West Nile Virus and malaria are well-known mosquito-borne viruses, but there is another dangerous illness that can be deadly to humans and horses — with Michigan being one of five states where outbreaks have occurred in recent years.

Elizabeth Carr, professor and diplomate of the American College of Veterinary Internal Medicine at Michigan State University’s College of Veterinary Medicine answers questions about Eastern equine encephalitis (EEE), while offering suggestions on how to keep horses (and yourself) safe.

What exactly is EEE, and what are the symptoms?

Eastern Equine Encephalitis is a mosquito-borne, viral disease that affects the central nervous system of those it infects. Early in the disease, clinical signs are relatively non-specific, including fever, loss of appetite, depression, malaise and stiffness.

Later in the disease, neurologic signs may develop; for horses, these may include ataxia (unsteady movements), cranial nerve deficits, head pressing, circling and blindness. Affected animals may become hyperexcitable or profoundly depressed and drowsy. 

Do all mosquitoes carry EEE?

The virus persists in asymptomatic birds, with mosquitoes acting as a vector to transmit to other birds or mammals. While many species of mosquito can carry EEE, the virus seems to have a specificity for certain vector species (Culex melanura and certain Aedes spp.). Humans, horses and other species become infected when fed upon by an infected mosquito.

Compared to other mosquito-borne diseases, how dangerous is EEE?

Depending on the virulence of the virus and the immune system of the horse, infected horses may be asymptomatic, develop mild signs of fever, loss of appetite and depression or progress to having neurologic signs. The mortality rate for horses that develop neurologic deficits is relatively high, with 75% to 100% dying or being euthanized.

Similarly, humans infected with EEE may be asymptomatic, show mild signs or develop brain injury. Children and the elderly are more likely to develop clinical signs of EEE and about one-third of individuals who develop neurologic signs will die. Individuals who survive may have residual brain damage. 

Are other animals/humans at risk of contracting EEE? What groups are most susceptible?

Other than birds, horses and humans are the most common species affected by EEE.  Rarely other species are infected, including deer, cattle, pigs, camelids and dogs to name a few.

Michigan received lots of rain in 2021, and standing water is associated with mosquito prevalence. Should we expect spikes in EEE cases?

There does not appear to be a relationship between inches of rainfall and incidence of EEE in Michigan. 

How can we protect ourselves and our animals against EEE?

The number one way to protect your horses, mules and donkeys is to vaccinate against EEE in the spring. The vaccine is very effective in preventing disease and yearly vaccination is recommended. Minimize exposure to mosquitoes by reducing standing water, which is a mosquito breeding ground; using topical insect repellants, protective “fly sheets” and fans to help prevent mosquito feeding on horses; and housing horses indoors during the prime mosquito feeding times of dawn and evening.

Currently, there is no vaccine available for humans. Like for horses, prevention includes using insect repellants, avoiding being outdoors during prime mosquito feeding times and removing standing water from the local environment.