Western equine encephalitis

male Culex tarsalis mosquito [western ecephalits mosquito] with a broken wing

The short answer

Western equine encephalitis (WEE) is a rare illness caused by a virus that spreads to people through the bite of an infected mosquito, mostly the mosquito species Culex tarsalis in western regions of the country. The virus normally circulates between mosquitoes and birds. People and horses can be infected when a mosquito bites them, but they are dead-end hosts and do not pass the virus on. Most people who are infected never feel sick. A small number develop a serious infection of the brain, and the risk of that is highest in infants and older adults. Human cases are now very uncommon, with no cases reported in the country since 1999.

The short version: WEE spreads through mosquito bites, not from person to person. Most infections cause no symptoms. Serious brain infection is rare but most dangerous for infants and adults over 60. There is no human vaccine and no specific medicine, so care is supportive. A vaccine does exist for horses.

What it is

Western equine encephalitis is a disease caused by the Western equine encephalitis virus, an alphavirus in the family Togaviridae. “Encephalitis” means inflammation of the brain, which is the serious outcome the virus can cause in a minority of infections.

The virus was a significant public health problem in the middle of the twentieth century. There were periodic outbreaks among people in the 1930s and 1940s, and reported human cases began to fall in the late 1960s and 1970s. Since 1964 there have been fewer than 700 confirmed human cases in the country, and none have been reported since 1999. Historically, most cases occurred in Colorado, Texas, North Dakota, and California. The reasons the virus has become so quiet are not fully understood.

How you get it

You get WEE from the bite of an infected mosquito. The main vector in western regions is Culex tarsalis, a mosquito common in irrigated farmland and other areas with standing water. Aedes melanimon acts as a secondary vector and can help carry the virus to people and horses.

The virus is maintained in a natural cycle between mosquitoes and birds. Mosquitoes pick up the virus when they feed on infected birds, especially passerine birds such as house sparrows and house finches, which serve as amplifying hosts. Infected mosquitoes then bite more birds, keeping the cycle going, and sometimes bite people and horses.

What does not transmit it:

It does not spread from person to person.
It does not spread by touching, handling, or being near infected birds or horses.
People and horses are dead-end hosts. They can become sick but do not build up enough virus in their blood to infect a mosquito that bites them, so they do not continue the chain of transmission.

Symptoms

Most people infected with the virus do not get sick at all. When symptoms do occur, they usually begin about 5 to 10 days after the bite.

Milder illness can include:

Fever and chills
Headache
Body aches and muscle pain
Malaise, weakness, and tiredness

A smaller number of people go on to develop severe disease, in which the infection reaches the brain. Signs of severe illness can include:

High fever
Stiff neck (a sign of meningitis)
Confusion, drowsiness, or altered mental state
Seizures
Coma, and in some cases death

Reported case fatality for the encephalitic form has been on the order of a few percent, with most deaths occurring in older patients. Some people who recover from severe disease, particularly infants, can have lasting neurological effects.

Who is at higher risk

Anyone bitten by an infected mosquito can be infected, but the risk of severe disease is not spread evenly. The people most likely to develop serious illness are:

Infants and very young children, especially infants under 1 year, who are the most vulnerable of all
Adults older than 60

People who spend time outdoors at dawn and dusk in areas where the virus circulates, such as agricultural and irrigated regions in the west, have more exposure to Culex tarsalis, which feeds mainly at those times.

When to see a doctor: Seek medical care promptly for a high fever with a severe headache, a stiff neck, confusion, unusual drowsiness, seizures, or a sudden change in alertness or behavior, particularly in an infant or an older adult. These can be signs of encephalitis and need urgent evaluation.

Diagnosis and treatment

WEE is diagnosed by a healthcare provider based on symptoms, recent exposure to mosquitoes in areas where the virus occurs, and laboratory testing. Testing usually looks for antibodies (IgM) to the virus in blood or in cerebrospinal fluid collected by a spinal tap. Because the virus is closely related to other viruses such as St. Louis encephalitis virus, antibody tests can cross-react, and confirmatory testing may be needed.

There is no antiviral medicine that cures WEE. Treatment is supportive care, which means managing symptoms and complications. People with severe disease often need to be hospitalized for care such as intravenous fluids, medicine for pain and fever, help with breathing, and nursing support.

On vaccines: there is no vaccine approved to prevent WEE in people. A vaccine does exist for horses, and veterinarians recommend routine vaccination of horses in areas where the virus circulates, because the disease can be severe or fatal in unvaccinated animals. The horse vaccine does not protect people.

How to protect yourself

Because there is no human vaccine, prevention means avoiding mosquito bites and reducing mosquitoes around your home.

Use an EPA-registered insect repellent and follow the label directions. [/guides/insect-repellent]
Wear long sleeves and long pants when you are outdoors, especially at dawn and dusk when Culex tarsalis is most active. [/guides/protective-clothing]
Repair window and door screens so mosquitoes cannot get indoors. [/guides/screens-and-doors]
Empty or change standing water at least weekly in items like buckets, plant saucers, birdbaths, and troughs. [/guides/standing-water]
Vaccinate horses on the advice of a veterinarian if you keep them in areas where the virus occurs. [/guides/protecting-horses]
Support local mosquito control efforts in your community. [/guides/mosquito-control]

Sources

Centers for Disease Control and Prevention. About Western Equine Encephalitis Virus. https://www.cdc.gov/wee/about/index.html. Accessed 20 Jul 2026. (transmission by Culex tarsalis and birds, western occurrence, most infections asymptomatic, fewer than 700 cases since 1964, no cases since 1999, historical state distribution)
Centers for Disease Control and Prevention. Clinical Features, Diagnosis, and Treatment of Western Equine Encephalitis. https://www.cdc.gov/wee/hcp/clinical-diagnosis-treatment/index.html. Accessed 20 Jul 2026. (incubation 5 to 10 days, symptoms, highest risk in infants under 2 and adults over 60, IgM serology, supportive care)
Centers for Disease Control and Prevention. Symptoms, Diagnosis, and Treatment (Western Equine Encephalitis Virus). https://www.cdc.gov/wee/symptoms-diagnosis-treatment/index.html. Accessed 20 Jul 2026. (no vaccine or medicine to prevent WEE in humans, no approved treatment, supportive management including IV fluids and pain medication)
Centers for Disease Control and Prevention. Data and Maps for Western Equine Encephalitis. https://www.cdc.gov/wee/data-maps/index.html. Accessed 20 Jul 2026. (rarity of reported human cases)
Bhatt A, et al. Western Equine Encephalitis. StatPearls, National Center for Biotechnology Information. https://www.ncbi.nlm.nih.gov/books/NBK470228/. Accessed 20 Jul 2026. (case fatality around 4 percent, infants affected more than adults, IgM and IgG diagnosis, cross-reaction with St. Louis encephalitis, dead-end hosts)
Ronca SE, et al. Western equine encephalitis virus: a comprehensive review of epidemics, transmission, hosts, and strategies for mitigation. Virulence. https://pmc.ncbi.nlm.nih.gov/articles/PMC12584650/. Accessed 20 Jul 2026. (enzootic bird and Culex tarsalis cycle, Aedes melanimon as secondary vector, passerine amplifying hosts)
U.S. Department of Agriculture, Animal and Plant Health Inspection Service. Equine Encephalitis (EEE/WEE/VEE). https://www.aphis.usda.gov/livestock-poultry-disease/equine/encephalitides. Accessed 20 Jul 2026. (equine vaccine availability and severity in unvaccinated horses)

[/diseases/eastern-equine-encephalitis]
[/diseases/west-nile-virus]
[/diseases/st-louis-encephalitis]
[/diseases/venezuelan-equine-encephalitis]
[/species/culex-tarsalis]
[/guides/mosquito-borne-diseases-overview]

Educational reference only. This is not medical advice. If you or someone you care for may have a mosquito-borne illness, contact a licensed healthcare provider.

Last reviewed 20 July 2026.