EEE Confirmed in Michigan Horses and Deer

Four cases of Eastern equine encephalitis (EEE) were recently confirmed in two neighboring west Michigan counties, and another in the northeast corner of the Michigan's Lower Peninsula. The disease was also confirmed in three of seven free-ranging white-tailed deer, making Michigan only the second state in the country to detect EEE in cervids. Steve Halstead, DVM, Michigan state veterinarian, suggests that it is not to late in the season for horse owners to take measures to protect horses against this deadly disease, such as making sure their horses have received EEE vaccinations and reducing mosquito populations around the barn that spread the disease.

Three equine cases of EEE were confirmed in Kent County during September and one in Montcalm County on Oct. 1. The deer were found in both counties. All five horses and seven deer died as a result of the disease.

Eastern equine encephalitis is not as common in Michigan as it is in southern states, although we consider it to be endemic and always a threat. "We didn't have any cases reported last year, and in 2003 only three cases had been reported," Halstead said. "Interestingly, two of those three cases were also co-infected with West Nile Virus (WNV)."

Halstead said that 2002 was the biggest year for mosquito-borne viruses in Michigan. Although only one equine case of EEE was confirmed in Michigan in 2002, 341 WNV cases were confirmed. As of Oct. 11, there had been 21 cases of WNV in 14 counties.

Animal-feeding mosquitoes that have fed on birds carrying the disease can transmit EEE and WNV to horses and other animals. Some birds are able to be infected with the disease and never show clinical signs and they can act as a continual encephalitis reservoir. However, the level of WNV or EEE in horses never reaches a high enough level to be contagious to other animals, making horses a dead-end host.

"We believe the number of (encephalitis) cases we see each year is dependent on the immunity of our bird population," Halstead said. "In 2002, a large number of birds contracted the virus (WNV) and then developed immunity to it. This next generation of birds probably doesn't have the immunity toward the disease yet, so we are sporadically seeing equine cases pop up."

Michigan usually sees cases of EEE and WNV surface in late summer and early fall. During the early part of the year, the viruses circulate through bird and mosquito populations and gradually spill over to horses and other animals.

Halstead said this year's weather may have been a contributing factor as well. "It was hot and we had heavy spring rains that left a lot of surface water," Halstead said. "July and August were hot and humid creating perfect incubator-like temperatures for the mosquitoes."

He recommends owners take precautions to prevent exposure to mosquitoes by reducing time spent outside during the hours of dawn and dusk. He added that it is still not too late in the season to vaccinate for the diseases. Additionally, he recommends repellents containing DEET for protection.

Since the state health department relies on local veterinarians to report these diseases, owners should report clinical signs consistent with EEE to their veterinarian immediately.

About the Author

Chad Mendell

Chad Mendell is the former Managing Editor for TheHorse.com .

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