Western equine encephalitis is a viral illness most commonly reported in the western United States and Canada. Most infected people have mild flu-like symptoms, including fever and vomiting, as well as symptoms affecting the nervous system in more severe cases. Older people and infants are at the greatest risk of deadly complications.
Western equine encephalitis is a rare disease now. An epidemic in 1941 affected more than 3,000 people, but since 1964, fewer than 700 cases have been reported in the U.S. Most occur west of the Mississippi River and the Rocky Mountains in states like California.
This virus can cause nervous system disease and death, and it is considered a potential bioterrorism threat by the Centers for Disease Control and Prevention.
What Is Western Equine Encephalitis?
This is an arboviral disease (i.e., a viral disease carried by arthropods, most often insects) caused by the western equine encephalitis virus. It is an alphavirus closely related to eastern equine encephalitis and Venezuelan equine encephalitis viruses. The western equine encephalitis virus can cause severe and even fatal disease. Minnesota alone reported over 800 cases during the 1941 outbreak, and 90 people died.
This virus affects both horses and humans and is spread by the bite of infected mosquitoes. It is chiefly spread by the Culex tarsalis mosquito. This mosquito can fly several miles away from the site where it hatched. Apart from Culex tarsalis, Aedes and Culiseta mosquitoes can also transmit this virus. Most cases occur from July to September.
Western equine encephalitis is most common in the plains regions of the western and central United States. It also has been reported in Central America and South America. The virus cycles between birds and mosquitoes in nature. Horses and humans are accidental hosts.
Though the virus also infects birds, you're not likely to get Western equine encephalitis by direct transmission from birds. Pregnant people may transmit the virus to the baby across the placenta, though. The Western equine encephalitis virus is not breathed out and airborne transmission does not occur.
This disease is more likely to affect you if:
- You live in an area where transmission is common.
- You visit such an area.
- You spend a lot of time outdoors.
Western Equine Encephalitis Symptoms
Symptoms of western equine encephalitis in humans usually manifest 5 to 15 days after infection from a mosquito bite. The usual symptoms include:
- Poor appetite
Most likely, you will experience a mild illness with headaches, vomiting, loss of appetite, and fever. If the western equine encephalitis virus multiplies rapidly and saturates the blood, it may cross the blood-brain barrier and cause meningitis (inflammation of the meninges, the membranes covering the brain) and encephalitis (inflammation of the brain itself). Resulting symptoms may include giddiness, intolerance of bright light (photophobia), confusion, agitation, drowsiness, unconsciousness (coma), rigidity of muscles (spasticity), fits (seizures), and learning difficulties. Nervous system complications are more likely in infants and the elderly.
These complications are the most dangerous aspect of Western equine encephalitis virus infection. This disease has a mortality of 3% to 7%. Among encephalitis survivors, 15% to 30% will continue to experience significant neurological problems.
Western Equine Encephalitis Diagnosis and Treatment
If you live in or visit an area where this disease is common, you should be alert to any symptoms and consult your doctor if they occur. This disease has symptoms similar to many other viral encephalitides, though, and diagnosis is challenging. If your nervous system is affected, your doctor may hospitalize you and order some tests, such as:
- Lumbar puncture (i.e., a spinal tap) for cerebrospinal fluid (CSF) examination
- A computed tomography scan (CT scan)
- A magnetic resonance scan (MRI scan)
- Antibody tests
The first three tests will indicate encephalitis if it is present, allowing your doctor to plan your treatment. The antibody tests can diagnose western equine encephalitis specifically, but positive results will take a few weeks to confirm.
There are no specific antiviral drugs effective against the Western equine encephalitis virus, so your doctor will prescribe supportive treatment to manage fever, pain, and other symptoms. Nervous system involvement may require intensive care, including mechanical ventilation, control of seizures, maintenance of hydration, and management of increased intracranial pressure (high pressure within the skull, a very dangerous condition).
Western Equine Encephalitis Prevention
A safe and effective veterinary vaccine is available for vaccinating horses. A vaccine for human use has been developed and proven effective and safe, but this vaccine is not being produced for clinical use. It is developed and used by the Department of Defense under the Special Immunizations Program (SIP). The vaccine, known as the TSI-GSD 210 vaccine, is only available to laboratory workers at risk of western equine encephalitis.
The best way to protect yourself from this illness, then, is to avoid mosquito bites. You should:
- Avoid outdoor activities at times when mosquitoes are most active: i.e., at dusk and dawn.
- Wear trousers and long-sleeved shirts to cover as much of your skin as possible.
- Fit screens on your doors and windows to prevent mosquitoes from getting in.
- Sleep inside a mosquito net. The net should be long enough to tuck under your mattress.
- Apply insect repellents on exposed skin areas. Effective insect repellents registered by the environmental protection agency (EPA) include diethyl toluamide (DEET), oil of lemon eucalyptus, picardin (KBR 3023), 2-undecanone, para-menthone-diol, and IR3535.
- Treat clothes, bednets, and camping gear with 0.5% permethrin. This insecticide kills or repels mosquitoes.
- Prevent mosquito breeding. Empty all water collections at least once a week. This includes birdbaths, buckets, planters, flowerpots, trashcans, and tires. Cover all water containers.
- Vaccinate horses.