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[Microbiology](https://durgical.com/en/microbiology/) · [Other viruses and prions](https://durgical.com/en/microbiology/groups/other-viruses-and-prions/)

# Equine encephalitis viruses and Mayaro virus

- Virus
- Enveloped
- For completeness

Mosquito-borne alphaviruses that cause fever and sometimes encephalitis in horses and people; Mayaro causes fever with joint pain.

Updated 2026-11-01

Also called: Venezuelan equine encephalitis, eastern equine encephalitis, western equine encephalitis, Mayaro fever

## Key points

- Alphaviruses (Togaviridae): positive-sense RNA, enveloped, mosquito-borne.
- Venezuelan equine encephalitis: the horse amplifies the virus; in eastern and western it is a dead-end host.
- Eastern equine encephalitis is the deadliest.
- Venezuelan equine encephalitis epidemic in La Guajira, 1995.
- Mayaro: fever with persistent arthralgia, a chikungunya differential.

## At a glance

**Shape**
Spherical, icosahedral capsid

**Genome**
Single-stranded, positive-sense RNA

**Envelope**
Enveloped

**Family**
Togaviridae, genus Alphavirus

**Size**
About 70 nm

## Morphology

- Envelope with the E1 and E2 glycoproteins
- Icosahedral capsid
- Same genus as chikungunya

## Transmission

- Bite of infected mosquitoes
- Venezuelan equine encephalitis: horses and donkeys develop high viraemia and amplify the virus in epidemics
- Eastern and western encephalitis: a bird-mosquito cycle; horses and humans are dead-end hosts
- Mayaro: Haemagogus mosquitoes of forest areas

**Reservoir:** Wild rodents (enzootic VEE), birds (eastern and western), non-human primates (Mayaro, possibly)

**Vector:** Mosquitoes of several genera: Culex, Aedes, Psorophora, Culiseta and Haemagogus, depending on the virus and region.

### Isolation precautions

What the care team adds to standard precautions.

- Standard

## Virulence factors

- Multiplies in lymphoid tissue and reaches the blood
- Some cross into the nervous system and cause encephalitis
- Eastern equine encephalitis is the deadliest and leaves neurological sequelae in many survivors

## Diseases it causes

- Venezuelan, eastern and western equine encephalitis [Viral encephalitis (herpes simplex encephalitis) →](https://durgical.com/en/conditions/viral-encephalitis/)
- Mayaro fever with arthralgia (a chikungunya-like illness)

## Clinical picture

- Fever, headache, myalgia and vomiting
- Encephalitis: drowsiness, seizures, behaviour change; more severe in children and older adults
- Mayaro: joint pain and swelling that can last weeks or months

## Diagnosis

- IgM in serum or cerebrospinal fluid
- RT-PCR in the acute phase
- Consider Mayaro in the differential of dengue, chikungunya and Zika

## Treatment

No specific antiviral: supportive treatment and neurological care in encephalitis.

Drug class only; doses and regimens are set by the clinician from the current guideline.

## Prevention

- Avoid mosquito bites: repellent, long clothing, bed nets
- Vaccinating horses against Venezuelan equine encephalitis and restricting equine movement during outbreaks
- Control of mosquito breeding sites

## In Colombia

In 1995 a Venezuelan equine encephalitis epidemic spread from Venezuela into La Guajira, with thousands of human cases in municipalities such as Manaure, Riohacha and Uribia; it was controlled with equine vaccination, movement restrictions and spraying. PAHO records Mayaro fever cases in Colombia in 1958-1960.

## Memory aid

"The Venezuelan Trojan horse": in VEE the horse brings the virus into town; in eastern and western the horse is just another victim of the birds.

## Test yourself

Think of the answer, then tap the question to see it.

## What role do horses play in Venezuelan equine encephalitis epidemics?

They are amplifying hosts: they develop high viraemia and feed the mosquitoes.

## Which genus do the equine encephalitis viruses and Mayaro virus belong to?

Genus Alphavirus, family Togaviridae, like chikungunya.

## Which Colombian region had a large Venezuelan equine encephalitis epidemic in 1995?

La Guajira.

## Which equine encephalitis is the deadliest?

Eastern equine encephalitis.

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## Related

- [Chikungunya virus](https://durgical.com/en/microbiology/chikungunya-virus/)
- [West Nile virus](https://durgical.com/en/microbiology/west-nile-virus/)
- [Dengue virus](https://durgical.com/en/microbiology/dengue-virus/)
- [Zika virus](https://durgical.com/en/microbiology/zika-virus/)

## Related conditions

- [Viral encephalitis (herpes simplex encephalitis)](https://durgical.com/en/conditions/viral-encephalitis/)

## What the colours mean

**Gram-positive**
Thick peptidoglycan wall: stains purple.

**Gram-negative**
Thin wall with an outer membrane: stains pink.

**Acid-fast**
Lipid-rich wall (mycobacteria): Ziehl–Neelsen stain.

[← Back to the group: Other viruses and prions](https://durgical.com/en/microbiology/groups/other-viruses-and-prions/)

Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: [Venezuelan Equine Encephalitis -- Colombia, 1995](https://www.cdc.gov/mmwr/preview/mmwrhtml/00039070.htm) (CDC (MMWR), 1995); [About Eastern Equine Encephalitis Virus | Eastern Equine Encephalitis Virus | CDC](https://www.cdc.gov/eastern-equine-encephalitis/about/index.html) (CDC, 2024); [Epidemiological Alert: Mayaro Fever](https://paho.org/sites/default/files/2019-06/2019-May-01-phe-epi-alert-mayaro.pdf) (PAHO/WHO, 2019).

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/microbiology/equine-encephalitis-viruses-and-mayaro-virus/