You have been logged out of VisualDx or your session has expired.

Please reload this page and sign into VisualDx to continue.

  VisualDx Mobile   Select Language

Get VisualDx Mobile

There are VisualDx mobile apps available for iOS and Android devices.

You will need a VisualDx account to use the mobile apps.

Already have an account? Sign In or
sign up for a free trial.

Users with VisualDx accounts earn CME credits for using VisualDx.

Already have an account? Sign In or
sign up for a free trial.

Create a Personal Account

E-mail (username)
Verify Password
First Name
Last Name

Personal Account Created

Mobile Access

You can now download VisualDx for your iOS and Android devices. Launch the VisualDx app from your device and sign in using your VisualDx personal account username and password.

CME Certification

Sign in with your personal account to earn and claim CME credits through VisualDx. Credits can be earned by building a differential or looking up a diagnosis.

Version: 7.13.1441   (build df7aed4)
Select Language

Select Region

Send us your feedback

This field is required

Oops! There was an issue during submission. Please try again. If the problem persists, email with your feedback.

Thank You!

We appreciate your feedback and you will be hearing from us soon.


Share This Page

Thank You!

We have sent an e-mail with a link to the current page.


E-mail This Patient Information Sheet

Thank You!

We have sent an e-mail with this patient information.


Image Contributors


  • Christine Ahn MD
    Carl Allen DDS, MSD
    Brandon Ayres MD
    Howard P. Baden MD
    Robert Baran MD
    Keira Barr MD
    Gregory J. Basura MD, Ph.D
    Donald Belsito MD
    Jeffrey D. Bernhard MD
    Jesse Berry MD
    Victor Blanco MD
    Benjamin R. Bohaty MD
    William Bonnez MD
    Sarah Brenner MD
    Robert A. Briggaman MD
    Robert Brodell MD
    Roman Bronfenbrener MD
    Walter Brooks MD
    William Buckley MD
    Philip Bulterys MD, PhD (candidate)
    Susan Burgin MD
    Sonya Burton MD
    Sean P. Bush MD, FACEP
    Jeffrey Callen MD
    Scott Camazine MD
    Michael Cardwell
    Shelley D. Cathcart MD
    Robert Chalmers MD, MRCP, FRCP
    Chia-Yu Chu MD, PhD
    Flavio Ciferri MD
    Maria Rosa Cordisco MD
    Noah Craft MD, PhD
    John T. Crissey MD
    Harold E. Cross MD, PhD
    Charles E. Crutchfield III MD
    Adriana Cruz MD
    Donna Culton MD, PhD
    Bart J. Currie MBBS, FRACP, DTM&H
    Chicky Dadlani MD
    Alexander Dane DO
    C. Ralph Daniel III MD
    Thomas Darling MD, PhD
    William Delaney MD
    Damian P. DiCostanzo MD
    Ncoza Dlova MD
    James Earls MD
    Libby Edwards MD
    Melissa K. Egge MD
    Charles N. Ellis MD
    Rachel Ellis MD
    David Elpern MD
    Nancy Esterly MD
    Stephen Estes MD
    E. Dale Everett MD
    Janet Fairley MD
    David Feingold MD
    Jennifer J. Findeis-Hosey MD
    Benjamin Fisher MD
    Henry Foong MBBS, FRCP
    David Foster MD, MPH
    Brian D. Foy PhD
    Michael Franzblau MD
    Vincent Fulginiti MD
    Sunir J. Garg MD, FACS
    Kevin J. Geary MD
    Lowell Goldsmith MD, MPH
    Sethuraman Gomathy MD
    Bernardo Gontijo MD, PhD
    Kenneth Greer MD
    Kenneth G. Gross MD
    Alan Gruber MD
    Nathan D. Gundacker MD
    Akshya Gupta MD
    Vidal Haddad MSC, PhD, MD
    Edward Halperin MD, MA
    Ronald Hansen MD
    John Harvey
    Rizwan Hassan MD
    Michael Hawke MD
    Jason E. Hawkes MD
    Peter W. Heald MD
    David G. Hicks MD
    Sarah Hocker DO
    Ryan J. Hoefen MD, PhD
    Li-Yang Hsu MD
    William Huang MD
    Sanjana Iyengar MD
    Alvin H. Jacobs MD
    Saagar Jadeja MD
    Shahbaz A. Janjua MD
    Joshua J. Jarvis MD
    Kit Johnson
    Zachary John Jones MD
    Robert Kalb MD
    A. Paul Kelly MD
    Henry Kempe MD
    Loren Ketai MD
    Sidney Klaus MD
    Ashwin Kosambia MD
    Jessica A. Kozel MD
    Carl Krucke
    Mario E. Lacouture MD
    Joseph Lam MD
    Alfred T. Lane MD
    Edith Lederman MD
    Nahyoung Grace Lee MD
    Pedro Legua MD, PhD
    Robert Levin MD
    Bethany Lewis MD
    Sue Lewis-Jones FRCP, FRCPCH
    Taisheng Li MD
    Christine Liang MD
    Shari Lipner MD, PhD
    Adam Lipworth MD
    Jason Maguire MD
    Mark Malek MD, MPH
    Jere Mammino DO
    Ricardo Mandojana MD
    Lynne Margesson MD
    Thomas J. Marrie MD
    Maydel Martinez MD
    Ralph Massey MD
    Patrick McCleskey MD
    Karen McKoy MD
    Thomas McMeekin MD
    Josette McMichael MD
    Somchai Meesiri MD
    Joseph F. Merola MD
    Mary Gail Mercurio MD
    Anis Miladi MD
    Larry E. Millikan MD
    Dan Milner Jr. MD
    Zaw Min MD
    Stephanie Montero
    Alastair Moore MD
    Keith Morley MD
    Dean Morrell MD
    Samuel Moschella MD
    Rehan Naseemuddin MD
    Taimor Nawaz MD
    Vic Newcomer MD
    John Nguyen MD
    Matilda Nicholas MD
    Thomas P. Nigra MD
    Steven Oberlender MD, PhD
    Maria Teresa Ochoa MD
    Art Papier MD
    Lawrence Parish MD
    Tanner Parrent MD
    Mukesh Patel MD
    Lauren Patty-Daskivich MD
    David Peng MD, MPH
    Robert Penne MD
    Nitipong Permpalung MD
    Miriam Pomeranz MD
    Doug Powell MD
    Harold S. Rabinovitz MD
    Christopher J. Rapuano MD
    Sireesha Reddy MD
    Angela Restrepo MD, PhD
    Bertrand Richert MD, PhD
    J. Martin Rodriguez, MD, FACP
    Theodore Rosen MD
    Misha Rosenbach MD
    Scott Schiffman MD
    Robert H. Schosser MD
    Glynis A. Scott MD
    Carlos Seas MD, MSc
    Deniz Seçkin MD
    Daniel Sexton MD
    Paul K. Shitabata MD
    Tor Shwayder MD, FAAP, FAAD
    Elaine Siegfried MD
    Gene Sienkiewicz MD
    Christye Sisson
    Philip I. Song MD
    Mary J. Spencer MD, FAAP
    Lawrence B. Stack MD
    Sarah Stein MD
    William Van Stoecker MD
    Frances J. Storrs MD
    Erik J. Stratman MD
    Lindsay C. Strowd MD
    Erika Summers MD
    Belinda Tan MD, PhD
    Robert Tomsick MD
    Hensin Tsao MD, PhD
    Richard P. Usatine MD
    Jenny Valverde MD
    Vishalakshi Viswanath MD
    Susan Voci MD
    Lisa Wallin ANP, FCCWS
    Douglas Walsh MD
    Ryan R. Walsh MD
    George Watt MD
    Clayton E. Wheeler MD
    Sally-Ann Whelan MS, NP, CWOCN
    Jan Willems MD, PhD
    James Henry Willig MD, MPH
    Karen Wiss MD
    Vivian Wong MD, PhD
    Sook-Bin Woo MS, DMD, MMSc
    Jamie Woodcock MD
    Stephen J. Xenias MD
    Nathaniel Yohannes
    Lisa Zaba MD
    Vijay Zawar MD
    Bonnnie Zhang MD
    Carolyn Ziemer MD
    Jeffrey P. Zwerner MD, PhD


  • Am. Journal of Trop. Med & Hygiene
  • Armed Forces Pest Management Board
  • Blackwell Publishing
  • Bugwood Network
  • Centers For Disease Control and Prevention
  • Centro Internacional de Entrenamiento e Investigaciones Mèdicas (CIDEIM)
  • Dermatology Online Journal
  • East Carolina University (ECU), Division of Dermatology
  • International Atomic Energy Agency
  • Massachusetts Medical Society
  • Oxford University Press
  • Radiological Society of North America
  • Washington Hospital Center
  • Wikipedia
  • World Health Organization
ContentsSynopsisCodesLook ForDiagnostic PearlsDifferential Diagnosis & PitfallsBest TestsManagement PearlsTherapyReferences
Western equine encephalitis
Other Resources UpToDate PubMed

Western equine encephalitis

Print Images (1)
Contributors: Zaw Min MD, FACP, Ricardo M. La Hoz MD
Other Resources UpToDate PubMed


Western equine encephalitis (WEE) is caused by a zoonotic arthropod-borne virus, WEE virus, which belongs to the Togaviridae family and genus Alphavirus. WEE virus is closely related to other arboviral members in the Togaviridae family, namely eastern equine encephalitis and Venezuelan equine encephalitis. Human cases of WEE have occurred in the western and midwestern United States, western Canada, and South America (Guyana, Ecuador, Brazil, Uruguay, and Argentina).

The enzootic cycle of WEE is maintained between passerine birds as the primary reservoirs and the mosquito vector Culex tarsalis. The epizootic transmission to horses and humans is mediated by specific mosquito bridge vectors such as Ochlerotatus melanimon in California, Aedes dorsalis in Utah and New Mexico, and Aedes campestris in New Mexico. Most cases of WEE infection have occurred in summer, from June to September in the United States and a month later in Canada. Similar to Venezuelan equine encephalitis virus, WEE virus can be transmitted through aerosols and so could be used as an agent in bioterrorism.

There has been a significant decline in human cases of WEE since 1988, and no cases have been reported since 2001 according to the US Centers for Disease Control and Prevention (CDC). This decline in incidence is thought to be due to improved mosquito control, a decline in the horse population, and high vaccination rates in horses.

Most human cases of WEE are asymptomatic. In symptomatic patients, a nonspecific febrile syndrome occurs after a 5-10 day incubation period.

Less than 1 in 1000 adult patients develop neuroinvasive encephalitis, but the risk is much higher in children (1:50) and infants (1:1). Typical features of encephalitis are headache, nausea, vomiting, neck stiffness, altered mental status, and seizures. Children and infants tend to have more severe neurologic symptoms.

The overall case fatality rate is estimated to be between 3% and 7% and is especially high in the extremes of age, ie, the young and the very elderly. Among survivors, neurologic sequelae are more common in infants younger than 1 year (sequelae such as mental retardation, developmental delay) and in elderly patients (sequelae such as parkinsonism, dementia).


A83.1 – Western equine encephalitis

47523006 – Western equine encephalitis

Look For

Subscription Required

Diagnostic Pearls

Subscription Required

Differential Diagnosis & Pitfalls

The major differential diagnoses are several mosquito-borne arboviral encephalitides commonly seen in the United States.
  • West Nile virus (WNV) encephalitis (see WNV, viral encephalitides) – There is widespread distribution in the continental United States, and it is the most common arboviral infection in the United States. The majority of cases occur in late summer through fall. Neuroinvasive WNV can present as encephalitis, meningitis, flaccid paralysis, or a mixed pattern.
  • St. Louis encephalitis (SLE) – Elderly patients are more prone to symptomatic infections. Prior to the onset of neurologic symptoms, patients experience fever, generalized malaise, and myalgias. Some patients have dysuria, hematuria, or sterile pyuria. Neurologic manifestations include meningitis and encephalitis. Patients may present with myoclonus, tremors, opsoclonus, ataxia, or nystagmus. Cerebrospinal fluid (CSF) studies show lymphocytic pleocytosis. Magnetic resonance imaging (MRI) findings are nonspecific, although MRI may show T2 signal intensity in the substantia nigra.
  • Eastern equine encephalitis (EEE) – Most human cases in the United States occur along the Atlantic and Gulf coasts during summer. It has the highest mortality rate (as high as 75%) among arboviral encephalitides in the United States. Typical MRI brain findings include hyperintense signal abnormalities in bilateral symmetric basal ganglia, thalami, and brain stem.
  • Venezuelan equine encephalitis (VEE) – It has a widespread geographic distribution from Florida to South America. It has an average case fatality rate of 10% but neurologic recovery is excellent in survivors. VEE virus could be utilized as a biowarfare weapon since it is infectious via aerosols.

Best Tests

Subscription Required

Management Pearls

Subscription Required


Subscription Required


Subscription Required

Last Updated: 09/30/2014
Copyright © 2018 VisualDx®. All rights reserved.
Western equine encephalitis
Print 1 Images
Western equine encephalitis : Fever, Headache, Malaise, Myalgia, Nuchal rigidity
Copyright © 2018 VisualDx®. All rights reserved.