Introduction

Venezuelan equine encephalitis (VEE), caused by the Venezuelan equine encephalitis virus (VEEV), is a disease that tends to occur in sporadic outbreaks throughout Latin America, causing tens of thousands of cases each year in this region. Mosquitoes carry VEEV, causing infections in other species, including mammals, birds, and reptiles.

As the name of the disease suggests, VEE causes encephalitis, inflammation of the brain, which can result in fevers, headaches, impaired mental function, and seizures. There is no approved treatment for VEEV, however, the fatality rate for the disease is low, with just 0.7% of those with the disease dying from it.

Children are most likely to experience severe disease and mortality. VEEV was initially discovered in 1938 where it was found to be present in the brain of a horse in Venezuela that had died of encephalitis. Several years later, in 1950, VEEV was found in humans for the first time, specifically in ill Colombians suffering from fevers.^1,2,3,4,5 

Historical outbreaks of Venezuelan equine encephalitis

Several outbreaks of VEE have occurred throughout history. Key outbreaks were those of 1962, 1969, and 1995.

The Venezuelan equine encephalitis outbreak in Colombia and Venezuela of 1962

In the months leading up to the autumn of 1962, cases of VEE were in horses in Venezuela. By October of 1962, an outbreak of VEE was reported in humans, occurring in the Northern regions of Venezuela. These cases of VEE spread throughout Venezuela, with approximately 14,000 people in Venezuela having been impacted by 1963. Subsequently, outbreaks in the parts of Colombia adjacent to Venezuela occurred. Cases of VEE continued in the following months, eventually coming to a stop, however, in 1964.⁶

The Venezuelan equine encephalitis outbreak in El Salvador and Guatemala of 1969

In 1969, an outbreak of VEE cases commenced on the border between El Salvador and Guatemala. This outbreak spread northward to Mexico, impacting tens of thousands of horses and humans. This outbreak continued until 1972, at which point 93 humans and 50,000 horses in Mexico had died.

Some cases in Mexico spread to southern parts of the United States of America, leading to the death of 1,500 horses in the country’s southern state of Texas, as well as 110 humans having EEV. Many mosquitoes were found to be carrying VEEV during this time. By vaccinating horses with the TC-83 vaccine, cases came to a halt by 1972.^7,8

The Venezuelan equine encephalitis outbreak in Colombia and Venezuela of 1995

During April of 1995 in Venezuela, outbreaks of VEE in horses were reported. Not long after these reports, cases of humans in the same area reporting to be ill with fevers were increasing, consistent with VEE. As months progressed, more and more cases of VEE were being reported in Northern regions of Venezuela.

By September 1995, cases had spread to Columbia, specifically Northern regions neighbouring Venezuela. During several months when this outbreak of VEE took place, rainfall in the affected regions was particularly heavy, and large quantities of mosquitoes, the species involved in the spread of VEEV, were collected.

To combat the rising cases of VEE, the Colombian government initiated a campaign where they vaccinated nearly 100,000 horses with the TC-83 vaccine, with no more cases being reported beyond October of 1995. Cases in Venezuela, however, continued until December of that year. This outbreak ultimately impacted 75,000 - 100,000 people across Colombia and Venezuela, making it the largest outbreak of VEE.^6,9

Factors contributing to outbreaks of Venezuelan equine encephalitis

Historical outbreaks of VEE have been associated with heavy rainfall. This is because mosquitoes, which transmit VEEV, are present in high quantities during rainfall. As a result, climate change could contribute to outbreaks of VEE as it may lead to changes in rainfall and also drive the northward distribution of mosquitoes, causing more outbreaks of VEE in northern regions.^1,2

Control and prevention of Venezuelan equine encephalitis

While no treatment is specifically approved for VEE, there are strategies to help control and prevent VEE, namely, limiting physical exposure to mosquitoes and vaccination.²

Limiting physical exposure to mosquitoes

Since mosquitoes transmit VEEV, limiting physical exposure to mosquitoes can help control and prevent VEE. Several strategies can be applied to limit physical exposure to mosquitoes:²

• Do not enter areas often populated by mosquitoes unless necessary
• Wear protective clothing
• Frequently use repellants

Vaccination

Currently, no vaccine has been specifically approved for use in humans against VEEV. However, a vaccine called TC-83 has been used previously, mostly for horses, to control outbreaks of VEE. Though not widely available, the TC-83 vaccine can also be used in humans. It is particularly helpful for those at a high risk of being exposed to VEEV, such as people working in laboratories where they may encounter VEEV. Booster injections of the TC-83 vaccine are needed for the vaccine to remain effective.²

FAQs

How have historical outbreaks of VEE impacted human and other animal populations?

Historical outbreaks of VEE have impacted hundreds of thousands of humans and other animals, particularly horses, across Latin America, reaching southern regions of the United States of America. Though VEE doesn’t always cause signs and symptoms, it can result in fevers, headaches, impaired mental function, and seizures. Fatality from VEE in humans is rare, however, the condition has claimed the lives of many horses.^2,6,7,8,9

Has climate change played a role in the occurrence of VEE?

Climate change causes changes in weather patterns which can lead to more rainy seasons and a subsequent increase in the quantity of mosquitoes that spread VEEV. Climate change can cause mosquitoes to migrate northward to result in more outbreaks of VEE towards northern regions.^1,2

What measures have been put in place to prevent or control the spread of VEEV?

Surveillance systems have helped control the spread of VEEV as they allow close monitoring of when new cases of VEE arise, and, therefore, help these cases be dealt with efficiently.³

As mosquitoes spread VEEV, limiting physical exposure to mosquitoes can help prevent VEE. This can be achieved by not entering areas where mosquitoes are often present unless necessary, wearing protective clothing, and frequently using repellants.²

The TC-83 vaccine has been used in historical outbreaks of VEE to stop the condition spreading in horses, which are commonly impacted by, and can die as a result of, VEE. The TC-83 vaccine is sometimes used in humans and can be particularly helpful for preventing VEE in those who are more likely to be exposed to VEEV, including people working in laboratories where VEEV may be present.²

Summary

Venezuelan equine encephalitis (VEE) is a disease that causes inflammation of the brain, which may result in fevers, headaches, impaired mental function, and seizures, but can resolve on its own. Venezuelan equine encephalitis virus (VEEV) causes VEE and is transmitted via mosquitos to infect humans, as well as mammals, birds, and reptiles.

Historically, horses have been most affected by VEE, specifically vulnerable to dying from the condition, though the TC-83 vaccine can combat this. Several outbreaks of VEE have occurred historically, but the largest one occurred in 1995 in Colombia and Venezuela, with it being estimated that 75,000 to 100,000 people had VEE during this time.