As spring sweeps across the country and temperatures rise, the risk for mosquito-borne diseases also rises. Horse owners begin to calculate the cost of spring vaccinations for their horse and the expense can quickly become overwhelming. Many horse owners begin to wonder “is this vaccination really necessary?” and some immunizations might get cut out of their vaccination program altogether. Although it may seem to strain your budget at the barn, preventing deadly diseases is more effective and often less expensive than the price of treatment. When establishing a vaccination program, horse owners must consider the farm’s location and general management practices to ensure that they’re protecting their herd as best they can. Core vaccines are the vaccines that all horses should receive, according to the American Association to Equine Practitioners, but there are also non-core vaccines that you may want to consider based on your horse’s potential risks. One vaccine that horse owners should seriously contemplate adding to their roster of core immunizations this spring is Eastern Equine Encephalomyelitis. More commonly abbreviated as EEE, the virus is difficult and costly to treat but can be effectively prevented through vaccination. EEE is a neurodegenerative disease that is exclusively insect borne.

The transmission of EEE can be traced back to the bite of a tiny mosquito. The mosquito was infected with the Togaviradae virus, which it may have gotten from an infected wild bird, and then proceed to bite and infect a horse. However, horses aren’t the only ones on the farm with cause for concern- humans can actually get EEE too! Luckily, horses are considered a dead-end host and a human can only contract the disease if they, too, are bitten by an infected mosquito. Furthermore, horses can’t infect each other with the disease. Once infected, detection and diagnosis of EEE in horses is crucial to your horse’s health.

Typically, the acute symptoms of Eastern Equine Encephalomyelitis are general and milder, but they are key in the early detection and diagnosis of the disease. The time from when exposure to the disease occurs until symptoms are first shown, also called the incubation period, for the disease can vary greatly but might be any time between 24 hours and a week in length. Initial signs of EEE in horses include, but are not limited to, any of the following:

• Depression
• Lethargy
• Increased Excitability
• Wandering
• Poor Appetite
• High Fever (104-106 degrees F) that may be sustained for 24-48 hours
• Weakness in Hind Limbs

If any of these symptoms appear in your horse, it is critical to alert your veterinarian. The vet may decide to draw a blood sample from the horse to rule out other more common diseases and confirm the diagnosis. The onset of clinical symptoms of EEE occur promptly after the incubation period and the final phase of symptoms starts soon after.

The late-onset symptoms of Eastern Equine Encephalomyelitis are much more severe than the acute signs of the disease described earlier. The late-onset symptoms are as listed:

• Lack of Coordination and Stumbling
• Head-Pressing
• Aimless Walking
• Twitching of the Face and/or Limbs
• Convulsions
• Paralysis
• Coma

These signs first present about two to four days after the acute symptoms began and they signal the final stage of EEE. The virus’s attack on the central nervous system is brutal during the last phase and typically fatal. After experiences of convulsions, or paralysis, or becoming comatose, the horse rarely survives with the daunting EEE mortality rate of about 90%. If your horse outlives EEE, their body will improve very gradually over weeks and months but permanent brain damage is regrettably common. However, these few survivors of the late-onset symptoms are often immune to getting the disease for the rest of their lives. During the final stage of EEE, treatment can include preventing dehydration through the use of electrolytes, antibiotics for secondary infections, and keeping the horse as calm as possible to prevent further pain or injury. At this point, euthanasia is often considered to ease the animal’s suffering. Yet, undoubtedly, the most important aspect of treatment for EEE is prevention.

Fortunately, the devastating effects of EEE can be prevented by vaccinating. If your horse has not previously been vaccinated against EEE, they’ll require a two shots series of vaccinations with a three to six week interval between doses. If your horse has been vaccinated for the disease before, they’ll likely only need to be vaccinated annually. As with all vaccines, timing is key but the proper time to vaccinate for EEE is region-dependent. In areas that have mosquitos year-round, many veterinarians will recommend vaccinating horses at four to six month intervals. In areas where mosquitos aren’t active year-round, an annual vaccine in the spring is enough to protect your horse.

While Eastern Equine Encephalomyelitis is an often-fatal neurodegenerative disease, it can be effectively prevented by vaccination. Although some may try to prevent the disease by limiting the horse’s exposure to mosquitos, this is a much less effective technique and the vaccine is the only tried and true method to prevent EEE. This spring, as you compile your list of core vaccines, please consider the devastating effects of EEE and vaccinate against it.

References:

1. Waite, Karen, Dr. “III Nervous System Diseases.” ANS 242: Introduction to Horse Management. Michigan State University, East Lansing. 19 Oct. 2015. Lecture.

2. “Eastern / Western Equine Encephalomyelitis: AAEP.” American Association of Equine Practitioners, 2020, aaep.org/guidelines/vaccination-guidelines/core-vaccination-guidelines/easternwestern-equine-encephalomyelitis.

3. Nadeau, Jenifer, Dr., and Sandra Bushmich, Dr. “Department of Animal Science, University of Connecticut.” Mosquito Borne Diseases: Eastern and Western Equine Encephalomyelitis and West Nile Virus – Prevention Is Just a Vaccine Away! Department of Animal Science, University of Connecticut, n.d. Web. 01 Dec. 2015.

Lisa Skylis is an MSU Alumna with a degree in Animal Sciences and a regular contributor to Saddle Up! Magazine. She is a horse enthusiast and avid supporter of therapeutic riding. Lisa is a professional freelance writer and inquiries can be sent to : skylisli@msu.edu.