What You Need to Know About Equine Encephalitis

Pet owners know all too well the dangers mosquitoes can pose to their four-legged friends. But did you know that cats and dogs aren’t the only pets that can catch mosquito-borne diseases? Mosquitoes also love to bite large domestic animals like horses, and infected mosquitoes can cause serious and sometimes fatal illness in our trusty steeds. Equine encephalitis is the most common disease spread to horses from mosquitoes, and it can also be the deadliest depending on the variant.

What is Equine Encephalitis?

Equine encephalitis (encephalomyelitis) is a virus spread to horses through the bites of infected mosquitoes. Sometimes called “sleeping sickness," this disease causes inflammation and damage to the brain, spinal cord, and central nervous system in horses, causing their physical and mental functions to slow drastically. In the most extreme cases, equine encephalitis can result in comas or death.

Horses are considered dead-end hosts since there is not enough virus accumulated in them to become contagious and pass on the disease. Other animals, namely birds, act as reservoir hosts. These animals harbor the virus without showing any severe symptoms, and they play a large role in allowing the disease to grow and spread.

Equine encephalitis comes in three alphaviruses: Eastern, Western, and Venezuelan.

Eastern and Western Equine Encephalitis (EEE or WEE)

Eastern and Western equine encephalitis are primarily prevalent in the Southeastern and Southwestern United States, but cases have been reported throughout the US. Eastern equine encephalitis is one the deadliest mosquito-driven diseases that horses can acquire, with a mortality rate of 85-90% within days of presenting symptoms. Western equine encephalitis has a mortality rate of 20-40%, though untreated symptoms can cause lifelong injuries.

Symptoms of both Eastern and Western equine encephalitis range from both physical and behavioral symptoms. Physical symptoms are often more apparent than behavioral symptoms, but any drastic changes in your horse’s physical wellbeing or behavior should alert you that something is wrong.

Physical symptoms might include:

• Moderate fever
• Difficulty swallowing food and water
• Loss of appetite
• Seizures
• Blindness
• Facial paralysis

Behavioral symptoms might include:

• Head pressing and circling (behaviors horses usually do when stressed or suffering neurologically)
• Extreme fatigue
• Hyperexcitability
• Dementia
• Self-mutilation (running into walls, biting, stomping, kicking, etc.)

Venezuelan Equine Encephalitis (VEE)

Venezuelan equine encephalitis is similar to EEE and WEE with the major difference being that horses develop high viremia. This means that horses are no longer dead-end hosts in the virus’s lifecycle. Instead, horses act as amplifying hosts as infected horses can re-infect mosquitoes that bite them and shed the disease in their milk, furthering the spread of the virus.

Symptoms for VEE are similar to EEE and WEE, with the inclusion of diarrhea as an additional physical symptom. The mortality rate for horses that contract VEE is 40-90%, a huge range compared to the other alphavirus variants. Because of this uncertainty, it is crucial to get your horse tested for VEE if:

1. they were recently in an area where VEE is prevalent
2. they are developing symptoms similar to EEE and WEE

Treatment and Prevention

Due to the high mortality rate, treatment options are limited for horses that contract equine encephalitis. The Equine Disease Communication Center (EDCC) suggests that supportive care is the best strategy to treat symptoms and ease your animal’s discomfort. Fortunately, there are vaccines available to prevent this disease from harming your horses. The American Association of Equine Practitioners suggests annual vaccine and booster scheduling for EEE and WEE, both of which are highly recommended for horses traveling and training within the United States. Vaccines for VEE are optional and typically recommended if traveling to an area where VEE is prevalent.

For more information on which equine diseases are most prevalent in your region, visit the EDCC’s Reportable Diseases webpage.

Don’t let mosquitoes have free reign on your farm! Contact your local Squad today to protect your stables from mosquitoes and ticks.