Introduction

Western Equine Encephalitis (WEE) is a rare vector-borne viral disease, transmitted through mosquito bites. The disease, as the name suggests, affects both horses and humans. The virus can cause in extreme cases, encephalitis, inflammation of the brain, which is a serious condition, especially for certain groups of vulnerable people, such as infants and pregnant women. The prevalence of WEE is at its magnitude in the summer months in Western and Central America. The most recent outbreak was in Argentina, with a human case being reported in December 2023, the first in over 20 years.¹ In most cases, infection with the WEE virus will lead to the onset of mild symptoms such as fever and flu-like symptoms, however, in vulnerable groups, the impact can be severe and in some cases fatal. 

Western equine encephalitis and pregnancy

Similarly to other vector-borne viruses, the WEE virus is able to cross the placental barrier if a woman were to become infected during pregnancy. This may sound familiar to the Zika virus outbreak, which is transmitted similarly via the mosquito acting as a vector. Following a bite from a mosquito with the virus, the disease may progress within the host. 

The most vulnerable group of people susceptible to the most severe symptoms, encephalitis, are children under 5 years old. With mortality rates being the highest in the youngest of this age group.² Ultimately, this means that developing foetus are the most at risk of developing encephalitis due to the ability of the viral particles to cross the placental barrier leading to irreversible brain damage in most cases. This permanent change in the physiology of the brain can have severe life-limiting consequences, such as severe learning difficulties, impaired cognitive functioning, and effects on mobility and physical development. 

The impact on pregnant women

Albeit rare, pregnant women residing in countries where the disease is present must be aware of the risks associated with becoming infected and the likelihood of transplacental transmission. Additionally, the risk to the unborn child is maximal in infants with chances of severe irreversible brain damage and life-limiting consequences highest in this age group. The pregnant women themselves are at a higher risk of acquiring infection due to decreased immunity during pregnancy. If the virus is transmitted to the developing foetus, the effects on human development overall are severe. Permanent, irreversible brain damage will alter the capacity and quality of life in this vulnerable group, highlighting the importance of limiting the risks of transmission to pregnant women.

Western equine encephalitis. what is it?

The Culex and Aedes mosquito species are known to transmit the WEE virus, explaining the epidemiology of the infection rates. Human cases of the virus are rare, however, they do still occur in certain areas where the vector (mosquito species) is an inhabitant. Transmission occurs during the summer months and when rainfall is at its peak. Mosquitoes thrive and reproduce in hot, humid environments, making the months of April to September the peak season for cases across certain parts of America and Canada.³ The virus is part of a family of viruses known as Alphaviruses, these viruses are categorised by their ability to infect the brain tissue. The virus is transmitted from the mosquito to the human host via a bite which allows the viral particles to enter the bloodstream of the equine or human host, where it is able to replicate and pathogenesis may then occur within the subject.⁴ 

Signs and symptoms of western equine encephalitis

In many people infected with the WEE virus, host subjects are asymptomatic or suffer mild to moderate symptoms. The incubation period of the virus is between 2 and 14 days from the encounter with the mosquito. Mild symptoms include fever, malaise, chills, and nausea. Moderate symptoms include headache, joint pain, neck stiffness, and vomiting. In severe cases, where the virus has infected the brain tissue, neurological symptoms arise including 

confusion, dizziness, seizures, coma, and in some cases, death. Encephalitis is the most severe outcome of the disease and it is rare for the disease to progress to this stage in adults.

Epidemiology of western equine encephalitis and infection rates

WEE is a rare disease and one which most people may have never heard of. However, due to recent outbreaks and cases emerging at an accelerated rate in the Americas, it is useful to gain knowledge and information regarding transmission, prevention, and the epidemiology of the disease, so that it may be prevented and limited. In December 2023, a case was recorded in Argentina, this is the first case to ever be confirmed in the area. By the end of January 2024, there were a probable 279 cases in the area, highlighting increased transmissibility amongst humans and an increased risk to human health. The disease has also been reported recently in Uruguay, with two confirmed cases as of February 2024.⁵ The situation is being closely monitored to limit further spread across the continent, and awareness is being raised to ensure the public can take any precautionary measures that may prevent infection.

The infectivity of the WEE virus varies, with a major factor determining infection and disease severity being age. Approximately 0.5% of adults become infected upon exposure to the virus, compared to a 4% infection rate in children. Mortality rates are also lower in adults, with approximately 10% of severe cases resulting in fatality in adults, and a much higher mortality rate of 35% in infected children with severe symptoms.⁴ Infection rates are also much higher in children than in infants. The infection rate for children after exposure is 1 in 4, whereas in infants the infection rate is significantly higher with a 1:1 ratio to exposure.³ 

Treatment of western equine encephalitis

Currently, there is no vaccine available to prevent the severe consequences of WEE infection. Unfortunately, there is also no effective specific treatment for cases of infection. Over-the-counter painkillers are prescribed to combat the symptoms and alleviate any pain and discomfort during this time. In severe cases, hospitalisation may be required to monitor and treat the symptoms of severe disease, to limit the permanent consequences that arise from damage to the brain tissue.¹

Prevention and management

It is necessary for residents and travellers to countries where infection is possible to ensure that they take adequate precautionary measures to limit the chance of infection. As the disease is vector-borne, and requires a bite to the skin from a mosquito, it is obvious that the main preventative measures include limiting exposure to mosquito bites. Ways in which people can prevent insect bites include:

• Ensuring the skin is covered by clothing, limiting access
• Installing nets and curtains to windows and doors to reduce the chances of mosquitoes gaining entry to the property of the residence
• Installing mosquito nets around the bed to limit the chances of being bitten during the night
• Applying mosquito repellents to the skin, DEET is the most effective substance
• Mosquito-repellent plug-in devices are available which may also help to deter mosquitoes from entering the property
• Avoiding stagnant pools of water, as these places are breeding grounds and preferred habitats for the mosquitos
• Wherever an outbreak is reported, remain indoors and utilise all of the preventative measures above to avoid infection
• At-risk groups, pregnant women and children, are to avoid travel to places where there are confirmed cases of Western Equine Encephalitis⁵

The importance of medical care

Many people infected with WEE will remain asymptomatic, and may never know they have been infected with the virus. Furthermore, as most cases yield mild flu-like symptoms, cases may not be recorded as people may not require medical treatment. Additionally, as the disease is rare, it may not be likely that a person presenting with mild to moderate symptoms be tested for the disease itself. However, in severe cases where neurological symptoms arise, it is vitally important that these individuals seek and obtain medical treatment, to prevent any irreversible brain damage that can be caused by encephalitis. In the case that a pregnant woman becomes infected with WEE, close medical monitoring will be delivered, to limit the damage to the mother and the unborn child who is at the most risk of developing irreversible brain damage and fatality.

Summary

Western Equine Encephalitis (WEE) is a rare tropical disease caused by a bite from an infected mosquito. Due to an unusual increase in cases in Argentina and Uruguay, highlighting the potential for further transnational transmission, it is important to raise awareness of these outbreaks to those at risk. The risk of severe disease across the general population is low, however, in some groups of people, the risk is accentuated and can have devastating consequences for those infected. Children, infants, and foetuses have an elevated risk of developing encephalitis. Encephalitis can lead to permanent brain damage, affecting cognitive functioning, and mental development and ultimately can lead to life-limiting conditions for which there is no treatment or cure. Furthermore, fatality rates in the risk groups highlighted are higher than in any other age group, therefore it is of utmost importance that these individuals exercise all precautionary and preventive measures if in an area where WEE is prevalent.