Key points

• Western equine encephalitis (WEE) virus is spread through the bite of an infected mosquito.
• No human cases have been reported in the United States since the late 1990s.
• Most people with WEE virus infection do not get sick, though some will develop fever, headache, body aches, or confusion.
• There are no vaccines to prevent or medicines to treat WEE.
• Reduce your risk by taking steps to prevent mosquito bites.

Overview

Western equine encephalitis is caused by a virus most commonly spread to people through the bite of an infected mosquito. Most people infected with western equine encephalitis virus do not feel sick. However, the virus can cause some people to develop symptoms of a febrile illness (fever) or severe neurologic disease.

Western equine encephalitis virus has caused sporadic outbreaks of disease in horses and people from the late 1930s through the early 1980s in the United States. The last outbreak in the United States in people was in 1987, and since then, there have only been five cases of western equine encephalitis virus identified in the United States. The last reported case of western equine encephalitis occurred in 1999. Historically, the virus has been found in parts of North America, Central America, and South America.

Risk of infection is highest during mosquito season, which starts in the summer and continues through fall. You can reduce your risk of infection by preventing mosquito bites.

No symptoms in most people. Most people infected with western equine encephalitis virus do not develop any symptoms.

Febrile illness (fever) in some people. Some people who are infected develop a fever along with other symptoms, such as headache, body aches, fatigue, or vomiting. Most people with a febrile illness due to western equine encephalitis virus will recover completely within a few days.

Serious symptoms in a few people. Some people who are infected develop a severe illness affecting the central nervous system such as encephalitis (inflammation of the brain), myelitis (inflammation of the spinal cord), or meningitis (inflammation of the membranes that surround the brain and spinal cord).

• Symptoms of severe illness include high fever, headache, neck stiffness, dizziness, light sensitivity, confusion, agitation, drowsiness, coma, and seizures.
• Severe illness can occur in people of any age. However, older adults and young children, particularly infants and children aged 2 years and younger, are at greater risk for severe illness.
• Recovery from severe illness might take several weeks or months. Some effects to the central nervous system might be permanent and are more common in infants and young children.
• About 3 to 15 out of 100 people with severe disease will die.

Talk with your healthcare provider if you think you or a family member might be infected with western equine encephalitis.

Healthcare providers diagnose western equine encephalitis virus infection based on:

• Signs and symptoms
• History of possible exposure to mosquitoes that can spread western equine encephalitis virus
• Laboratory testing of blood or spinal fluid

Your healthcare provider can order tests to look for western equine encephalitis virus infection or other infections that can cause similar symptoms.

No specific medicines are available to treat western equine encephalitis. Antibiotics do not treat viruses.

Rest, fluids, and over-the-counter pain medications may relieve some symptoms.

In severe cases, patients often need to be hospitalized to receive supportive treatment, such as intravenous fluids, pain medication, and nursing care.

Key points

• Western equine encephalitis virus is spread by the bite of an infected mosquito.
• There are no vaccines to prevent western equine encephalitis in people.
• The best way to prevent infection is to protect yourself from mosquito bites.

Prevention tips

Western equine encephalitis virus is spread to people through the bite of an infected mosquito. Mosquitoes bite during the day and night. Anyone who spends time in a place where western equine encephalitis virus is circulating is at risk. Historically, the virus has been found in parts of North America, Central America, and South America. No human cases of western equine encephalitis have been reported in the United States since 1999.

There are no vaccines to prevent western equine encephalitis in people.

The best way to prevent western equine encephalitis is to protect yourself from mosquito bites.

• Use insect repellent
• Wear long-sleeved shirts and pants
• Treat clothing and gear with 0.5% permethrin according to label instructions
• Take steps to control mosquitoes indoors and outdoors
• If traveling, choose lodging with air conditioning, screens on windows and doors, or use a mosquito net if you will be sleeping outdoors