Springtime = Mosquitoes

Spring has sprung early in many regions of the country. With the cherry blossoms, green grass and tulips also comes April showers and mosquitoes. In 2011, there were 65 cases of eastern equine encephalomyelitis reported in 11 states throughout the northeastern and southeastern regions, as well as mid-western regions such as Wisconsin and Michigan1. There was also one case of western equine encephalomyelitis reported in Arizona in 20111. Is your American Quarter Horse at risk of contracting one of these mosquito-borne diseases?

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The risk of exposure and geographic distribution of eastern equine encephalomyelitis varies from year to year with changes in distribution of insect vectors and reservoirs important to the natural ecology of the virus2.? However, unvaccinated horses may be at serious risk of contracting mosquito-borne illnesses, even in areas where the disease may not be common.? Eastern equine encephalitis is considered to be endemic in all areas of North America by the American Association of Equine Practitioners, and all horses should be vaccinated against it.

Symptoms in horses that have eastern equine encephalomyelitis can be varied but usually begin with fever, depression and listlessness, which then progress to more serious neurological signs such as incoordination, stumbling, circling, head pressing, coma and often death3. Once the horse becomes infected with the eastern equine encephalomyelitis and develops neurological signs, the disease is fatal in roughly 90 percent of cases4. There is no effective treatment, and seizures resulting in death usually occur within 48-72 hours of the horse’s first indication of illness3.

Vaccinations are the most effective way to help protect horses against eastern equine encephalomyelitis and other encephalitic diseases, such as western equine encephalomyelitis and West Nile virus. According to AAEP?s vaccination guidelines, eastern equine encephalomyelitis, western equine encephalomyelitis and West Nile virus are considered core diseases for all horses, along with tetanus and rabies2. For adult horses previously vaccinated against eastern equine encephalomyelitis, annual revaccination should be completed in the spring prior to mosquito season. In animals at high risk or with impaired immunity, more frequent vaccination or appropriately timed vaccination is recommended in order to help induce protective immunity during periods of likely exposure. It is best to consult a veterinarian on your horse’s vaccination program, as timing and frequency may vary depending on age, geographic location, use and whether the horse has been previously vaccinated.

Pfizer Animal Health offers the INNOVATOR? line of vaccines, including the Mosquito Shot ? (WEST NILE-INNOVATOR? + EWT), which helps protect against eastern equine encephalomyelitis, western equine encephalomyelitis, West Nile and tetanus in a single vaccine. Protection from eastern and western equine encephalomyelitis and tetanus is also available in combination with FLUVAC INNOVATOR?. The INNOVATOR vaccines, along with other Pfizer Animal Health equine vaccine combinations, are included in the Equine Immunization Support Guarantee.

The Pfizer Animal Health Equine Immunization Support Guarantee program provides up to $5,000 for reasonable diagnostic and treatment costs if a horse properly vaccinated by a veterinarian contracts the corresponding equine disease. Disease protection backed by the Equine Immunization Support Guarantee includes infection from West Nile virus, equine influenza virus, tetanus, eastern equine encephalomyelitis virus, western equine encephalomyelitis virus and Venezuelan encephalomyelitis virus. Vaccinations must be performed by a licensed veterinarian with an established client-patient relationship to be eligible.

Ensuring that all horses get their spring vaccinations is the best way for horse owners to help protect their horses.


  1. Disease Maps 2011. U.S. Department of the Interior. U.S. Geological Survey. Available at: diseasemaps.usgs.gov/eee_us_veterinary.html. Updated December 14, 2011. Accessed?? March 26, 2012.
  2. Core Vaccination Guidelines. American Association of Equine Practitioners. 2008. Available at: www.aaep.org/core_vaccinations.htm. Accessed March 26, 2012.
  3. Crans WJ. Questions Regarding Eastern Equine Encephalitis and Horse. Rutgers Center for Vector Biology. October 1 1993.? Available at: www.rci.rutgers.edu/~insects/heee.htm. Accessed on March 26, 2012.
  4. Eastern/Western Equine Encephalomyelitis. American Association of Equine Practitioners. 2008. Available at: www.aaep.org/eee_wee.htm. Accessed March 26, 2012.

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