Introduction

What is Western equine encephalitis (WEE)?

Western equine encephalitis (WEE) is caused by a virus iand is transmitted to humans through the bite of an infected mosquito.¹

Encephalitis is a severe condition in which the brain becomes inflamed and swollen. Early symptoms of WEE often include a sudden fever, headache, vomiting, and fatigue. Progression of the disease can further result in seizures, disorientation, and coma.

The virus which causes WEE is an alphavirus from the Togaviridae family which includes several viruses which can cause equine encephalitis infections.¹

The virus may translocate to the central nervous system and cross the blood-brain barrier if it replicates sufficiently within its host. This may result in cerebral and meningeal inflammation (encephalitis) and cell death (necrosis).

The fatality rate for WEE is approximately six – eight%, with elderly patients having the highest risk for fatality. In 13% of cases¹, WEE results in permanent motor and/or intellectual difficulties. There is also the possibility of epilepsy in four – 11% of cases.¹

There is no specific treatment for WEE. Individuals suffering from acute cases may receive supportive care in a hospital or other medical setting.

Brief overview of WEE transmission

Humans contract WEE from the bite of an infected mosquito.

Different species of mosquitos can be vectors for transmission. Some species which are vectors for human transmission are the Aedes, Culiseta, and Culex tarsalis.²

Animals such as mules, horses, pheasants, and other bird species have experienced outbreaks of WEE, often during human epidemics. Birds are ‘amplifying hosts’, meaning that the virus often replicates at a high rate in birds, making them a reservoir for the virus.

WEE is not spread through the air. However, the virus is able to cross the placenta and be transmitted from the mother to the foetus. Transmission via blood infusion is possible but unlikely.²

Overview of western equine encephalitis

Causative agent: western equine encephalitis virus (WEEV)

WEE is caused by the western equine encephalitis virus (WEEV) and is hypothesised to be a recombinant virus: sharing features of two other ancestral alphaviruses. The virus is closely related to the eastern equine encephalitis virus.

The virus is transmitted by mosquitoes infected with WEEV. Birds are the reservoir for the virus and are an ‘amplifying host’ in which the virus rapidly self-replicates.

Geographic distribution and seasonality

WEEV is found throughout the world; however, most reported cases are in the United States and South America. It is more prevalent in and around swampy areas, likely due to the role of mosquitoes as a vector for transmission. Moreover, it is transmitted more frequently during warmer seasons and periods of higher humidity.

Diagnosis of western equine encephalitis

Clinical evaluation

It is difficult to diagnose WEE, since patients present with a collection of symptoms commonly seen in many other diseases.

If a patient demonstrates neurological symptoms, neuroimaging examinations such as computed tomography (CT) or magnetic resonance imaging (MRI) may be used.²

Encephalitis can cause inflammation seen in the thalamus or basal ganglia but is not specific to WEE. A lumbar puncture done on patients with WEEV may show abnormalities (elevated opening pressure or a cell count with lymphocytic predominance). However, this is also non-specific and is seen in many forms of encephalitis and meningitis.

Laboratory tests/serology

It is difficult to isolate the WEE virus from serum or cerebrospinal fluid (CSF).²

IgG antibodies are usually detectable one – three weeks following infection, peaking after one – two months. IgG indicates exposure to the virus and may suggest a recent infection.

IgM antibodies correlate with an acute infection and are detectable one – three weeks from when symptoms started.²

Differential diagnosis

WEE can be difficult to distinguish from other infections, especially those which cause any form of encephalitis or meningitis. The test for WEE cross-reacts with that for St. Louis encephalitis virus making a diagnosis more difficult.

Importance of understanding WEE symptoms in humans

Most individuals who contract WEE will not experience serious illness. Often patients with WEE are asymptomatic or experience mild symptoms. Viremia (viral replication) increases the risk of encephalitis to 1:1200 in adults, 1:60 in children, and 1:1 in infants.³ Infants account for roughly 1/3 of WEE cases, and it is also possible for infections to occur in utero.³ There is a 10-day incubation period for the virus, after which symptoms may begin to manifest.

Infants and the elderly are more susceptible to acute cases of WEE and are more likely to develop encephalitis. It is estimated that around five – 15% of encephalitis cases caused by WEE are fatal. However, 50% of infants who survive encephalitis experience residual brain damage. 

In symptomatic WEE, individuals typically experience the following initial symptoms:

• Sudden onset fever (defined as a body temperature higher than 100.4 ^F or 38 ^C)
• Nausea
• Vomiting
• Fatigue
• Neck stiffness
• Headaches
• Chills
• Weakness
• Myalgia (muscle pain)¹

Most patients who experience symptoms will recover without medical assistance or further issues.

Patients who deteriorate further may experience the following symptoms:

• Disorientation/confusion¹
• Seizures
• Irritability/agitation
• Coma
• Vertigo
• Aversion to light (photophobia)
• Spasticity (abnormal muscle tightness from a prolonged muscle contraction)
• Somnolence (feeling tired or ready to fall asleep)

Elderly patients are the most likely to develop brain disease and the most likely to die due to complications.

Young children are the most likely to develop a permanent neurological condition. If acute neurological disease has occurred patients often suffer with lasting effects for years. Long-term symptoms can include fatigue, irritability, and headaches.

Treatment and management

There is no specific, effective treatment for WEE and is primarily supportive, meaning that patient care is centred on preventing and managing complications. No vaccines currently exist for WEE.²

Treating encephalitis may require managing increased intracranial pressure and/or utilising mechanical ventilation. Seizures, which are common in WEE, require administration of anticonvulsants. Individuals who experience seizures because of WEE are more likely to develop lifelong seizures.²

Prevention and control measures

Since there are no specific treatment options for WEE, prevention is critical. Reducing the likelihood of infection is best achieved by preventing mosquito bites, especially in areas where WEE is endemic.

Avoiding mosquito bites can be challenging but it is possible with careful preparation. Even with preventative measures, it is still possible to be exposed to multiple mosquito bites in a short time. Try using some of the following protective measures to reduce the incidence of infected mosquito bites:

Wear clothes which covers your skin

Leaving skin exposed increases the likelihood of mosquito bites. Long sleeves, long trousers, and socks which extend past the ankle are key to covering up skin. Make sure the fabric is tightly woven to prevent mosquitoes from biting through. Tuck your trousers into your socks or boots, so that it is not possible for mosquitoes to find their way in.

Use insect repellent

There are a range of different insect products which can repel mosquitoes and reduce the likelihood of bites. DEET, permethrin, oil of lemon eucalyptus, or OFF! can all reduce the risk of contracting WEE via mosquito bite. Make sure you use an insect repellent which is reliable and supported by high-quality scientific research.

Also, be sure to always follow the label instructions because different insect repellents are only effective for a certain period of time. Bathing and swimming also require you to reapply more frequently. Additionally, permethrin can be applied to clothes and gear and it is possible to purchase permethrin-treated clothes.

Be aware of mosquito activity

Mosquitoes are most active between dawn and dusk because they feed during this time. Avoid outdoor activities during this period to reduce the incidence of mosquito bites.

Prevent bites during sleep

In many countries, it is advised to use mosquito nets to prevent bites during sleep. Mosquito nets can be placed around beds, or you may sleep in a building which has insect screens over the windows (which prevent mosquitoes from coming in). Traps can also reduce the number of mosquitoes indoors.

Be conscious of standing water

Standing water, especially children’s swimming pools, is a breeding ground for mosquitoes and can be very dangerous. Be sure to drain pools and other sources of standing water.

Summary

Most individuals who contract WEEV do not experience acute symptoms or encephalitis and only a small percentage of those who develop WEE will experience encephalitis; however, especially in young children and elderly patients, encephalitis can have lifelong effects.

There is no treatment or vaccine for WEE so treatment is primarily supportive. A differential diagnosis is also difficult to obtain because there are a lack of specific test options.

Outbreaks of WEE are limited, rare, and concentrated in areas which are hot, humid, and swampy. You should always be cautious when travelling to areas where WEE is endemic and mosquitoes are prevalent. The best way to manage WEE is through prevention of mosquito bites.