DULUTH — While biologists were busy studying the impact of West Nile Virus on ruffed grouse, they discovered yet another another deadly disease in the upland bird — eastern equine encephalitis.

The Minnesota Department of Natural Resources reported Monday, Nov. 18, that the virus was confirmed in three grouse shot by hunters in Itasca County, the first time the virus has been confirmed to cause illness in any Minnesota wild animal.

The disease, like West Nile, is carried by mosquitoes.

The hunters who harvested the grouse brought them to DNR staff in late October after they noticed abnormal behavior in the birds — they didn’t or couldn’t fly away. When field dressing the birds, the hunters also noticed reduced muscle mass.

The grouse were sent to the University of Minnesota’s Veterinary Diagnostic Laboratory, which also found one grouse had swelling of the brain.

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“It’s too soon to say how widespread the (eastern equine encephalitis) virus might be in grouse populations,’’ said Michelle Carstensen, Minnesota DNR wildlife health program leader, in a statement.

The disease is generally rare in humans. People bitten by infected mosquitoes seldom develop any symptoms. But the virus can be serious if they do. Symptoms of illness may include a sudden onset of fever, chills and muscle or joint aches. Cases with severe illness may begin with fever, headache and vomiting that may progress into disorientation, seizures and coma.

The Minnesota Department of Health warns that "eastern equine encephalitis is the most severe mosquito-borne disease in the United States. Approximately one in three persons who develop severe illness die. Most of those who survive will have permanent neurological damage." Thirteen people have died from the disease this year in the U.S. The department recommends avoiding mosquito exposure when outdoors.

DNR experts said care should be used when processing the animal to avoid cuts that could cause potential infection. Any game that appears abnormal — either in the field or after dressing — should not be consumed. Hunters with questions about what they harvest can contact a nearby DNR area wildlife office.

Dr. Arno Wuenschmann of the University’s lab said he at first suspected that West Nile virus caused the encephalitis. But molecular tests conducted on the grouse in collaboration with the Animal Health Diagnostic Center at Cornell University proved the eastern equine encephalitis virus was to blame.

The virus is typically found in the eastern United States and along the Gulf Coast, but also has been found in other states, including Michigan and Wisconsin. Until now, the DNR had confirmed that wolves and moose in Northeastern Minnesota had been exposed to the virus but never found animals of either species actually sick with the disease.

In 2018 the DNR began asking hunters to submit grouse samples for West Nile virus testing. Samples collected the first year showed 12% of the birds had been exposed to West Nile virus but none had been exposed to encephalitis.

The virus primarily moves between small songbirds and a specific species of mosquito in freshwater hardwood wetlands. Birds in this cycle usually remain healthy despite being bitten and infected.

Illness caused by the equine encephalitis virus in humans, horses, dogs and other types of birds is rare because the mosquito that transmits the virus feeds almost exclusively on little birds. But if a mammalian host or other type of bird is bitten and encephalitis develops, the fatality rate can be high.

No human cases of eastern equine encephalitis have been reported in Minnesota but the Board of Animal Health reported that the virus was found in a horse this year and three horses in 2001. Most of the horse cases occurred near tamarack bogs or hardwood swamps in northern and eastern Minnesota.