Vaccinating your horse for eastern equine encephalitis

Learn more about the core vaccinations your horse is receiving, including eastern equine encephalomyelitis.

April 11, 2017 - Author: , , and Lisa Skylis, MSU

Horse eating grass

As horse owners calculate the annual cost of veterinary care for their horse, the cost of vaccinations can seem overwhelming. It is expensive to have a veterinarian come out for vaccinations, and some vaccinations require multiple boosters that are administered over several weeks. Many horse owners begin to wonder if vaccinations are really necessary, and may cut some out of their vaccination program. However, finances should not be the driving factor in these decisions. Horse owners must also consider their farm’s location and general management practices of the farm, and make these healthcare decisions in consultation with their veterinarian.

Core vaccines are the vaccines all horses should receive, according to the American Association to Equine Practitioners. There are also non-core vaccines that should be decided upon based on your horse’s potential risks through the process discussed above. One disease that is considered a core vaccine is eastern equine encephalitis, which is difficult to treat but prevented efficiently through vaccination. Eastern equine encephalitis is a neurodegenerative disease that is exclusively insect-borne.

The transmission of eastern equine encephalitis begins with mosquitos that bite a wild bird with the Togaviradae virus and then proceed to bite and infect a horse. Your horse cannot infect you with the disease. Although eastern equine encephalitis is classified as a zoonotic disease, horses are a dead-end host and a human can only obtain the disease if they, too, are bitten by an infected mosquito.

The acute symptoms of eastern equine encephalitis are general and often more mild, but they are key in the early detection and diagnosis of the disease. The incubation period (time from when exposure to the disease occurs until symptoms are first shown) for the disease varies greatly, but may be any time between 24 hours and two weeks. Initial signs of eastern equine encephalitis in horses include, but are not limited to, any of the following:

  • Depression
  • Lethargy
  • Increased excitability
  • Wandering
  • Poor appetite
  • High fever (104-106 degrees Fahrenheit) that may be sustained for 24-48 hours
  • Weakness in hind limbs

Alert your veterinarian if any of these symptoms are present. The vet may elect to draw a blood sample from the horse to rule out other diseases and confirm diagnosis. The onset of acute clinical symptoms of eastern equine encephalitis occur promptly after the incubation period and the final phase of symptoms begin after that.

The late-onset symptoms of eastern equine encephalitis are much more severe than the acute signs of the disease described earlier. The late-onset symptoms are:

  • Lack of coordination and stumbling
  • Head-pressing
  • Aimless walking
  • Twitching of the face or limbs
  • Convulsions
  • Paralysis
  • Coma

These signs are present about two to four days after the acute symptoms began and signal the final stage of eastern equine encephalitis. After experiences of convulsions, paralysis or becoming comatose, the horse rarely survives with the mortality rate being about 90 percent. Horses rarely survive this phase, but if they do they may improve gradually over weeks and months and permanent brain damage is common. Survivors of late-onset symptoms are immune to receiving the disease after infection for up to two years.

At the last phase of symptoms, treatment can include antibiotics for secondary infections, preventing dehydration through the use of IV electrolytes and keeping the horse calm to prevent further discomfort or injury; euthanasia is often recommended at this point to ease the animal’s pain. The most important aspect of treatment for eastern equine encephalitis is prevention, as most infected horses die or are euthanized.

The proper time to vaccinate is region-dependent. Since Michigan doesn’t have mosquitos year round, it is recommended by most vets to do the vaccination annually in the spring. Eastern equine encephalitis vaccinations are thought to be effective anywhere from six to eight months, thus if your horse spends the winters in Florida, a second vaccination is normally given six months after the initial vaccination.

In summary, eastern equine encephalitis is a usually-fatal neurodegenerative disease that is effectively prevented by vaccination. While some may try to prevent the disease by limiting the horse’s exposure to mosquitos, this is normally ineffectual and the vaccine is the only tried and true method to prevent eastern equine encephalitis. When calculating the costs of the numerous vaccines your horse requires, please consider the devastating effects of eastern equine encephalitis and vaccinate against it.

For more ways to share science with youth in your life, please explore the Michigan State University Extension Science and Engineering webpage. For more information about 4-H learning opportunities and other 4-H programs, contact your local MSU Extension office

MSU Extension and the Michigan 4-H Youth Development program help to create a community excited about STEM (Science, Technology, Engineering, and Mathematics). 4-H STEM programming seeks to increase science literacy, introducing youth to the experiential learning process that helps them to build problem-solving, critical-thinking and decision-making skills. Youth who participate in 4-H STEM are better equipped with critical life skills necessary for future success. To learn more about the positive impact of Michigan 4-H youth in STEM literacy programs, read our 2015 Impact Report: “Building Science Literacy and Future STEM Professionals.”

Tags: 4-h, 4-h horses & ponies, animal science, horses, msu extension


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