Understanding Venezuelan Equine Encephalitis Transmission

  • Yujin Wang Master of Science – BSc, University of Sheffield, England

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What is Venezuelan Equine Encephalitis?

Venezuelan Equine Encephalitis (VEE) is an infectious zoonotic disease that can affect all equine species (horses, donkeys, zebras) and humans. VEE was first discovered in horses in the 1930s in South America.1 This disease is mainly passed by mosquito bites. After a mosquito bites an infected animal, it can carry on and spread the VEE virus to the next biting object. VEEV infection in equines is usually fatal. It can cause serious central nervous system disorders or even death. Interestingly, although the VEE virus can infect humans as well, the symptoms are much milder. More importantly, infection in humans with VEE virus in adults usually leads to a low mortality (<1%), and flu-like illness, such as headache, and high fevers.1

However, currently, there is no specific treatment or approved vaccine for VEE in humans.2 So, understanding the transmission of VEE is critical for preventing this disease. In this article, we will explore the route of VEE transmission, including the roles of mosquito vectors, animal reservoirs, and environmental factors.

Profile of VEE virus

The classification of VEE virus

According to the Animal Health Surveillance, the VEE virus is a member of the Alphavirus genus within the Togaviridae family, including a diverse group of viruses that are primarily transmitted by biting insects.3 VEE virus is known to cause severe neurological disease in equids (horses, donkeys, and mules), and children are more likely to develop encephalitis than adults.4,5 This zoonotic virus poses a significant threat to public health, particularly in regions where mosquito populations are abundant.

The subtypes of the VEE virus

Different subtypes of viruses usually target different proteins and pathways which leads to different pathogenic phenotypes (disease presentations or symptoms). VEE virus exhibits genetic diversity, with multiple strains classified into different subtypes based on antigenic variations. Among these subtypes, the epizootic strains (subtype IAB and IC) are responsible for outbreaks in equids. In contrast, the enzootic strains (subtypes ID, IE, and II-VI) are associated with endemic transmission cycles involving mosquitoes. Understanding the genetic diversity and geographic distribution of VEE virus strains is essential for limiting the disease spread and outbreak response efforts.5,6

The transmission of VEE virus

The VEE virus is primarily transmitted by mosquito vectors, such as Aedes and Culex in different environments. These mosquitoes acquired the virus by feeding on the blood of hosts (infected equids). Once infected, mosquitoes can pass the virus to others during subsequent blood meals, keeping the transmission cycle.

In addition to mosquito-borne transmission, the VEE virus can also be transmitted from infected mosquitoes to their offspring, contributing to the maintenance of the virus in mosquito populations. This type of transmission is known as vertical transmission (from mother to child), which plays an important role in the persistence of the VEE virus during epidemic periods and may impact the intensity and duration of transmission seasons.4

Furthermore, the VEE virus can infect a wide range of hosts, including humans, birds, and other mammals. While equids are the main hosts during epizootic outbreaks, humans can become infected through mosquito bites or exposure to infected animals or contaminated materials. Although human infections are rarely fatal, they can lead to severe nervous disorder, particularly in cases of encephalitic illness(inflammation of the brain).7

By reading here, you probably know what are the routes of VEE virus transmission. If you still have not got a clear idea, do not worry, here is a little summary of previous paragraphs. There are four routes which are:

  • Mosquito Vector Transmission: Mosquitoes play a pivotal role in the transmission cycle of this disease. Mosquito species from the Aedes and Culex are the primary vectors responsible for transmitting the virus to other hosts.
  • Vertical Transmission: Infected female mosquitoes pass this virus to their offspring. These offspring then pass the VEE virus through the first transmission pathway.
  • Contact with Infected Animals: VEE virus can also be transmitted through direct or indirect contact with infected animals. Equids are the primary hosts during epizootic outbreaks, and human infections often occur through contact with infected horses, donkeys, or mules. Additionally, handling of contaminated materials from infected animals may also increase the risks for transmission in humans.
  • Environmental Factors: Environmental factors such as climate, habitat, and seasonality influence are all factors that may affect VEE transmission. Mosquito vectors are impacted by temperature, humidity, and rainfall patterns, which in turn affect the intensity and duration of transmission seasons. Understanding the environmental drivers of VEE transmission is essential for predicting and managing disease outbreaks.
Environmental FactorDescription
ClimateInfluences mosquito abundance and activity, with warmer temperatures and higher humidity promoting mosquito breeding and viral transmission.
HabitatCharacteristics of breeding habitats such as seeper, sewer, swampland and vegetation are all affect the populations and distribution.of  mosquito vector
SeasonalitySeasonal fluctuations in mosquito activity affect the timing and intensity of VEEV transmission, with peak transmission occurring during warmer months.

How will Venezuelan Equine Encephalitis impact public health?

Venezuelan Equine Encephalitis (VEE) has significant challenges to public health due to its potential for causing outbreaks and severe illness in both humans and animals. The impact of VEE on public health is multifaceted and requires comprehensive strategies to prevent and reduce the risk of this disease.8

Firstly, VEE can lead to outbreaks of varying magnitudes, ranging from sporadic cases to large-scale epidemics. These outbreaks can overwhelm healthcare systems, strain resources, and disrupt essential services. The sudden onset of VEE outbreaks can also create panic and uncertainty within communities, affecting mental health and well-being.

Secondly, VEE can result in severe neurological complications in humans, including encephalitis and meningitis. These complications can lead to long-term disability, cognitive impairment, and even death in severe cases. Additionally, individuals who survive VEE infection may experience persistent neurological disorders,  requiring ongoing medical care and support.

Furthermore, VEE can have significant economic implications, particularly in regions where agriculture and livestock play an important role in the economy. Outbreaks of VEE in equines can result in significant losses for farmers, breeders, and agricultural industries. Moreover, restrictions on the movement of animals during VEE outbreaks can disrupt trade and commerce, impacting livelihoods and economic stability.

In addition to the direct health and economic impacts, VEE can also strain public health resources and infrastructure. Responding to VEE outbreaks requires coordinated efforts from healthcare providers, public health agencies, veterinarians, and other stakeholders. This includes implementing surveillance systems, conducting outbreak investigations, and implementing control measures to limit the spread of the virus.


Venezuelan Equine Encephalitis (VEE) is a viral disease transmitted primarily by mosquitoes, affecting both humans and animals. The VEE virus belongs to the Alphavirus genus and can cause severe neurological complications, including encephalitis and meningitis, in infected individuals. Different virus subtypes target different proteins and pathways. Acknowledging different subtypes of the VEE virus is important to predict the outbreak risk and prevent the disease spread.

Transmission of VEE virus occurs through mosquito vectors, with species from the Aedes and Culex genera serving as primary vectors. Additionally, vertical transmission from infected female mosquitoes to their offspring contributes to the maintenance of the virus in mosquito populations. Animal reservoirs, particularly equids, play pivotal roles in VEE virus transmission cycles.

Human infections with the VEE virus can occur through mosquito bites, contact with infected animals, or exposure to contaminated materials. The epidemiology of VEE is influenced by factors such as climate, habitat, and seasonality, which shape mosquito vector populations and viral amplification.

VEE outbreaks can lead to significant public health challenges, including strain on healthcare systems, economic losses in agriculture, and disruptions to trade and commerce. Preventive measures such as mosquito control, equine vaccination, and public education are crucial for mitigating the impact of VEE and protecting vulnerable populations. Vigilance and collaboration among healthcare providers, public health agencies, and communities are essential for effectively managing VEE outbreaks and reducing the burden of the disease on public health.


What are the symptoms of Venezuelan Equine Encephalitis?

This disease can cause various symptoms but commonly include fever, headache, nausea, vomiting, and muscle pain. In severe cases, individuals may experience neurological symptoms such as confusion, seizures, and coma.

Can you get Venezuelan Equine Encephalitis by direct contact?

Venezuelan Equine Encephalitis is not directly contagious between humans. It primarily spreads through the bite of infected mosquitoes. However, individuals infected with the VEE virus can transmit the virus to mosquitoes, which can then infect other humans or animals.

How is Venezuelan Equine Encephalitis diagnosed?

Venezuelan Equine Encephalitis can be diagnosed through laboratory blood testing or cerebrospinal fluid samples. Techniques such as polymerase chain reaction (PCR) and virus isolation are used to detect the presence of the Venezuelan Equine Encephalitis Virus or antibodies produced in response to the virus.

Can Venezuelan Equine Encephalitis be prevented?

For humans, yes, this disease can be effectively prevented by using insect repellents, wearing protective clothing, and avoiding outdoor activities during peak mosquito activity. There are vaccines used for animals, but no approved vaccine for humans yet. Equine vaccination programmes can also help prevent VEE in horses and reduce the risk of human transmission.


  1. Levine BA, Nurudeen SK, Gosselin JT, Sauer M V. Endemic Venezuelan equine encephalitis in the Americas: hidden under the dengue umbrella. Future Virol [Internet]. NIH Public Access; 2011 [cited 2024 Feb 27]; 6(6):721. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3134406/ 
  2. Haines CA, Campos RK, Azar SR, Warmbrod KL, Kautz TF, Forrester NL, et al. Venezuelan Equine Encephalitis Virus V3526 Vaccine RNA-Dependent RNA Polymerase Mutants Increase Vaccine Safety Through Restricted Tissue Tropism in a Murine Model. Zoonoses (Burlington, Mass.) [Internet]. NIH Public Access; 2022 [cited 2024 Feb 27]; 2(1). Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8900488/ 
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  5. Go YY, Balasuriya UBR, Lee C. Zoonotic encephalitides caused by arboviruses: transmission and epidemiology of alphaviruses and flaviviruses. Clin Exp Vaccine Res [Internet]. Korean Vaccine Society; 2014 [cited 2024 Feb 27]; 3(1):58. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3890452/ 
  6. Forrester NL, Wertheim JO, Dugan VG, Auguste AJ, Lin D, Adams AP, et al. Evolution and spread of Venezuelan equine encephalitis complex alphavirus in the Americas. PLoS Negl Trop Dis [Internet]. Public Library of Science; 2017 [cited 2024 Feb 27]; 11(8):e0005693. Available from: https://journals.plos.org/plosntds/article?id=10.1371/journal.pntd.0005693.
  7. Venezuelan Equine Encephalitis (VEE) | School of Veterinary Medicine [Internet]. [cited 2024 Feb 27]. Available from: https://ceh.vetmed.ucdavis.edu/health-topics/venezuelan-equine-encephalitis-vee.
  8. Yuill TM. ZOONOSES. Encyclopedia of Virology [Internet]. Elsevier; 1999 [cited 2024 Feb 27]; 1987–97. Available from: https://linkinghub.elsevier.com/retrieve/pii/B0122270304003150.

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This content is purely informational and isn’t medical guidance. It shouldn’t replace professional medical counsel. Always consult your physician regarding treatment risks and benefits. See our editorial standards for more details.

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Yujin Wang

Master of Science – MSc, University of Sheffield, England

Yujin is a first-year master’s student in Health Technology Assessment and Reimbursement. She has several years of experience in medical and health reimbursement in public sectors. She is passionate in health related research and health promotions.

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