Mosquitoes are more than just a nuisance in the backyard, pasture, campground, and park. They also can spread harmful diseases to humans and animals. Many people have heard that mosquitoes play a role in the transmission of heartworm infections, malaria, and Zika virus. In Michigan, two mosquito-borne viral diseases, West Nile Virus and Eastern Equine Encephalitis, pose a threat to both humans and animals, especially horses.

West Nile Virus (WNV) and Eastern Equine Encephalitis (EEE) are viruses that are spread by mosquitoes or other biting insects, known as arboviruses. While both WNV and EEE can be found throughout the United States, EEE is found more often in the eastern part of the country, and WNV is heading farther north in the United States.

Because both diseases are spread by mosquitoes, they are seasonal and most often diagnosed in late summer through early fall. Rainfall and temperature patterns play a critical role in how successful these viruses are in the infection of birds and mosquitoes, and ultimately, animals and humans. Therefore, the number of cases and affected species can vary widely year to year.

For example, in 2017, Michigan had 7 cases of EEE and 15 cases of WNV in horses including the first-ever reported mosquito-transmitted equine diseases in the Upper Peninsula. In 2019, Michigan experienced one of the worst outbreaks of EEE ever documented in the state, with 10 human cases—including 6 deaths—and 50 cases in animals from 20 counties. As of October 2, 2020, two human cases of EEE and 36 animal cases have been confirmed across 15 Michigan counties.

Given this variability, vaccination is the most important prevention strategy for EEE and WNV infection in horses.

Birds are amplifying hosts for both viruses. Certain birds are more vulnerable to WNV than others. In particular, the American crow, jays, and yellow-billed magpies often die from WNV infection, while robins, finches, and sparrows have fewer symptoms and maintain the virus in the wild. Bird “die offs” have been associated with WNV when levels of circulating virus are high. Here in Michigan, in addition to crows, blue jays, and ravens, WNV has been detected in 28 species of birds, and is primarily transmitted by a species of mosquito called the Culex sp, which likes to breed in collections of stagnant water.

EEE virus may be detected in multiple species of birds in the Eastern United States. Outbreaks and deaths from EEE have been seen in pheasants, domestic turkeys, ostriches, emus, rock doves, house sparrows, and penguins. In addition to birds, rodents and snakes also may serve as reservoirs for EEE. EEE is transmitted by various types of mosquitoes, in particular Cutiseta melanura, which prefers to feed on birds.

For both WNV and EEE, the virus is amplified in infected birds. Female mosquitoes pick up the virus by feeding on infected birds. The virus lives in the mosquitoes and is transferred to a variety of incidental hosts including humans, horses, and other mammals, such as cats, dogs, livestock, and wildlife.

Vaccination is the most important prevention strategy for EEE and WNV infection in horses. There are no vaccines approved for use in humans. Horse owners and the public can protect themselves and animals by applying mosquito repellent, eliminating standing water, and bringing animals indoors from early evening until after sunrise when mosquitoes are most active.

Information on the prevention of arbovirus infections in humans, signs and symptoms, treatment, and the current number of detections in Michigan is available on Michigan’s Emerging Diseases website, which is managed by MDHHS. Additional information is also available through the Centers for Disease Control and Prevention.

The Michigan State University Veterinary Diagnostic Laboratory offers tests for both WNV and EEE. Samples from sick or dead domestic or exotic animals can be submitted by veterinarians. More details on testing options and other information for the veterinary community are available in the Summer 2018 issue of the laboratory’s newsletter, Diagnostic News, and in the Laboratory’s test catalog.

The Laboratory also performs testing for WNV and EEE for Michigan wildlife, including birds, for the Michigan Department of Natural Resources. Residents are encouraged to report sick or dead wildlife to the Michigan Department of Natural Resources.

Originally published October 9, 2018. Reviewed and updated October 6, 2020.