The Buzz About EEE: What Horse Owners Need to Know
July 17, 2020By Christina Keim
Eastern equine encephalitis, better known as EEE, is caused by an arbovirus that lives in the bird population on the East Coast. It spreads from birds to horses via the bite of an infected mosquito, with equine cases usually occurring in mid to late summer. But most modern horse owners have never seen a horse affected by what used to be called “swamp fever” for one simple reason: for horses there is an effective vaccine.
“If living in an area where EEE is endemic, the most important thing you can do for your horse is vaccinate based on the schedule recommended by your veterinarian,” says Jessica Starcevich, M.S., staff entomologist with Spalding Labs.
But EEE makes headlines in the Northeast almost every year because it is zoonotic—meaning that the virus can spread from mosquitoes to humans—and unfortunately for us, vaccination against EEE is not an option. Although horses cannot spread the disease to humans directly or indirectly, misinformation can cause public concern around possible exposure to the virus from local equine populations. Therefore, it is critical that equestrians have correct facts about EEE to help assuage fear.
The best EEE prevention strategy is two pronged: equine vaccination and a thorough mosquito control plan. By staying informed and taking active steps to reduce the spread of EEE, equestrians play an integral role in their local communities.
Evaluating Risk
The American Association of Equine Practitioners considers EEE a core vaccine, because a horse is at risk of contracting the disease even if they aren’t exposed to other horses. In addition, it is often fatal—mortality in unvaccinated horses that contract EEE is between 50-90%, and those that do survive infection often have permanent neurologic deficits necessitating euthanasia.
As its name implies, risk of contracting the virus is highest in the eastern coastal states and along the Gulf Coast; it has been found as far west as Texas and as far north as Michigan.
“EEE is not very common in New England, but infected mosquitoes and cases of EEE in people and horses were reported in New England states last year,”
says Margaret Gabour, DVM, an associate with Seacoast Equine in Stratham, N.H. “EEE is spread by mosquitoes, so all horses have exposure.”
An unvaccinated or undervaccinated horse (i.e., one that has not completed the initial vaccination series or has not been kept boostered) that contracts EEE will start to show signs of the disease within a week of exposure via infected mosquito bite. Symptoms include fever, lethargy, behavioral changes, and other neurologic abnormalities including head pressing, muscle tremors or convulsions, paralysis, and blindness. Once a horse becomes sick, there is little even your veterinarian can do to help.
“Sometimes administering medications to keep the horse hydrated and reduce pain and fever may improve comfort and allow the horse time to recover,” says Gabour. “Unfortunately, most horses that contract EEE are eventually euthanized or succumb to the disease.”
When it comes to human exposure, working outdoors at dawn and dusk (when mosquitoes are most active) or in wet, swampy areas (where mosquitoes breed and congregate) will increase risk of being bitten by an infected mosquito. According to data from the Vector Disease Control International, EEE symptoms in humans tend to occur four to 10 days after being bitten and include headache, fever, vomiting, muscle aches, and joint pain and fatigue. They list the mortality rate in humans at 33%, the highest among arboviruses common to the U.S.
How is EEE spread?
Despite its name, horses are just one of a number of mammal species that can contract EEE, all of which are considered “dead end hosts.” This means that the virus does not build to high enough levels in the mammal’s bloodstream to spread it further. In its natural cycle, EEE goes mainly between birds and mosquitoes; the primary vector is a species of mosquito called Culiseta melanura. This species acquires EEE after feeding on an infected bird, then spreads the virus to other birds as it continues to feed. Most birds are unaffected by the virus, serving only as carriers, although researchers at Cornell University have determined that EEE can cause death in pigeons, pheasants, and emus.
“Culiseta melanura primarily feed on birds,” explains Starcevich. “In early spring when that cycle is going normally between that mosquito species and the birds it feeds on, the incidence of EEE in horses and humans is pretty low.”
But later in the summer, as wet areas dry up and both birds and mosquitoes start traveling farther to feed, the virus begins to show up in other mosquito species, such as Coquillettidia perturbans. This species feeds on both birds and mammals and plays a major role in spreading EEE outside of avian populations. Several members of the Aedes genus have also been considered possible vectors.
What is perhaps most important to understand is that though both horses and humans can get sick with EEE, neither species can spread the disease to anyone else. Incidence of EEE in non-avian species increases during the mid to late summer because more diverse types of mosquito species are coming into contact with infected birds.
What can I do to prevent EEE infection?
When it comes to keeping your horse safe from EEE, all experts agree: vaccinate, vaccinate, vaccinate.
“In New England, horses should be vaccinated annually in early spring prior to the beginning of mosquito season,” says Gabour. “Typically, annual vaccination is all that is needed. In times of EEE outbreak or in geographic regions with long mosquito seasons, horses may be vaccinated more often.”
Pregnant mares should be boostered for EEE in the final month of their pregnancy, which will confer immunity to their newborn foal. To maintain protection, foals should be given a dose of vaccine between four and six months of age, followed by a booster about a month later. This is the same protocol used on an adult horse of unknown vaccination history, or coming from a region where the virus is uncommon. In general, the EEE vaccine provides protection for a year.
Taking active steps to reduce the mosquito population in and around your farm, along with appropriate use of repellents on horses and humans, are the best ways to reduce risk of human exposure to EEE.
Mosquitos breed in water, and some species only require miniscule amounts to complete the cycle. In general, the mosquito breeding cycle ranges from 5-7 days to 10-14 days, depending on temperature and the specific species. As a rule of thumb, wet areas need to be drained or dried within a week to avoid becoming potential mosquito nurseries.
“Especially in mid-summer, the faster you can get it cleaned up, the better,” says Starcevich.
Start by taking a walk around your property, looking carefully for any areas where water can pool; drain and turn over items that collect water, or throw them away if not needed. Pay special attention to places like tire swings, planters, pool covers, discarded tires, buckets, water pans for pets, clogged gutters, downspouts and even the tops of sealed containers. The goal is to drain water where you can, and treat it where you can’t.
Rain barrels, bird baths, fountains, and stock tanks—anything that permanently stores standing water—should be treated with a larvicide containing S-methoprene as the active ingredient. This chemical is commonly used even on human food crops and is considered to be one of the safer options for non-target species.
“S-methoprene is an insect growth regulator that can be added to things that cannot be easily dumped weekly,” says Starcevich. “Treated water will not harm birds or bees.”
Finally, use repellents to help prevent adult mosquitoes from feeding on you and your horses. Permethrin based sprays work best for horses, and should be used regularly when mosquito activity increases toward mid to late summer. For humans, long sleeved shirts and pants will help reduce bites, as will the use of sprays containing DEET, picaridin, or IR3535. These repellents have proven effective against mosquito species that spread EEE, as well as several other common biting pests.
Final Thoughts
The EEE virus may be a part of our local environment, but it need not strike fear or panic into equine managers. Ensuring that horses are fully vaccinated and administering annual boosters prior to the onset of mosquito season will offer them the best chance of protection from this deadly virus. Creating and implementing an effective mosquito control plan on your property will help to reduce the risk of human exposure, as well as allow for a more pleasant outdoor experience.
