Epizootic Equine Encephalitis – A Viral Zoonotic Threat

Epizootic Equine Encephalitis – A Viral Zoonotic Threat

Epizootic Equine Encephalitis refers to a group of severe viral diseases affecting the central nervous system of horses and mules. The main forms of the disease are Eastern Equine Encephalitis (EEE), Western Equine Encephalitis (WEE), and Venezuelan Equine Encephalitis (VEE). These diseases are caused by viruses from the Togaviridae family, genus Alphavirus, and are zoonotic—meaning they can also be transmitted to humans. They primarily occur in the Americas but may pose a risk in other regions due to climate change and globalization.

 

Etiological Agent and Transmission

EEE, WEE, and VEE viruses are transmitted by mosquitoes (e.g., Culex, Aedes, Coquillettidia), which become infected by feeding on reservoir hosts—mostly wild birds for EEE and WEE, and rodents for VEE. Horses serve as incidental hosts and do not transmit the virus to other animals or humans.

In humans, infection occurs through bites from infected mosquitoes, especially in endemic areas. VEE is also unique in that it can spread directly through contact with secretions or excretions of infected animals.

 

Clinical Signs in Horses

After an incubation period of 5 to 14 days, horses may develop general, neurological, and often fatal symptoms. Common clinical signs include:

  • Fever (up to 41°C), weakness, loss of appetite, restlessness;

  • Neurological signs: ataxia, blindness, balance disorders, seizures, hypersensitivity;

  • Aggression or lethargy, difficulty rising, collapse;

  • Limb paralysis, coma.

Mortality rates vary by virus type:

  • EEE: very high (70–90%)

  • WEE: lower (20–50%)

  • VEE: variable, may reach 50–80%.

Survivors often suffer permanent neurological damage.

 

Diagnosis

Diagnosis is based on clinical signs, history of travel or vector exposure, and laboratory tests, including:

  • Serological ELISA tests for IgM/IgG antibodies;

  • PCR for viral genetic material detection;

  • Histopathology of the brain (inflammatory changes, neuron necrosis).

Differential diagnosis should exclude other neurological diseases such as rabies, equine herpesvirus encephalitis, and bacterial infections (Listeria, Borrelia).

 

Treatment

No specific antiviral treatment exists. Therapy is supportive and symptomatic, including:

  • Anti-inflammatory drugs (NSAIDs, corticosteroids);

  • Sedatives and anticonvulsants;

  • Fluid therapy and nutritional support;

  • Protection from secondary injuries (e.g., from recumbency).

Treatment success depends on early intervention and the overall condition of the horse.

 

Prevention

Vaccination is the primary preventive measure:

  • Inactivated vaccines (monovalent or combined, e.g., EEE/WEE/VEE);

  • Annual vaccinations, ideally before mosquito season;

  • Foals vaccinated starting at 4 months of age, with a booster after 4 weeks.

Additional control measures include:

  • Use of repellents and insect screens;

  • Removal of standing water to reduce mosquito breeding sites;

  • Grazing animals away from wetlands during peak mosquito activity.

 

Zoonotic and Economic Significance

Equine encephalitis viruses pose serious threats not only to animal health but also to humans. Human EEE infections can be severe, with high fatality rates and lasting brain damage. These diseases are reportable to veterinary and public health authorities.

For horse owners, epizootic encephalitis leads to significant costs due to vaccination, treatment, and losses from animal deaths and removal from use.

Epizootic equine encephalitis is a serious mosquito-borne viral encephalopathy. High mortality, lack of specific treatment, and zoonotic potential make it a disease of major concern. Regular vaccination, biosecurity, and mosquito control are critical to protecting the health of horses and humans alike.