Understanding Western Equine Encephalitis Transmission

Introduction

Western Equine Encephalitis (WEE) is a vector-borne disease caused by the western equine encephalitis virus, which has a wide geographical distribution from Canada to the south of Argentina. The term vector-borne means the infection gets transmitted to humans or any other animal through an intermediate animal, in our case, culex and aedes mosquitoes. Infection generally occurs during hot and humid phases of the year, such as spring and summer, when the climate is perfect for mosquitoes to thrive. In most cases, the disease is mild to moderate in intensity, but in rare incidents, it could be life-threatening.¹

Protection of oneself and others is of the utmost importance in cases of a disease like this. In the next section, we will cover how this disease spreads, who helps spread the disease, disease transmission, symptoms, and the solutions available.

Understanding western equine encephalitis

The history dates back to the 1930s when the virus was isolated from the brain of a horse in the western state of California. The discovery was the result of an investigation into a fatal disease that spread among equine animals across the state. Later, in the late 1930s, the virus was subsequently discovered in humans.² Though in recent years the havoc of the disease has subsided greatly, from 1930 to 1950 it raged across the North American continent.³

Symptoms and impact on health

In the majority of cases, the disease shows mild to moderate symptoms such as mild to moderate fever, lethargy, headache, drowsiness, loss of appetite with occasional symptoms of restlessness or anxiety, and severe diarrhoea. On rare occasions, when a patient does not recover, it may lead to the fatal aspects of the disease: meningitis, or inflammation of the membrane covering the brain, and encephalitis, or inflammation of the brain. The presence of a rash has not been reported so far.²

Meningitis and encephalitis could lead to complex conditions such as confusion, irritation from bright lights, loss of balance, ringing of the ears, coma, and subsequent death. Though western equine encephalitis may occur to anyone, children and older adults are prone to severe disease manifestation. 

Causative agent

The WEE virus, belongs to the family Togaviridae and the genus Alphavirus. It is also classified as an arbovirus, which means the WEE virus needs an arthropod (insect) as a medium to spread among other animals. In the case of WEE, the arthropod is a mosquito, and the insect that carries the virus is called a vector. The culex and aedes mosquitoes are the natural vectors of the disease.²

Transmission dynamics

In medical sciences, a host is a specific organism that allows another organism to thrive inside it, irrespective of mutual benefit. Here, animals such as horses, cattle, pigs, and lastly, humans are the hosts that allow the WEE virus to multiply inside their bodies. Reservoirs are those organisms inside which these infective agents can live and spread. In the case of WEE, the reservoirs are small wild birds and mosquitoes. So those who love to delve into nature must be careful, as the chances of getting bitten by a mosquito get higher while roaming near forests and wetlands.⁴

Geographical distribution

According to the Centre for Disease Control (CDC), the virus can be found worldwide, but according to scholarly articles, this specific type of virus is restricted only to the global West. However, closely related viruses could be found elsewhere.⁴

Modes of transmission

The transmission of the WEE virus occurs in several stages of complex processes. It is thought that the natural reservoir of the virus is small wild animals, such as pheasants. If some culex mosquito takes up the virus from an infected bird, then a series of cyclic events will start. 

Epizootic cycle

In the epizootic cycle, the virus is circulated among the natural reservoir, other reservoirs, and the host. So in this transmission cycle, which is mostly seen during spring and summer, the mosquito takes up the virus from a natural reservoir animal such as a pheasant and bites another animal that might live near human civilization, such as a sparrow. Now, from the bird, the virus either gets transmitted to farm animals such as horses and pigs or to humans through a mosquito again. This is one aspect of the epizootic cycle.⁴

Another aspect could be that the virus transmission continues among birds only where no humans or other animals are infected.

Enzootic cycle

The enzootic cycle is where the virus gets circulated among the reservoir species and other mosquitoes. It is thought that this phase occurs during the winter. In this cycle, the mosquitoes carrying the virus infect one after another allowing the virus to thrive and grow. In some cases, the reservoirs are also called amplifying hosts.⁴

Rarely, for example during an epidemic among animals, other wild animals such as rabbits, wild boars, or, on extremely rare occasions, members of the reptile family might get infected with the virus. 

Human-to-human transmission

Human-to-human transmission has not been reported so far. In the case of WEE in farm animals such as horses, it is still considered safe for humans as horses serve as the dead end of WEE transmission. This characteristic makes the disease rare among humans but fairly common in animals.^1,4

Factors influencing transmission

Environmental factors

The life cycle of mosquitoes is highly dependent upon a few factors, such as temperature, the presence of stagnant water bodies, humidity, etc. Culex mosquitoes lay 200–300 eggs at a time, and the presence of stagnant water bodies such as marshland supported by a warm and humid climate helps insects in their rapid breeding. An increase in temperature allows the virus to complete its life cycle earlier than expected inside a mosquito. Though this is a matter of debate, global warming may induce new areas to see an unprecedented rise in mosquito-borne disease burden.⁵

Traditionally, it is always thought that rural areas have more events of mosquito-borne disease reporting, but it is quite surprising that urban slums in a city are as potent as rural areas in propagating an infectious disease. 

International trade

Although chances are quite low, equine animal trade may lead to the transport of infected animals from one country to another. Multinational trade amplifies the chances of this phenomenon, and even more so if combined with global warming. 

Migratory birds

Though it is highly unclear which bird species serve the reservoirs, migration by birds could be proven to be an important factor in disease transmission.⁵

Prevention and control strategies

Vaccination programmes

Unfortunately, no vaccine is available for humans yet. The vaccine is only available for horses. The vaccine for horses is called the WEEV vaccine and is given annually. The horses are vaccinated prior to the commencement of the summer season, and booster doses are available accordingly.⁶

Mosquito control measures

Since mosquitoes are the most important agents in disease transmission, several tactics could be applied to control mosquito breeding. Both long-term and short-term solutions are available.⁶

• Elimination of Breeding Sites: Mosquitoes prefer to lay eggs in stagnant water bodies. So, anything that may act as a potential breeding ground, such as plastic cups, water drums, swimming pools, etc., should be occasionally cleaned
• Insecticide Spraying: A large variety of insecticides are available on the market that can be used. Most of these insecticides are inorganic in nature and may cause damage to the environment if used irrationally. So, preference should be given to organic insecticides
• Personal protection: mosquito-repellent creams and sprays are now available worldwide. Applying such creams to the bare parts of the body before going outside will decrease the chances of mosquito bites. The use of mosquito nets is always recommended
• Biocontrol: Certain fish, such as guppy, feed on insect larvae. The introduction of such species into stagnant water bodies, such as ornamental ponds, will disrupt the breeding grounds of mosquitoes

Public awareness and education

Recognise the symptoms if anyone near you might have been suffering from the condition. Contact your GP and tell them about the symptoms. Though this disease shares symptoms with a large number of other infectious diseases, it may take some time to come to a definite conclusion. Consider personal protection when the season starts. Go through the guidelines by the CDC and WHO.

Summary

Prevention of Western Equine Encephalitis requires an understanding of its transmission via mosquitoes. Outbreaks have happened all over North America since the 1930s, where both people and horses show mild-to-severe symptoms. Preventive measures include vaccinating horses, controlling mosquito populations, and educating the public. International trade and climate change are two issues that greatly influence the spreading of this infection and complicate the control of such. Reducing WEE transmission requires further investigation, but moreover, it requires cooperation and awareness.