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West Nile fever virus


Synonym(s): WNV




  • Family: Flaviviridae.
  • Genus:Flavivirus.
  • Group: Japanese Encephalitis.


  • Latin: flavus - yellow.

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Clinical Effects



  • High level viremia occurs in birds.
  • Many species of mosquitoes can be hosts.
  • Virus has also been isolated from ticks; their role in transmission is not clear; transtadial transmission may be important for virus maintenance.
  • Overwintering mechanism not known.


  • Virus circulates primarily between mosquitoes and birds.
  • Inoculation into incidental host results in transient viremia, in horses this is often followed by infection of CNS.
  • Infection of CNS is enhanced by other factors that compromise the blood-brain barrier.
  • Replication occurs in the cellular cytoplasm.


  • Arthropod-borne transmission.
  • Peak of incidence in summer, early autumn/fall.
  • Horses are not a source for human infection.

Pathological effects

  • Neuronal and glial infection   →   destruction due to direct viral injury.
  • Inflammation also   →    neuronal and astrocyte destruction.
  • Dysfunction of neurons without cell death may occur.
  • Subclinical infection or mild febrile disease may result in lieu of encephalitis.

Other Host Effects

  • Humans and horses are incidental hosts, and not a source of virus for spread.
  • Pathogenicity and virulence may be strain-related.


Control via environment

Vector abatement

  • Efforts should be made to reduce the population of mosquito vectors and to minimize the exposure of susceptible horses to mosquitoes.
  • Mosquito population reduction can be achieved by using larvicidal or adulticidal chemical treatments and removal of mosquitoes breeding sites.
  • Physical and chemical barriers can be placed between at-risk horses and mosquitoes in order to decrease the exposure during epizootic.
  • Summer-weight blankets and mosquito repellents can be helpful.
  • Horses should be stabled at night and during other periods of great mosquito activity and the barns rendered as vector-free as possible. 

Movement restrictions

  • Horses do not serve as a source of the virus for mosquitoes. However, there are still concerns about the potential threat posed by close proximity to infected animals.
  • Only restriction of movement of sick horses is justified since there is no evidence that WNV produces a long-term viremia in horses.
  • In a recent experiment, using one species of mosquito, uninfected mosquitoes remained uninfected after they were fed on experimentally infected horses during the viremic stages.
  • Horses housed outdoors at night time, or close to mosquito infested water, are at higher risk of infection.


  • Licensed vaccines are available in Europe, USA and Canada.

The available EEE, WEE and VEE vaccines in the US are not cross protective for WNV infection.

  • It has been recommended that the initial and booster be given at least 4 weeks before the start of mosquito season. However, the duration of immunity appears to be short and the period when clinical signs are seen is later than the start of the mosquito season, so it might be more beneficial that the second vaccination be timed for 2 weeks before the period when the first equine cases are likely to be first seen. 
  • The optimal frequency of subsequent vaccine boosters is unknown at this time.
  • Horses that have clinical signs do not benefit from vaccination and vaccination may confound testing. 
  • The exact antibody titer that is required to protect against WNV infection is unknown. In addition, the exact duration of protection after vaccination or natural infection is also unknown at the present time. 
  • Some horses are poor responders to vaccination.
  • Repeat vaccination in recovered and vaccinated horses at least annually and perhaps more in areas where the vector season persists for more than 3-4 months.
  • In an experiment to asses the efficacy of one vaccination (Recombitek Equine West Nile Virus (Merial)), 19 horses were exposed to WNV-infected mosquitoes; 8 of 10 non-vaccinated horses developed viremia, while only 1 of 9 vaccinated horses developed viremia.
  • In an outbreak of WNV in horses in North Dakota, 27% of the affected horses were vaccinated against WNV.
  • In Florida, a small number of fully vaccinated horses developed the disease.
  • Also, many horses received only one injection during the outbreak and a significant number of them developed the disease.



  • Recombitek Equine West Nile Virus(Merial).
  • West Nile-Innovator (Fort Dodge).
  • West Nile-Innovator + EW (Fort Dodge).
  • West Nile-Innovator + EWT (Fort Dodge).


Other countermeasures

  • Quarantine of affected animals not necessary; isolation is recommended.

European Union (EU)

  • As of the 23rd November 2001, the European Commission of the EU updated requirements relating to horses being imported from the US; a supplementary certification must be added to the horse's health certificate stating that the horse complies with one of the following conditions:
    • It was not vaccinated against WNV or;
    • It was vaccinated against WNV with an inactivated vaccine on at least two occasions at an interval of between 21-42 days, the last vaccination being carried out no later than 30 days prior to dispatch on (date).


  • Regulations affected as of 17th September 2000:
    • Any equine being transported from the affected areas to premises either within or outside of the affected areas must be accompanied by a certficate stating that in the past 30 days, no case of West Nile fever has been identified within a radius of at least 50 km of the animal's premises of origin, and that for the previous 2 weeks the animal has not come into contact with any equine that has resided at premises where a case of West Nile fever has been confirmed in the previous 30 days.
    • If a case if West Nile fever has occurred within 50 km of the animal's premises of origin in the previous 30 days, the certificate must state that before its departure, the animal spent at least 21 days in quarantine, the animal's temperature was taken every day and remained normal. The animal must have had a blood test, either 17 days or 21 days after the start of the quarantine period, and the sample must have tested negative for the West Nile virus antibodies by either one or two ELISA tests.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Raleigh P L et al (2012) Suveillance for antibodies to West Nile virus in Ireland. Vet Rec 170 (7), 180 PubMed.
  • Jiménez-Clavero et al (2010) West Nile virus serosurveillance in horses in Doñana, Spain, 2005 to 2008. Vet Rec 167 (10), 379-380 PubMed.
  • Durand B et al (2005) Serosurvey for West Nile virus in horses in sounthern France. Vet Rec 157 (22), 711-713 PubMed.
  • Davidson A H, Traub-Dargatz J L, Rodeheaver R M et al (2005) Immunologic responses to West Nile virus in vaccinated and clinically affected horses. JAVMA 226 (2), 240-245 PubMed.
  • Vest D J, Cohen N C, Berezowski C J, Morehead J P, Blodgett G P & Blanchard T L (2004) Evaluation of administration of West Nile virus vaccine to pregnant broodmares. JAVMA 225 (12), 1894-1897 PubMed.
  • Salazar P, Traub-Dargatz J L, Morley P S et al (2004) Outcome of equids with clinical signs of West Nile virus infection and factors associated with death. JAVMA 225 (2), 267-274 PubMed.
  • Schuler L A, Khaitsa M L, Dyer N W & Stoltenow C L (2004) Evaluation of an outbreak of West Nile virus infection in horses: 569 cases (2002). JAVMA 225 (7), 1084-1089 PubMed.
  • Siger L, Bowen R A, Karaca K et al (2004) Assessment of the efficacy of a single dose of a recombinant vaccine against West Nile virus in response to natural challenge with West Nile virus-infected mosquitoes in horses. Am J Vet Res 65 (11), 1459-1462 PubMed.
  • Ward M P, Levy M, Thacker H L et al (2004) Investigation of an outbreak of encephalomyelitis caused by West Nile virus in 136 horses. JAVMA 225 (7), 1027-1028 PubMed.
  • Abutarbush S M, OConnor B P, Clark C, Sampieri F & Naylor J M (2004) Clinical West Nile virus in 2 horses in western Canada.Can Vet J 45 (4), 315-317 PubMed Full Text Article.
  • Porter M B, Long M T, Getman L M et al (2003) West Nile Virus encephalomyelitis in horses: 46 cases (2001). JAVMA 222 (9), 1241-1247 PubMed.
  • Bunning M L, Bowen R A, Cropp C B et al (2002) Experimental infection of horses with West Nile virus. Emerg Infec Dis (4), 380-386 PubMed Full Text Article.
  • Snook C S (2002) Equine west Nile virus: diagnosis and treatment. Comp Contin Educ Pract Vet 24 (4), 338-340.
  • Long M T (2002) West Nile virus. Standards of care. Equine Diag Treatment (3), 1-6.
  • Snook C S, Hyman S S, Del Perio F et al (2001) West Nile virus encephalomyelitis in eight horses. JAVMA 218, 1576-1579 PubMed.
  • Wamsley H L, Alleman A R, Porter M B & Long M T (2001) Findings in cerebrospinal fluids of horses infected with West Nile virus: 30 cases (2001). JAVMA 221 (9), 1303-1305 PubMed.
  • Kramer L D & Bernard K A (2001) West Nile virus infection in birds and mammals. Ann N Y Acad Sci 951, 84-93 PubMed.
  • Marfin A A & Gubler D J (2001) West Nile encephalitis - an emerging disease in the United States. Clin Infect Dis 33 (10), 1713-1719 PubMed Uni of Chicago Press.
  • Kulasekera V L, Kramer L et al (2001)West Nile virus infection in mosquitoes, birds, horses and humans, Staten Island, New York, 2000. Emerg Infect Dis (4), 722-725 PubMed CDC.
  • Ostlund E N, Crom R L et al (2001) Equine West Nile encephalitis, United States. Emerg Infect Dis (4), 665-669 PubMed CDC.
  • Ostlund E N, Andresen J E & Andresen M (2000) West Nile encephalitis. Vet Clin North Am Equine Pract 16, 427-442.
  • Rappole J H, Derrickson S R & Hubalek Z (2000) Migratory birds and spread of West Nile virus in the Western Hemisphere. Emerg Infect Dis (4), 319-328 PubMed CDC.
  • Nolen R S (2000) West Nile virus survives winter; no surprise, says CDC. JAVMA 216 (8), 1199-1200 PubMed.
  • Cantile C, Ciguardo G, Eleni C & Arisici M (2000) Clinical and neuropathological features of West Nile virus equine encephalomyelitis in Italy. Equine Vet J 32, 31-35 PubMed.

Other sources of information

  • Leighton F A & Abutarbush S M (2003) The West Nile Virus Epidemic in North America: 1999-2002. In: Large Animal Veterinary Rounds (1).
  • University of Kentucky (2003) West Nile 2002. In: Equine Dis Q 12 (1), 4-5.
  • Long M T et al (2002) Equine West Nile Encephalitis: Epidemiological and Clinical Review for Practitioners. In: Proc AAEP.Orlando, Florida, USA. 48, 1-6.
  • Smith B P (2002) Large Animal Internal Medicine. 3rd edn. Mosby, USA. pp 890-891.
  • University of Kentucky (2002) West Nile Virus Update. In: Equine Dis Q 11 (1), 4.
  • Issel C & McConnico R (2001) West Nile Update. In: Equine Dis Q (2), 3.
  • Monath T P & Heinz F X (1996) Flaviviruses. In: Fields Virology. Eds: B N Fields, D M Knipe & P M Howley. Lipincott-Raven, USA.
  • Rice C M (1996) Flaviviridae - the viruses and their replication. In: Fields Virology. Eds: B N Fields, D M Knipe & P M Howley. Lipincott-Raven, USA.





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