Overview of Venezuelan equine encephalitis 

The Venezuelan Equine Encephalitis virus (VEE) is transmitted primarily by mosquitoes and can infect all equine species, including horses, donkeys, and zebras. Humans of all ages can also catch this disease, experiencing flu-like symptoms and, in severe cases, life-threatening illness, or, in rare instances, death. 

Epidemiology 

VEE was named in 1938 after its discovery in Venezuela by Albert Sabin and Robert Phillips, who found the virus in horses with an unknown illness.¹ This prompted the name Venezuelan Equine Encephalitis, due to the neurological symptoms observed in the horse. 

VEE is an endemic disease, which means it regularly occurs in a specific area. In this case, it frequently affects parts of Central and South America.¹ The virus thrives in regions with a tropical or sub-tropical climate, especially during the rainy season when mosquito populations are higher. This is because there is a higher risk of people being bitten and thus, increases the transmission of the disease. 

Whilst VEE outbreaks outside this area have been rare, they are not impossibilities. For example, from 1969-1972, there was an outbreak in Texas in which several hundred humans were infected.² Climate change has the potential to bring the virus to other parts of the world, as warmer climates create the ideal habitat for its transmission. Therefore, it is important to understand the transmission of the virus to humans, the symptoms of the disease and the preventative measures that can be taken to prevent human cases of VEE. 

Transmission of Venezuelan equine encephalitis 

The virus is transmitted to humans via mosquitoes that initially bite an infected animal and subsequently, bite and feed on another animal or human. It is during the feeding process that the transmission occurs. An animal that carries the disease from an infected individual to a non-infected individual through biting is called a ‘vector’. 

There are two types of VEE transmission to humans: Enzootic and Epizootic. 

Enzootic

Enzootic transmission occurs naturally between mosquitoes. The vectors carrying the disease are the natural transmission of VEE between mosquitos. 

In this case, the reservoir hosts (animals that are the drivers of disease) include rodents and bird species. When mosquitos feed on these animals, they become infected and can transmit the virus to other mosquitoes during subsequent feedings. 

Humans become infected when bitten by an infected mosquito. The speed of transmission depends on the type of virus and the density of the mosquito population. This type of infection tends to remain in small areas. 

Epizootic

In epizootic transmission, equine species, such as horses, zebras and donkeys become infected with VEE and the virus replicates to high levels in their blood. These equine species are highly attractive to mosquitos and other blood-feeding animals. Multiple animals can feed on them at one time, all with the potential to become infected with the virus. 

This is the most common type of infection that can spread rapidly through large populations. 

Interestingly, once humans are infected with VEE, they do not produce enough of the virus in their blood to pass this virus onto mosquitos and other blood-feeding animals. Therefore, humans are dead-end hosts of VEE and largely do not contribute to the spread of the virus. 

Symptoms and diagnosis of human Venezuelan equine encephalitis 

Symptoms of the disease include: 

• Chills 
• Fever
• Headache 
• Nausea
• Vomiting 
• Lumbosacral (middle to lower back) pain 
• Myalgia (muscle pain)

Non-neurologic infections are typically self-limiting, with most individuals experiencing a full recovery within a few weeks of onset. The overall fatality rate of the disease is less than 1%. 

In severe cases, VEE infection may progress to encephalitis. Encephalitis is a condition where the brain becomes swollen and inflamed, posing a life-threatening situation.³ 

Additional symptoms may develop after the initial flu-like symptoms, and they include: 

• Seizures or fits 
• Difficulty with speech 
• Weakness/Loss of movement of the body 
• Loss of consciousness 
• Confusion 
• Sensitivity to light 
• Stiff neck 
• Coma 
• Change in behaviour or personality 

It must be noted here that severe cases of VEE are relatively rare and most people with the virus experience symptoms that are mild to moderate, and eventually make a full recovery. 

Acute encephalitis can cause chronic neurological problems such as motor disorders, affective disorders, and dysarthria (difficulty speaking due to weak speech muscles).³ It is important that healthcare is accessed as quickly as possible in these cases for appropriate evaluation and treatment.

Humans of all ages can catch and develop severe VEE. However, data suggests that more severe infections occur in children within a range of 4-14% compared to less than 1% in adults.^1,4

Diagnosis is typically made based on symptoms and various laboratory methods that require specialised laboratory equipment and expertise- this may vary depending on the availability of resources. 

Treatment, management, and prevention of human Venezuelan equine encephalitis 

There are currently no antiviral treatments for the VEE infection. However, people with VEE can receive care to manage their symptoms, keep them under control and aid recovery. The effectiveness of this varies depending on the severity of the infection in each case. 

Treatment

Possible supportive care includes: 

• Fluids and Nutrition: People with VEE may require intravenous (IV) hydration and nutrition, especially if they are struggling with nausea and vomiting and are, therefore, unable to eat or drink
• Hospitalisation: people who have a severe case of VEE may require hospitalisation to monitor symptoms, most often in the case of the neurological symptoms listed above
• Preventing Secondary Infection: people with VEE are at risk of developing secondary infections such as pneumonia and antibiotics may be administered if secondary infections are suspected or confirmed

Prevention

Due to the absence of antiviral treatments, public health measures are crucial for managing VEE cases and preventing transmission. 

These measures include vector control methods such as personal protective measures to avoid mosquito bites. This includes: 

• The application of DEET (the active ingredient in many mosquito repellents) at ≤35% strength for adults and ≤10% for children⁵
• Wearing long sleeve tops and long trousers 
• Using mosquito nets

All methods can reduce the risk of being bitten and therefore, the risk of contracting a mosquito-borne disease. 

Management

Currently, there is not a widely distributed vaccine for VEE. It is primarily administered to individuals in the military or working with the virus in a laboratory environment. This vaccine does not fully immunise the patient and causes side effects. 

There is a vaccine that is utilised in horses and other equine species. This can be used to decrease the infection cycle. People in endemic areas must be educated on the importance of their animals to receive this vaccine to prevent the spread of the disease, emphasising the importance of community education. 

Impact of human Venezuelan equine encephalitis on the economy

In regions where the virus is endemic (regularly occurring), the economic impact of the virus can be significant. This is seen in many ways: 

• Human health costs – in severe cases of infection, specialised healthcare is required, which is costly. Furthermore, there may be indirect costs such as loss of workdays and the long-term impact of encephalitis, which can affect an individual’s quality of life, potentially leading to requiring long-term healthcare
• Veterinary Costs – the cost of vaccinating affected animals against the disease or treating the disease once it has arisen can be quite high
• Agricultural and Livestock Losses – when horses are infected this can affect humans by increasing their risk of catching the disease but also in that horses can experience severe illness and death. This impacts people who use horses for transportation, recreation, or work. Additionally, other livestock, such as cattle can also get the disease
• Tourism and Trade – if human cases of VEE are high in an area, this can have a large impact on trade. Travel restrictions may be in place for that are reluctant to travel to an area where they could get the disease

Summary

• VEE is a virus that can infect humans and cause flu-like symptoms 
• Some cases of VEE can lead to a serious condition of Encephalitis and are more likely to occur in children 
• Encephalitis can be a life-threatening disease that can lead to long-term impacts if not treated accordingly 
• There is currently no widely-distributed vaccine for the VEE virus
• Control measures have been put in place in areas where VEE is reoccurring 
• These control measures include the vaccination of equine species, careful monitoring of the disease and mosquito prevention measures. 
• Careful monitoring of the disease is important for travellers and the trade of the area VEE is prevalent in
• It is important to understand the symptoms of the disease and to be aware of its prevention, especially for those native to or travelling to Central and Southern America 
• Understanding and awareness can prevent the spread of VEE and benefit public health, the economy and other aspects of society