Western equine encephalitis (WEE) is a virus spread by a bite from an infected mosquito. While WEE is rare, an infection can be serious or fatal.
WEE is caused by a virus. The virus is passed through the bite of an infected mosquito.
Factors that may increase your chance of WEE include: Living in or visiting the plains regions of western and central United StatesDoing activities outdoors and not using insect repellent
Most people with WEE do not have any symptoms.
If symptoms do occur, they appear within 5-10 days after infection and include: HeadacheFeverNeck stiffnessChillsFatigueJoint and muscle painVomiting
WEE can lead to more serious, life-threatening symptoms like inflammation of the brain (encephalitis), seizures, and
. These serious symptoms are more common in infants and older adults.
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In addition to taking your medical history and doing a physical exam, your doctor will ask you: What kind of symptoms you are experiencingWhere you have been living or travelingWhether you have been exposed to mosquitoes
Your doctor may need to test your bodily fluids. This can be done with: Blood tests
Cerebrospinal fluid tests
to evaluate the fluid that surrounds the brain and spinal cord
Imaging tests to evaluate the brain can be done with: MRI scanCT scan
Because the infection is viral, there is no specific treatment for WEE. Treatment will focus on managing your symptoms and related complications through: IV fluidsAntiseizure medicationsMedications to decrease brain swelling
Breathing support—mechanical ventilation
There is no vaccine for humans. There is a vaccine for horses. Prevention of WEE focuses on controlling mosquitoes and avoiding mosquito bites. Steps you can take to avoid mosquito bites include: Stay inside between dusk and dark, when mosquitoes are most active.Wear long pants and long-sleeved shirts when outside.Use an insect repellent with DEET.Repair screens to prevent mosquitoes from entering the house.Use proper mosquito netting at night. Look for netting treated with insecticide.Remove standing water (such as birdbaths, clogged gutters) to prevent mosquito breeding.
Fact sheet: Western equine encephalitis. Centers for Disease Control and Prevention website. Available at:
http://www.cdc.gov/ncezid/dvbd. Updated November 7, 2005. Accessed January 4, 2013.
Meningitis and encephalitis fact sheet. National Institutes of Neurological Disorders and Stroke website. Available at:
http://www.ninds.nih.gov/disorders/encephalitis_meningitis/detail_encephalitis_meningitis.htm. Updated February 16, 2011. Accessed January 4, 2013.
Reimann CA, Hayes EB, DiGuiseppi C, et al. Epidemiology of neuroinvasive arboviral disease in the United States, 1999-2007.
Am J Trop Med Hyg.
Western equine encephalitis fact sheet. Minnesota Department of Public Health website. Available at:
http://www.health.state.mn.us/divs/idepc/diseases/weencephalitis/wee.html. Accessed January 4, 2013.
Last reviewed November 2015 by David L. Horn, MD
Please be aware that this information is provided to supplement the care provided by your physician. It is neither intended nor implied to be a substitute for professional medical advice. CALL YOUR HEALTHCARE PROVIDER IMMEDIATELY IF YOU THINK YOU MAY HAVE A MEDICAL EMERGENCY. Always seek the advice of your physician or other qualified health provider prior to starting any new treatment or with any questions you may have regarding a medical condition.
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