and adherent and sub-meningeal oedema with petechial or diffuse haemorrhages was observed.

Histological lesions in the brain were similar to those observed for other equine encephalitic viruses, a nonsuppurative mild to moderate encephalitis and vasculitis with perivascular cuffing and monocytic cellular infiltration. During the outbreak in Italy reported by Cantile et al. (1998) the predominant lesions were in the lower brainstem and ventral horns of the thorax and lumbar spinal cord with focal gliosis and haemorrhage occasionally observed. Lesions are consistent with an ascending neurological dysfunction but not specific for WNV infection.

Steele et al. (2000) has described the pathology of WNV infection among native and exotic birds in the USA. Haemorrhages of the brain, splenomegaly, meningeoencephalitis and myocarditis were the most prominent lesions. Gross and histological lesions were common in the cerebellum including haemorrhages, Purkinje cell necrosis, gliosis and inflammatory infiltrates. Changes were less severe in other portions of the brain. Crows did not exhibit the extensive lesions of the brain observed in other birds. Lesions observed in other tissues included lymphocytic myocarditis, focal necrosis of the liver and spleen, pancreatitis, pulmonary haemorrhage and inflammation of the adrenal glands. Viral antigen was detected in a wide range of avian tissues and cells confirming the pantropic nature of the invading strain of WNV.

Targeting of the Purkinje cells of the cerebellum observed in WNV infection of mammals and birds is considered unique to the flaviviruses (Komar, 2000).

Diagnosis

Top of page

Laboratory tests are essential for establishing diagnosis of WNV infection. Case definitions for WNV infection in humans and equines including clinical signs and confirmatory laboratory evidence are available in 'Epidemic/Epizootic West Nile Virus in the United States: Revised Guidelines for Surveillance, Prevention and Control' (Centers for Disease Control and Prevention, 2001).

Care is required when handling virus because of the zoonotic potential of WNV. Laboratories working with known WNV isolates should adhere to established containment requirements. Caution should be exercised when collecting material from live or dead specimens, particularly when central nervous tissue is examined. Precautions should include wearing two layers of waterproof gloves and a facemask.

A definitive diagnosis is possible by detection of viral RNA using polymerase chain reaction (PCR) or by isolation in tissue culture; for example, Vero cells or neonatal mouse inoculation. Preferred tissues from equines are brain or spinal cord, although isolation of WNV from brain tissue can be difficult. Equine blood and cerebrospinal fluid (CSF) from clinically sick animals is not a reliable source for the determination of the presence of virus by PCR or virus isolation because viraemia occurs before clinical signs are observed (Johnson et al., 2001). Multiple unfixed specimens from the cerebrum, brainstem and representative segments of the spinal cord should be obtained for analysis. Multiple tissue samples from other mammals should include samples of brain and kidney. Specimens from dead birds should include kidney, brain and heart tissue. Material should be chilled during transit and sent by overnight delivery service. Additional specimens from brain and spinal cord should be fixed in formalin and submitted for histological examination.

Isolates are identified using WNV-specific monoclonal antibodies, virus neutralisation assays or reverse transcriptase-polymerase chain reaction (RT-PCR). An RT-nested PCR (RT-nPCR) has proved to be a reliable and rapid method for detecting WNV in both equine and avian tissues (Johnson et al., 2001). WNV antigen can be detected in fixed tissues using immunohistochemical (IHC) techniques with WNV-specific antisera. In situ hybridisation detection of WNV nucleic acid in avian tissue has been reported (Steele et al., 2000). Species-specific antigen capture ELISA is available to detect antigen in avian tissues and mosquito pools (Centers for Disease Control Workshop, 2001).

Serological evidence of recent WNV infection is confirmed by a 4-fold or greater rise in plaque-reduction neutralising (PRNT) antibody in paired sera. The first serum should be drawn as soon as possible after the onset of clinical signs and the second between 14 and 21 after the first. Neutralising antibody may not be present until 2 weeks or more after exposure to WNV; so it is possible that clinical signs will be observed before a serum is PRNT positive. Other tests include detection of specific immunoglobulin M (IgM) to WNV by IgM-capture enzyme-linked immunosorbent assay (MAC-ELISA) in sera or CSF, haemagglutination inhibition (HI) and complement fixation (CF). PRNT can be applied to sera and CSF from all species and adapted to reflect antibody activity to currently circulating strains. However, it does require the use of live virus.

IgM antibody induced during the acute phase of WNV infection is short lived; approximately 3 months in horses (Ostlund et al., 2000). IgM-capture ELISA is a valuable tool for the detection of recent infection in all species. It is species-specific and must be modified for each new species to be tested. Whilst HI and ELISA are used extensively for diagnosis and serological prevalence studies they do cross react with other flaviviruses and should be used only as a screening test. Positive samples should be confirmed by neutralisation and serological samples should be screened against a panel of arboviral antigens, depending on the geographic distribution of known pathogens in the area.

List of Symptoms/Signs

Top of page
SignLife StagesType
Cardiovascular Signs / Tachycardia, rapid pulse, high heart rate Sign
Digestive Signs / Anorexia, loss or decreased appetite, not nursing, off feed Sign
Digestive Signs / Dysphagia, difficulty swallowing Sign
Digestive Signs / Excessive salivation, frothing at the mouth, ptyalism Sign
Digestive Signs / Grinding teeth, bruxism, odontoprisis Other:All Stages Sign
Digestive Signs / Hepatosplenomegaly, splenomegaly, hepatomegaly Poultry:All Stages Sign
Digestive Signs / Tongue weakness, paresis, paralysis Sign
General Signs / Abnormal proprioceptive positioning, knuckling Poultry:All Stages,Other:All Stages Sign
General Signs / Ataxia, incoordination, staggering, falling Poultry:All Stages,Other:All Stages Sign
General Signs / Decreased, absent thirst, hypodipsia, adipsia Sign
General Signs / Dehydration Other:All Stages Sign
General Signs / Dysmetria, hypermetria, hypometria Sign
General Signs / Dysmetria, hypermetria, hypometria Sign
General Signs / Exercise intolerance, tires easily Other:All Stages Sign
General Signs / Fever, pyrexia, hyperthermia Other:All Stages Sign
General Signs / Forelimb weakness, paresis, paralysis front leg Other:All Stages Sign
General Signs / Generalized lameness or stiffness, limping Sign
General Signs / Generalized weakness, paresis, paralysis Other:All Stages Sign
General Signs / Head, face, ears, jaw weakness, droop, paresis, paralysis Other:All Stages Sign
General Signs / Hemiparesis Other:All Stages Sign
General Signs / Inability to stand, downer, prostration Other:All Stages Sign
General Signs / Increased mortality in flocks of birds Poultry:All Stages Sign
General Signs / Lack of growth or weight gain, retarded, stunted growth Poultry:All Stages Sign
General Signs / Neck weakness, paresis, paralysis, limp, ventroflexion Poultry:All Stages Sign
General Signs / Opisthotonus Sign
General Signs / Paraparesis, weakness, paralysis both hind limbs Other:All Stages Sign
General Signs / Reluctant to move, refusal to move Poultry:All Stages,Other:All Stages Sign
General Signs / Sudden death, found dead Poultry:All Stages Sign
General Signs / Tetraparesis, weakness, paralysis all four limbs Other:All Stages Sign
General Signs / Torticollis, twisted neck Poultry:All Stages Sign
General Signs / Trembling, shivering, fasciculations, chilling Other:All Stages Sign
General Signs / Underweight, poor condition, thin, emaciated, unthriftiness, ill thrift Poultry:All Stages,Other:All Stages Sign
General Signs / Weakness of one hindlimb, paresis paralysis rear leg Other:All Stages Sign
General Signs / Weakness, paresis, paralysis of the legs, limbs in birds Poultry:All Stages Sign
General Signs / Weakness, paresis, paralysis, drooping, of the wings Poultry:All Stages Sign
General Signs / Weight loss Other:All Stages Sign
Nervous Signs / Abnormal behavior, aggression, changing habits Sign
Nervous Signs / Circling Other:All Stages Sign
Nervous Signs / Coma, stupor Other:All Stages Sign
Nervous Signs / Disoriented, memory loss Other:All Stages Sign
Nervous Signs / Dullness, depression, lethargy, depressed, lethargic, listless Other:All Stages Sign
Nervous Signs / Excessive or decreased sleeping Sign
Nervous Signs / Excitement, delirium, mania Sign
Nervous Signs / Head pressing Sign
Nervous Signs / Head tilt Other:All Stages Sign
Nervous Signs / Hyperesthesia, irritable, hyperactive Sign
Nervous Signs / Muscle hypotonia Other:All Stages Sign
Nervous Signs / Propulsion, aimless wandering Sign
Nervous Signs / Seizures or syncope, convulsions, fits, collapse Sign
Nervous Signs / Seizures or syncope, convulsions, fits, collapse Sign
Nervous Signs / Tremor Other:All Stages Sign
Ophthalmology Signs / Abnormal pupillary response to light Sign
Ophthalmology Signs / Anisocoria Sign
Ophthalmology Signs / Blindness Sign
Ophthalmology Signs / Blindness Sign
Ophthalmology Signs / Lacrimation, tearing, serous ocular discharge, watery eyes Sign
Ophthalmology Signs / Nystagmus Sign
Pain / Discomfort Signs / Back pain Other:All Stages Sign
Pain / Discomfort Signs / Ocular pain, eye Other:All Stages Sign
Pain / Discomfort Signs / Pain, head, face, jaw, ears Other:All Stages Sign
Pain / Discomfort Signs / Pain, neck, cervical, throat Other:All Stages Sign
Reproductive Signs / Agalactia, decreased, absent milk production Sign
Respiratory Signs / Abnormal breathing sounds of the upper airway, airflow obstruction, stertor, snoring Sign
Respiratory Signs / Dyspnea, difficult, open mouth breathing, grunt, gasping Sign
Respiratory Signs / Increased respiratory rate, polypnea, tachypnea, hyperpnea Sign
Skin / Integumentary Signs / Alopecia, thinning, shedding, easily epilated, loss of, hair Sign
Skin / Integumentary Signs / Pruritus, itching skin Sign
Skin / Integumentary Signs / Skin edema Sign

Disease Course

Top of page

Among horses, signs range from inapparent infections to fatal encephalitis with mortality of 30-45% in clinical cases. The most common signs reported by Ostlund et al. (2000) include marked abnormalities of gait, primarily of the hind limbs, with varying degrees of ataxia and muscular weakness, progressing on occasion to recumbency. Other neuromuscular signs include circling, head-tilt and tremors of the face, limbs, trunk and shoulder. Sick horses continue to eat and drink but behavioural changes such as depression or anxiety and nervousness are observed.

Until recently, reports of birds showing clinical disease during epizootics of WNV infection have been rare. Fatal neurological disease was a feature among wild and exotic birds during outbreaks in the USA (Steele et al., 2000) and among domestic flocks of geese in Israel (Weinberger et al., 2001). In the USA, mortality was high amongst corvids (crows and jays) but many other native bird species were also affected. Sick birds showed weakness often lie in sternal recumbency. Neurological signs were the principal manifestation however, including tremors, abnormal head position, ataxia, wobbly gait and circling.

Epidemiology

Top of page

WNV is amplified during periods of adult mosquito blood feeding by a continuous transmission cycle between mosquito vectors, particularly Culex spp. and avian reservoir hosts. Hard and soft ticks may serve as substitute vectors in areas lacking mosquitoes. Two transmission cycles are recognised, a rural or sylvatic cycle comprising wild, usually wetland birds and bird feeding mosquitoes, and an urban cycle involving birds and mosquitoes, which feed upon birds and humans. The significance of the urban cycle was apparent during the recent human epidemics in Bucharest, Volvograd and New York City. Environmental factors, including water management, sanitation and substandard housing contribute to mosquito breeding, virus amplification and disease transmission in urban areas.

A sufficient number of vectors must feed on an infectious host to ensure that some survive the viral incubation period of approximately two weeks allowing them to feed again and transmit to a susceptible host. (Komar, 2000). Peak virus activity in birds, occurs during seasons of high temperature and rainfall that coincide with high vector density and increased vector feeding capacity. Other modes of transmission are not confirmed, although direct bird to bird transmission has been suggested (Komar, 2000).

The role of migrating birds in the transmission of WNV, particularly to temperate zones, has received considerable attention following the epidemics in Europe and North America (Rappole et al., 2000). Northward spring migrations from Africa across the Middle East, Turkey and the Black Sea provide a route to introduce WNV to southern Europe and Russia. Possible mechanisms for overwintering of WNV in temperate zones include prolonged infection in hibernating mosquitoes and low level trans-ovarial transmission (Hubalek and Halouzka, 1999). Molecular and antigenic studies of isolates from around the world have provided clues as to the source and migration of WNV; the virus isolated from cases in New York has a close homology to virus isolated from domestic geese in Israel in 1998 (Lanciotti et al., 2000).

Impact: Economic

Top of page

Initial accounts of the disease in endemic areas of Africa (Taylor ,1956) indicated that infection among humans occurred during early childhood and was asymptommatic or resulted in only mild disease. The epidemics during the 1990s in Europe, Southern Russia, North America and the Middle East have involved increased fatality associated with encephalitic infection among adults. Mortality among horses has been reported recently in France, Italy, USA, and North Africa and among a variety of wild, domesticated and exotic bird species in Israel and the USA. An estimate of public expenditure attributed to the outbreak in New York State during 1999 exceeded US $15 million (Komar, 2000). Restrictions on the export of horses from affected states in the USA were imposed temporarily by several countries including the European Union during 1999 and 2000. Several major national and international equestrian competitions due to be held in the northeastern USA during 2000 were cancelled.

Zoonoses and Food Safety

Top of page

Affected individuals develop a flu-like illness characterized by acute fever, headache, sore throat, chills, generalised lymphadenopathy, nausea and myalgia. The incubation period ranges from 5-15 days with rapid onset of signs and a convalescent period of 1-2 weeks. There have also been occasional reports of hepatitis, pancreatitis and myocarditis. More severe illness has been reported during recent epidemics, involving acute aseptic meningitis or encephalitis particularly among elderly patients, leading in a small percentage of cases to coma and death. Signs in these patients include fever, headache, vomiting, confusion, rash, stiff neck and profound muscle weakness requiring respiratory support.

Disease Treatment

Top of page

There is no specific treatment for clinical WNV infection in humans or animals. Therapies may be applied to reduce pain, inflammation, to provide supportive care, prevent injury and minimise the adverse consequences associated with recumbency. Nonsteroidal anti-inflammatory medications assist in reducing inflammation of the central nervous system and alleviating pain. Slings, plus nutrient and fluid support are also of value.

Prevention and Control

Top of page

The control and prevention of arboviral disease may be accomplished through an integrated management program undertaken by trained and experienced personnel (Rose, 2001). Components include surveillance, source reduction, larvicide and biological control, resistance monitoring, public relations and education. The emergence of WNV infection causing human encephalitis, particularly in non-endemic areas, has prompted increased surveillance. A sophisticated monitoring system has been developed in the USA with the collaboration of medical, public health and veterinary resources at federal, state and local level (Centres for Disease Control Workshop, 2001). It involves active and passive surveillance for human cases of viral encephalitis, veterinary surveillance for disease in horses and other mammals and surveillance in populations of wild and sentinel birds and mosquitoes. Surveillance in the vector populations determines the minimum infection rate (MIR) expressed as the number infected per 1000 specimens examined, and provides warning of a probable disease outbreak. Such a program requires specialised laboratory diagnostic procedures, detailed epidemiological monitoring, data recording and analysis supported by trained personnel. Not all countries have the resources available, or have more urgent medical priorities and rely on passive medical and veterinary surveillance during the summer months when the incidence of disease is highest.

Source reduction to eliminate mosquito larval habitats and prevent mosquito breeding is achieved by improved sanitation such as the elimination of standing pools of water, management of irrigation projects, swamp and marshlands and public education. The most commonly used biological controls are the mosquito fish, Gambusia affinis and G. holbrooki. If the above are not feasible or have failed then 'larviciding', the application of chemicals to kill larva and pupae by ground or aerial treatment can be tried. Larvicidal chemicals may be applied in various formulations and include temephos, methoprene, oils and bacterial larvicides. Application of 'adulticides' to kill adult mosquitoes by ground or aerial spraying is the least effective control method but may be used as one of last resort during an outbreak. Adulticides include organophosphates, malathion and naled, natural and synthetic pyrethrums such as pyrethrins, permethrin, resmethrin and sumithrin. They can be applied as an ultra low volume (ULV) spray from truck-mounted equipment, fixed wing or rotary aircraft. The use of chemical insecticides has aroused considerable public controversy with respect to their potential human and environmental toxic effects. Education, particularly via local television and radio can overcome public apprehension and may be used to explain preventive measures prior to and during an outbreak.

Exposure of horses to mosquito vectors can be minimised by stabling in vector-proofed buildings plus the use of repellants such as N, N-diethyl-metatoluamide (DEET).

In August 2001 the United States Department of Agriculture (USDA) issued a conditional license for the use of an inactivated adjuvant equine WNV vaccine manufactured by Fort Dodge Animal Health. Commercial flocks of geese in Israel received an attenuated vaccine derived from Israel turkey meningoencephalitis (ITM) virus during 1999 (Komar, 2000). ITM, a flavivirus is administered to young goslings to provide cross protection to WNV infection. Studies are underway to develop vaccines for humans and horses that examine the safety and efficacy of DNA and live attenuated WNV vaccines.

References

Top of page

African Union-Interafrican Bureau for Animal Resources, 2011. Panafrican Animal Health Yearbook 2011. Pan African Animal Health Yearbook, 2011:xiii + 90 pp. http://www.au-ibar.org/index.php?option=com_flexicontent&view=items&cid=71&id=109&Itemid=56&lang=en

Batieha A, Saliba EK, Graham R, Mohareb E, Hijazi Y, Wijeyaratne P, 2000. Seroprevalence of West Nile, Rift Valley, and sandfly arboviruses in Hashimiah, Jordan. Emerging Infectious Diseases, 6(4):358-362.

Berthet FX, Zeller HG, Drouet MT, Rauzier J, Digoutte JP, Deubel V, 1997. Extensive nucleotide changes and deletions within the envelope glycoprotein gene of Euro-African West Nile viruses. Journal of General Virology, 78(9):2293-2297; 22 ref.

Cannon AB, Luff JA, Brault AC, MacLachlan NJ, Case JB, Green ENG, Sykes JE, 2006. Acute encephalitis, polyarthritis, and myocarditis associated with West Nile virus infection in a dog. Journal of Veterinary Internal Medicine, 20(5):1219-1223.

Cantile C, Guardo Gdi, Eleni C, Arispici M, 2000. Clinical and neuropathological features of West Nile virus equine encephalomyelitis in Italy. Equine Veterinary Journal, 32(1):31-35; 26 ref.

Castillo-Olivares J, Wood J, 2004. West Nile virus infection of horses. Veterinary Research, 35(4):467-483.

Centers for Disease Control, 2001. Epidemic/Epizootic West Nile Virus in the United States: Revised Guidelines for Surveillance, Prevention and Control. Atlanta, Georgia, USA: CDC, 104 pp.

Ching CY, Casals J, Bowen ET, Simpson DI, Platt GS, Way HJ, Smith CE, 1970. Arbovirus infections in Sarawak: the isolation of Kunjin virus from mosquitoes of the Culex pseudovishnui group. Annals of Tropical Medicine and Parasitology, 64(3):263-268.

Chowers MY, Lang R, Nassar F, Ben David D, Giladi M, Rubinshtein E, Itzhaki A, Mishal J, Siegman Igra Y, Kitzes R, Pick N, Landau Z, Wolf D, Bin H, Mendelson E, Pitlik SD, Weinberger M, 2001. Clinical characteristics of the West Nile fever outbreak, Israel, 2000. Emerging Infectious Diseases, 7(4):675-678.

Dauphin G, Zientara S, 2007. West Nile virus: recent trends in diagnosis and vaccine development. Vaccine [4th International Veterinary Vaccines and Diagnostic Conference, Oslo, Norway, 25-29 June 2006.], 25(30):5563-5576. http://www.sciencedirect.com/science/journal/0264410X

Filipe AR, de Andrade HR, 1990. Arboviruses in the Iberian Peninsula. Acta Virologica, 34(6):582-591.

Guenno Ble, Bougermouh A, Azzam T, Bouakaz R, 1996. West Nile: a deadly virus?. Lancet (British edition), 348(9037):1315; 5 ref.

Hannoun C, Panthier R, Corniou B, 1969. Epidemiology of West Nile infections in the south of France. In: Bardos, V, ed. Arboviruses of the California Complex and the Bunyamwera Group, Proceedings of the Symposium, 1966. Bratislava, Pub. House of the Slovak Academy of Sciences, 379-387.

Hayes CG, 1989. West Nile fever. The arboviruses: epidemiology and ecology. Volume V., 59-88; 151 ref.

Hayes CG, 2001. West Nile virus: Uganda, 1937, to New York City, 1999. Annals of the New York Academy of Science, 951:25-37.

Hayes EB, Komar N, Nasci RS, Montgomery SP, O'Leary DR, Campbell GL, 2005. Epidemiology and transmission dynamics of West Nile virus disease. Emerging Infectious Diseases, 11(8):1167-1173.

Heinz FX, Collet MS, Purcell RH, Gould EA, Howard CR, Houghton M, Moorman RJM, Rice CM, Thiel HJ, 2000. Family Flaviridae. In: van Regenmortel MH, Fauquet CM, Bishop DHL, Carstens EB, Estes MK, Lemon SM, Maniloff J, Mayo MA, McGeoch DJ, Pringle CR, Wicker RM, ed. Virus Taxonomy: Seventh Report of the International Committee on Taxonomy of Viruses. San Diego, California, USA: Academic Press, 859-878.

Hubalek Z, Halouzka J, 1999. West Nile fever - a reemerging mosquito-borne viral disease in Europe. Emerging Infectious Diseases, 5(5):643-650.

Johnson DJ, Ostlund EN, Pedersen DD, Schmitt BJ, 2001. Detection of North American West Nile virus in animal tissue by a reverse transcription-nested polymerase chain reaction assay. Emerging Infectious Diseases, 7(4):739-741.

Kecskeméti S, Bajmócy E, Bacsadi Â, Kiss I, Bakonyi T, 2007. Encephalitis due to West Nile virus in a sheep. Veterinary Record, 161(16):568-569. http://veterinaryrecord.bvapublications.com/archive/

Komar N, 2000. West Nile viral encephalitis. Revue Scientifique et Technique Office International Des Epizooties, 19(1):166-176.

Kramer LD, Styer LM, Ebel GD, 2008. A global perspective on the epidemiology of West Nile virus. Annual Review of Entomology, 53:61-81. http://www.annualreviews.org

Kutzler MA, Bildfell RJ, Gardner-Graff KK, Baker RJ, Delay JP, Mattson DE, 2004. West Nile virus infection in two alpacas. Journal of the American Veterinary Medical Association, 225(6):921-924.

Lanciotti RS, Kerst AJ, Nasci RS, Godsey MS, Mitchell CJ, Savage HM, Komar N, Panella NA, Allen BC, Volpe KE, Davis BS, Roehrig JT, 2000. Rapid detection of West Nile virus from human clinical specimens, field-collected mosquitoes, and avian samples by a TaqMan reverse transcriptase-PCR assay. Journal of Clinical Microbiology, 38(11):4066-4071.

Lanciotti RS, Roehrig JT, Deubel V, Smith J, Parker M, Steele K, Crise B, Volpe KE, Crabtree MB, Schereet JH, 1999. Origin of the West Nile virus responsible for an outbreak of encephalitis in the northeastern United States. Science, 286(5448):2333-2337.

L'vov DK, Butenko AM, Gromashevskii VL, Larichev VP, Gaidamovich SI, Vyshemirsky OI, Zhukov AN, Lazorenko VV, Salko VN, Kovtunov AI, Galimzianov KM, Platonov AE, Morozova TN, Khutoretskaya NV, Shishkina EO, Skvortsova TM, 2000. Isolation of two strains of West Nile virus during an outbreak in southern Russia, 1999. Emerging Infectious Diseases, 6(4):373-376.

Marberg K, Goldblum N, Sterk VV, Jasinka-Klingberg W, Klingberg MA, 1956. The natural history of West Nile fever. I. Clinical observations during an epidemic in Israel. American Journal of Hygiene, 64(3):259-269.

McIntosh BM, Jupp PG, Dos Santos I, Meenehan GM, 1976. Epidemics of West Nile and Sindbis viruses in South Africa with Culex (Culex) univittatus Theobald as vector. South African Journal of Science, 72(10):295-300.

Murgue B, Murri S, Zientara S, Durand B, Durand JP, Zeller H, 2001. West Nile outbreak in horses in Southern France, 2000: The return after 35 years. Emerging Infectious Diseases, 7(4):692-696.

OIE, 2003. West Nile fever in Oman. Disease Information, 16(48).

OIE, 2004. West Nile fever in Belize in october 2003. Disease Information, 17(10).

OIE, 2005. West Nile fever in Cuba. Virus detection in equids. Disease Information, 18(7).

OIE, 2009. World Animal Health Information Database - Version: 1.4. World Animal Health Information Database. Paris, France: World Organisation for Animal Health. http://www.oie.int

OIE, 2012. World Animal Health Information Database. Version 2. World Animal Health Information Database. Paris, France: World Organisation for Animal Health. http://www.oie.int/wahis_2/public/wahid.php/Wahidhome/Home

Ostlund EN, Andresen JE, Andresen M, 2000. West Nile encephalitis. Veterinary Clinics of North America Equine Practice, 16(3):427-441.

Ostlund EN, Crom RL, Pedersen DD, Johnson DJ, Williams WO, Schmitt BJ, 2001. Equine West Nile encephalitis, United States. Emerging Infectious Diseases, 7(4):665-669.

Peiris JSM, Amerasinghe FP, 1994. West Nile Fever. In: Berad GW, Steele JH, ed. Handbook of Zoonoses. Section B: Viral, edition 2. Boca Raton, Florida, USA: CRC Press, 139-148.

Powell DG, 2002. West Nile virus update. Equine Disease Quarterly, 10(2):2-3.

Rappole JH, Derrickson SR, Hubalek Z, 2000. Migratory birds and spread of West Nile virus in the Western Hemisphere. Emerging Infectious Diseases, 6(4):319-336.

Rose RI, 2001. Pesticides and public health: integrated methods of mosquito management. Emerging Infectious Diseases, 7(1):17-23.

Scherret JH, Poidinger M, Mackenzie JS, Broom AK, Deubel V, Lipkin WI, Briese T, Gould EA, Hall RA, 2001. The relationships between West Nile and Kunjin Viruses. Emerging Infectious Diseases, 7(4):697-705.

Schmidt JL, El Mansoury HK, 1963. Natural and experimental infection with West Nile virus. Annals of Tropical Medicine and Parasitology, 57(4):415-427.

Smithburn KC, Hughes TP, Burke AW, Paul JH, 1940. A neurotropic virus isolated form the blood of a native of Uganda. American Journal of Tropical Medicine, 20(4):471-492.

Steele KE, Linn MJ, Schoepp RJ, Komar N, Geisbert TW, Manduca RM, Calle PP, Raphael BL, Clippinger TL, Larsen T, Smith J, Lanciotti RS, Panella NA, McNamara TS, 2000. Pathology of fatal West Nile virus infections in native and exotic birds during the 1999 outbreak in New York City, New York. Veterinary Pathology, 37(3):208-224.

Taylor RM, Work TH, Hurlbut HS, Rizk F, 1956. A study of the ecology of West Nile virus in Egypt. American Journal of Tropical Medicine and Hygiene, 5(4):579-620.

Trevejo RT, Eidson M, 2008. Zoonosis update: West Nile virus. Journal American Veterinary Medical Association, 232:1302-1309.

Trock SC, Meade BJ, Glaser AL, Ostlund EN, Lanciotti RS, Cropp BC, Kulasekera V, Kramer LD, Komar N, 2001. West Nile virus outbreak among horses in New York state, 1999 and 2000. Emerging Infectious Diseases, 7(4):745-747.

Tsai TF, Popovici F, Cernescu C, Campbell GL, Nedelcu NI, 1998. West Nile encephalitis epidemic in southeastern Romania. Lancet (British edition), 352(9130):767-771; 31 ref.

Weinberger M, Pitlik SD, Gandacu D, Lang R, Nassar F, Ben David D, Rubinstein E, Izthaki A, Mishal J, Kitzes R, Siegman-Irga Y, Giladi M, Pick N, Mendelson E, Bin H, Shohat T, Chowers MY, 2001. West Nile fever outbreak, Israel 2000: epidemiologic aspects. Emerging Infectious Diseases, 7(4):686-691.

Links to Websites

Top of page
WebsiteURLComment
Audubon - West Nile virus informationhttp://www.audubon.org/bird/wnv/index.htmlA concise summary of West Nile disease and its impact on North American birds and wildlife. The site contains references with links and a series of maps showing the spread of West Nile virus throughout USA from 1999 to 2004.
Center for Disease Control (CDC), Atlantahttp://www.cdc.gov/
CFSPH: Animal Disease Informationhttp://www.cfsph.iastate.edu/DiseaseInfo/index.php"Animal Disease Information" provides links to various information sources, including fact sheets and images, on over 150 animal diseases of international significance.
FAO - West Nile Fever Presentationhttp://www.fao.org/docs/eims/upload/276313/ak752e00.pdf
OIE Manual of Diagnostic Tests and Vaccines for Terrestrial Animalshttp://www.oie.int/en/international-standard-setting/terrestrial-manual/access-online/The Manual of Diagnostic Tests and Vaccines for Terrestrial Animals (Terrestrial Manual) aims to facilitate international trade in animals and animal products and to contribute to the improvement of animal health services world-wide. The principal target readership is laboratories carrying out veterinary diagnostic tests and surveillance, plus vaccine manufacturers and regulatory authorities in Member Countries. The objective is to provide internationally agreed diagnostic laboratory methods and requirements for the production and control of vaccines and other biological products.
OIE Terrestrial Animal Health Codehttp://www.oie.int/en/international-standard-setting/terrestrial-code/access-online/The OIE Terrestrial Animal Health Code sets out standards for the improvement of terrestrial animal health and welfare and veterinary public health worldwide, including through standards for safe international trade in terrestrial animals and their products. The health measures in the Terrestrial Code should be used by the veterinary authorities of importing and exporting countries to provide for early detection, reporting and control agents pathogenic to terrestrial animals and, in the case of zoonoses, for humans, and to prevent their transfer via international trade in terrestrial animals and terrestrial animal products, while avoiding unjustified sanitary barriers to trade.
US Department of Health and Human Services - Centers for Disease Control and Preventionhttp://www.cdc.gov/ncidod/dvbid/westnile/resources/wnv-guidelines-apr-2001.pdfEpidemic/Epizootic West Nile Virus in the United States: Revised Guidelines for Surveillance, Prevention, and Control
US Geological Survey. Center for Integration of Natural Disaster Informationhttp://cindi.usgs.gov/hazard/event/west_nile/west_nile.htmlUS cases of West Nile virus; human, veterinary and vector-based.
USDA - Animal and Plant Health Inspection Service - West Nile virushttp://www.aphis.usda.gov/oa/wnv/index.html
World Health Organizationhttp://www.who.int
WWT Factsheet - West Nile virus diseasehttp://www.wwt.org.uk/uploads/documents/1341561742_RWDM4DiseaseFactsheetsWestNilevirusdisease.pdf

Distribution Maps

Top of page
You can pan and zoom the map
Save map