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Over the course of the past couple weeks, there has been multiple accounts of acute and rapidly progressive neurologic disease in horses of Marquette County as well as horses experiencing sudden death. Not all of the horses have been examined by a veterinarian and therefore the cause of death cannot be determined; however, two horses that were presented to our hospital were tested for multiple agents that could cause the severe disease seen. The testing results have come in and both horses are positive for Eastern Equine Encephalitis (EEE).

EEE is an arbovirus that is transmitted by mosquitos. While equid species such as horses and donkeys are the most commonly affected, other species such as humans can contract the virus. The virus is only spread by mosquitoes; therefore, humans cannot contract the disease from their horse nor can horses transmit to each other. It is also uncommon for a mosquito to pick up the virus from a horse and transmit it to another nearby horse due to low virus load in the horse’s bloodstream. Some species of birds can carry the virus asymptomatically for mosquitos to contract from them.

Symptoms of EEE in horses include fever, depression, pressing their head into corners, incoordination, blindness, seizures, and death. EEE has a high mortality rate of 80-90%. Fortunately, there is a vaccine for horses. The vaccine can be administered as a combination vaccine that protects them from other viruses such as Western Equine Encephalitis (WEE), West Nile Virus (WNV) and tetanus. This vaccine is typically only protective for 6-9 months, which means booster vaccinations may be warranted depending on when they were administered during the year. In the Upper Peninsula, transmission of the disease is at higher risk in late summer/early fall. Another preventative measure that can help your horse is insect management. While this does not replace vaccination, it can help significantly reduce their exposure to potentially infectious mosquitoes.

While uncommon, it is possible for humans to contract the virus from mosquitoes and fall ill. The virus is only transmitted by mosquitoes. It cannot be transmitted from human to human. Severe cases of the disease can manifest as fever, headache, chills and vomiting which can progress to seizures and death. Approximately of symptomatic humans will die and those who survive can develop brain damage. There is no specific treatment or vaccinations for humans; therefore, proper prevention of mosquito bites is of the utmost importance. It is important to contact your physician if you are concerned.

If you have any questions or concerns about your horse and their risk of EEE, please contact your veterinarian. Remember that vaccinations and insect management are very important to protect your horse from this deadly disease.

PLEASE read the following additional resources on EEE and WNV include


Dr. Rebecca Vollrath DVM CVA
Copper Country Veterinary Clinic
Houghton MI