Mosquitoes are more than just a nuisance in the great outdoors — they can also spread harmful diseases to humans and animals alike. Michigan State University’s Veterinary Diagnostic Laboratory tests for these harmful diseases to help veterinarians and public health officials identify and track cases.

Many people know that mosquitoes transmit heartworm infections, malaria and Zika virus but in Michigan, two mosquito-borne viral diseases — West Nile virus, or WNV, and Eastern equine encephalitis, EEE, — pose a fatal threat to humans and animals, especially horses. While EEE cases primarily occur in the Eastern U.S., WNV is found throughout the United States.

What do you need to know to protect yourself and your animals? The MSU Veterinary Diagnostic Laboratory shares three important pieces to stay safe.

1.     Mosquitoes and associated disease risks are seasonal.

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

In 2019, Michigan experienced one of the worst outbreaks of EEE ever documented in the state, with 10 human cases — including six deaths — and 50 cases in animals from 20 counties. In 2020, four human cases of EEE, including two deaths, and 41 animal cases were confirmed across 19 Michigan counties. While there were only 10 reported cases of WNV in Michigan birds in 2020, there were 32 confirmed human cases, according to data from Michigan Emerging Disease Issues website.

2.     Birds and mosquitoes are part of the transmission cycle.

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 replicates 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 through the mosquito’s bite.

Certain birds are more vulnerable to WNV than others. In particular, the American crow, jays, ravens 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. In Michigan, WNV has been detected in 28 species of birds, and is primarily transmitted by mosquitoes in the Culex genus which breed in rural or urban collections of stagnant water.

EEE virus may be detected in multiple species of birds in the eastern U.S. Outbreaks and deaths attributed to EEE have occurred in pheasants, domestic turkeys, ostriches, emus, rock doves, house sparrows and even penguins. In addition to birds, rodents and snakes can serve as hosts for EEE virus. Various mosquitoes may transmit the virus, but the mosquito Cutiseta melanura, which prefers to feed on birds, is considered the most important vector for EEE.

3.     Vaccinate horses and prevent mosquito bites on yourself and animals.

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

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

When testing for WNV and EEE, veterinarians send samples from sick or dead domestic or exotic animals to the MSU Veterinary Diagnostic Laboratory. In Michigan, veterinarians should contact the Michigan Department of Agriculture and Rural Development to see if testing may be done free of charge*.

The laboratory also performs testing for WNV and EEEV 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.

*More details on testing options and other information for the veterinary community are available in the Summer 2018 of the laboratory’s newsletter, Diagnostic News, and in the laboratory’s test catalog.