Western Equine Encephalitis (WEE) is a rare but serious viral illness caused by the Western Equine Encephalitis virus (WEEV), an arbovirus spread to humans and horses by infected mosquitoes. Historically, WEE has been reported primarily in the plains regions of the western and central United States and Canada. While human cases have become very infrequent since the late 1990s, the potential for outbreaks remains a public health consideration due to its past impact.
How Western Equine Encephalitis Spreads
The Western Equine Encephalitis virus maintains a complex transmission cycle in nature, involving specific mosquito species and birds. Mosquitoes, particularly those belonging to the Culex genus, such as Culex tarsalis, act as the primary vectors for the virus. These mosquitoes acquire the virus when they feed on infected wild birds, which serve as the natural reservoir hosts for WEEV.
After a mosquito bites an infected bird, the virus replicates within the mosquito. The infected mosquito can then transmit the virus to other birds, or to accidental hosts like humans and horses through subsequent bites. Humans and horses are considered “dead-end hosts” because they do not develop high enough levels of the virus in their bloodstream to transmit it back to feeding mosquitoes. Transmission of WEEV typically occurs during warmer months, from late spring through early summer, when mosquito populations are more active.
Recognizing Symptoms
Symptoms of Western Equine Encephalitis typically appear within 5 to 15 days after an infected mosquito bite. Many individuals infected with WEEV may not experience any symptoms at all, or only very mild ones. When symptoms do occur, they can initially resemble a flu-like illness.
These milder symptoms include fever, headache, general malaise, muscle aches, nausea, and vomiting. If the virus progresses to affect the central nervous system, more severe neurological symptoms can develop, indicating inflammation of the brain (encephalitis) or the membranes covering the brain and spinal cord (meningitis).
These serious signs may include high fever, stiff neck, confusion, disorientation, agitation, drowsiness, tremors, and seizures. In some cases, paralysis or coma can occur. Infants and very young children, as well as adults over 50 years of age, face a higher risk of developing severe disease with potentially lasting neurological deficits. The case-fatality rate for individuals who develop severe WEE can range from 3% to 15%.
Diagnosis and Management
Diagnosing Western Equine Encephalitis involves evaluating a patient’s clinical symptoms and history of potential mosquito exposure. Confirmation of WEEV infection relies on specific laboratory tests performed on samples of blood or cerebrospinal fluid (CSF). These tests typically look for the presence of specific antibodies, such as immunoglobulin M (IgM) antibodies. Positive IgM results are often confirmed with neutralizing antibody testing.
In some instances, medical imaging techniques like magnetic resonance imaging (MRI) or computed tomography (CT) scans may be used to assess for signs of brain inflammation or swelling. There is currently no specific antiviral treatment available for WEE infection. Medical care for patients with WEE focuses on supportive measures to manage symptoms and support bodily functions. This supportive care may include medications to control fever and seizures, ensuring adequate hydration through intravenous fluids, and providing respiratory support if breathing becomes difficult. Hospitalization is often necessary for individuals experiencing severe forms of the disease.
Protecting Yourself and Others
Protecting oneself and others from Western Equine Encephalitis primarily involves preventing mosquito bites. Using EPA-registered insect repellents that contain active ingredients like DEET or picaridin on exposed skin can provide effective protection. Wearing long-sleeved shirts and long pants, especially during dawn and dusk hours when Culex tarsalis mosquitoes are most active, reduces skin exposure. Staying indoors during these peak mosquito activity times also helps to minimize risk.
Controlling mosquito populations around the home is also an important preventive measure. This includes regularly eliminating standing water sources, which serve as mosquito breeding grounds. Common sources of standing water include old tires, buckets, clogged rain gutters, and bird baths, all of which should be emptied or cleaned frequently. Community-level mosquito control programs often play a role in reducing mosquito populations in affected areas. While there is no vaccine available for humans against WEEV, horses can be vaccinated to protect them from the disease, which is a common practice in endemic regions.