What Horse Owners Should Know About Eastern Equine Encephalomyelitis

Mosquitoes are pesky as it is, but their taste for a variety of hosts is what makes them dangerous. As they feed on different animals, they pick up and distribute parasites and pathogens. Horses can contract equine encephalomyelitis, an alphavirus that causes central nervous system inflammation, from an infected mosquito.
Eastern Equine Encephalitis (EEE) is found in states east of the Mississippi, and it’s highly fatal. Western Equine Encephalitis (WEE) is found in states west of the Mississippi, and while it’s less likely to be fatal, it can still cause serious, sometimes permanent neurologic damage. Here’s what you need to know to keep your horse safe.

Where Does Equine Encephalitis Come From?
Equine encephalitis can infect many species, but birds are the biggest reservoir of the viruses. Mosquitoes contract the virus when they feed on infected birds. Horses contract the virus when bitten by an infected mosquito. An infected horse does not harbor enough of the virus in their bloodstream to transmit it directly to other horses or animals.
It is possible for humans to contract the virus from a mosquito bite, though it’s uncommon. Just 9 cases of Eastern Equine Encephalitis were reported in humans in the United States in 2020.

What Are The Symptoms of Equine Encephalitis?
Symptoms of equine encephalitis set in about five days after transmission, though some horses clear the virus without symptoms.
The virus causes inflammation of the brain and spinal cord, resulting in neurological symptoms like muzzle twitching, weakness, paralysis, inability to swallow, head pressing, circling, and impaired vision. Over 90% of horses infected with EEE will die within several days, while WEE has a survival rate of over 70%.

How Can We Prevent Equine Encephalitis?
Vaccines are available for EEE and WEE. An unvaccinated horse or foal usually gets a two-dose series with 4-6 weeks between doses, then a booster at the start of mosquito season, followed by annual boosters. Your vet may recommend twice-yearly boosters for immunocompromised horses and those in heavily affected regions.
You should also manage the mosquito population in your horse’s environment by removing sources of standing water, keeping weeds under control, and cleaning manure frequently. Using a fan in the stables can keep mosquitoes away by disrupting their flying patterns and distributing carbon dioxide and odors that attract them. Certain plants like catnip, mint, lavender, and citronella can act as natural mosquito repellant around the barn.
Consult your veterinarian if you notice any neurological symptoms in your horse, as symptoms of EEE and WEE can be difficult to distinguish from those of other health conditions.