• Disease Overview
  • Synonyms
  • Signs & Symptoms
  • Causes
  • Affected Populations
  • Disorders with Similar Symptoms
  • Diagnosis
  • Standard Therapies
  • Clinical Trials and Studies
  • References
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  • Complete Report

West Nile Encephalitis


Last updated: January 29, 2008
Years published: 2002, 2004, 2005, 2008

Disease Overview

There are seventeen species of birds that are the known carriers of and transmit West Nile Encephalitis (WNE) to humans via the Culex, Aedes, and Anopheles mosquitoes. WNE first causes symptomatic or asymptomatic illness in wild migratory birds that act as viral replication factories. Wild birds infected with WNE contain high titers of the virus and remain viremic for 1-2 weeks, making them ideal hosts to perpetuate the disease. Mosquitoes transmit WNE from birds to humans. Horses, dogs, and other small animals may harbor WNE after being bitten; however, they are inefficient transmitters because viral titers are relatively low, and WNE viremia is short-lived in these animals.

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  • Eastern equine encephalitis
  • EEE
  • Japanese encephalitis
  • Kunjin fever
  • Venezuelan encephalitis
  • Viral encephalitis
  • WEE
  • Western equine encephalitis
  • West Nile Fever
  • West Nile virus
  • WNE
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Signs & Symptoms

Patients with WNE may present with a wide range of severity in symptoms and may features of encephalitis, aseptic meningitis (meningoencephalitis), or both. It is common for patients to have a mild febrile illness accompanied by headache, mental confusion, tremors, or flaccid paralysis. Symptoms are most obvious in people who are very young and people who are elderly. Many people who are infected (at least most children) are also often asymptomatic or have a nonspecific flu-like illness. A minority of people may develop neurologic disease.

The extent and severity of viral invasion of the central nervous system (CNS) determine the clinical expression of WNE. Most patients are febrile or have low-grade fevers. Patients presenting with encephalitis have mental confusion or disorientation and may have decreased consciousness. Patients with severe cases of WNE may present with stupor or coma. The most common ophthalmic manifestation for WNE is chorioretinitis and about 10% of patients have an enlarged liver and spelnomegaly. CNS findings include a stiff neck with meningeal signs, including positive Kernig or Brudzinski signs in patients with aseptic meningitis or meningoencephalitis. Generalized adenopathy and an enlarged submental node are also very common.

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Mosquito bites are the main culprits for the spread of WNE with an incubation period of WNE is 1-6 day but many patients may not provide a mosquito bite history. WNE usually occurs in the summer when mosquitoes, wild migratory birds, and humans are in close proximity outdoors. Mosquito bites, which are particularly prone to occur during feeding times (dawn and dusk) in the summer months, transmit WNE and prolonged contact or multiple mosquito bites enhances risk.

WNE may be transmitted in organ transplants and WNE virus has been found in breast milk.

WNE, as with other arthropod-borne viral encephalitis, traverses the blood-brain barrier and infects the brain parenchyma, clinically manifesting as viral encephalitis. WNE may also affect the leptomeninges, resulting in a clinical presentation of aseptic meningitis (viral meningitis). Patients with WNE may present with features of both encephalitis and aseptic meningitis (meningoencephalitis).

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Affected populations

In the US the initial cases of WNE were first identified in the greater New York area, but cases have now occurred in western states. Wild birds with WNE virus have also been identified in parts of Canada. WNE is common in the Middle East, Asia, and Africa WNE seropositivity of children in Egypt is approximately 50%. WNE is the most common cause of viral aseptic meningitis or encephalitis in patients presenting to emergency departments in Cairo. Both sexes are affected equally and it is more frequent in elderly patients.

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Electroencephalogram: This is the most sensitive way to make a presumptive diagnosis of HSV-1 encephalitis. An abnormal temporal lobe focus is present on EEG as early as the first few days of the disease.

EEG shows unilateral focal electrical abnormalities in the temporal lobe.

Lumbar puncture: CSF reveals mild-to-moderate pleocytosis with a lymphocytic predominance in WNE. CSF protein is variably elevated, and the CSF glucose level is not decreased.

The CSF lactic acid level is not elevated, and RBCs, excluding traumatic taps, are not present in WNE. CSF Gram stain and bacterial culture findings are negative.

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Standard Therapies

Medical Care

Provide supportive care to minimize the potential for cerebral edema. In comatose patients, pay special attention to maintaining a clear airway. Fluid replacement is also essential.

Surgical Care

No surgical care is necessary.

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Clinical Trials and Studies

Information on current clinical trials is posted on the Internet at www.clinicaltrials.gov. All studies receiving U.S. government funding, and some supported by private industry, are posted on this government web site.

For information about clinical trials being conducted at the National Institutes of Health (NIH) in Bethesda, MD, contact the NIH Patient Recruitment Office:

Tollfree: (800) 411-1222

TTY: (866) 411-1010

Email: prpl@cc.nih.gov

For information about clinical trials sponsored by private sources, contact:


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Asnis DS, Conetta R, Teixeira AA, et al: The West Nile Virus outbreak of 1999 in New York: the Flushing Hospital experience. Clin Infect Dis 2000 Mar; 30(3): 413-8

Batuello JT, Youngwerth J, Gabel R: Increased serum lipase in West Nile virus infection. N Engl J Med 2005 Jan 27; 352(4): 420-1

Cunha BA, Sachdev B, Canario D: Serum ferritin levels in West Nile encephalitis. Clin Microbiol Infect 2004; 10: 184-186.

Bréhin AC, Mouriès J, Frenkiel MP, Dadaglio G, Desprès P, Lafon M, Couderc T. Dynamics of Immune Cell Recruitment during West Nile Encephalitis and Identification of a New CD19+B220-BST-2+ Leukocyte Population. J Immunol. 2008 May 15;180 (10):6760-7.

Donoso Mantke O, Vaheri A, Ambrose H, Koopmans M, de Ory F, Zeller H, Beyrer K, Windorfer A, Niedrig M; European Network for Diagnostics of Imported Viral Diseases (ENIVD) Working Group for Viral CNS Diseases. Analysis of the surveillance situation Euro Surveill. 2008 Jan 17;13(3) pii: 8017.

Widman DG, Ishikawa T, Fayzulin R, Bourne N, Mason PW. Construction and characterization of a second-generation pseudoinfectious West Nile virus vaccine propagated using a new cultivation system. Vaccine. 2008 Mar 31.

Monreal R, Vega A, Simeone F, Hamm L, Palacios E, Maylin M, Lopez FA.

An immunosuppressed woman presenting with acute flaccid paralysis and progressive respiratory failure. J La State Med Soc. 2007 Nov-Dec;159(6):298-303; quiz 304, 342.

Takasaki T. [West Nile fever/encephalitis] Uirusu. 2007 Dec;57(2):199-205. Review. Japanese.

Goody RJ, Schittone SA, Tyler KL. Experimental reovirus-induced acute flaccid paralysis and spinal motor neuron cell death.

J Neuropathol Exp Neurol. 2008 Mar;67(3):231-9.

Shrestha B, Ng T, Chu HJ, Noll M, Diamond MS.The relative contribution of antibody and CD8(+) T cells to vaccine immunity against West Nile encephalitis virus.

Vaccine. 2008 Apr 7;26(16):2020-33. Epub 2008 Feb 20.

Patiris PJ, Oceguera LF 3rd, Peck GW, Chiles RE, Reisen WK, Hanson CV. Serologic diagnosis of West Nile and St. Louis encephalitis virus infections in domestic chickens.

Am J Trop Med Hyg. 2008 Mar;78 (3):434-41.

Wittich CA, Ward MP, Fosgate GT, Srinivasan R. Identification of hyperendemic foci of horses with West Nile virus disease in Texas. Am J Vet Res. 2008 Mar; 69(3):378-84.

Klinkhamer K, Lipman LJ. [Introduction of West Nile virus in the Netherlands]

Tijdschr Diergeneeskd. 2008 Feb 1;133(3):106-7. Review. Dutch.

Hall DA, Tyler KL, Frey KL, Kozora E, Arciniegas DB. Persistent neurobehavioral signs and symptoms following West Nile fever.

J Neuropsychiatry Clin Neurosci. 2008 Winter;20(1):122-3.

Douglas KO, Kilpatrick AM, Levett PN, Lavoie MC. A quantitative risk assessment of West Nile virus introduction into Barbados.West Indian Med J. 2007 Oct;56(5):394-7.

McMurtrey CP, Lelic A, Piazza P, Chakrabarti AK, Yablonsky EJ, Wahl A, Bardet W, Eckerd A, Cook RL, Hess R, Buchli R, Loeb M, Rinaldo CR, Bramson J, Hildebrand WH. Epitope discovery in West Nile virus infection: Identification and immune recognition of viral epitopes. Proc Natl Acad Sci U S A. 2008 Feb 26;105(8):2981-6. Epub 2008 Feb 25.

Rison RA. Re: Modafinil for treatment of fatigue in post-polio syndrome: a randomized controlled trial.

Neurology. 2008 Feb 26;70(9):736-7; author reply 737-8. No abstract available.

PMID: 18299530

Hamer GL, Kitron UD, Brawn JD, Loss SR, Ruiz MO, Goldberg TL, Walker ED. Culex pipiens (Diptera: Culicidae): a bridge vector of West Nile virus to humans. J Med Entomol. 2008 Jan;45(1):125-8.

Gerry AC, Nawaey TM, Sanghrajka PB, Wisniewska J, Hullinger P. Hematophagous Diptera collected from a horse and paired carbon dioxide-baited suction trap in southern California: relevance to West Nile virus epizootiology. J Med Entomol. 2008 Jan;45(1):115-24.

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