Mosquito-bourne virus found in area grouse

A mosquito-bourne virus called eastern equine encephalitis has been found recently in a couple of grouse in Itasca County.

In the midst of a rather long and cold firearms deer season I missed a significant story about the northern Minnesota grouse population.

Apparently, a couple of birds harvested in Itasca County this fall tested positive for a mosquito-bourne virus called eastern equine encephalitis (EEE).

“Now that we’ve found the EEE virus in Minnesota grouse, we will continue to monitor grouse populations for signs of the disease,” said Michelle Carstensen, wildlife health program leader for the Department of Natural Resources, said in a news release in November. “It’s too soon to say how widespread the EEE virus might be in grouse populations because we only have one year of grouse sampling results from 2018.”

It is unsettling how many of these diseases are out there and how they are starting to affect wild game that hunters chase and eat. Chronic Wasting Disease in deer is a huge problem in the southern part of the state and could very well spread throughout Minnesota and, worst case scenario, end deer hunting as we know it.

Now, grouse are falling victim to a virus that’s virtually unstoppable unless there is some plan to kill every mosquito in existence.

The DNR basically stumbled upon this discovery while trying to figure out if West Nile Virus, another gift from mosquitos, was behind the recent unpredictable swings of the grouse population in the state.

Historically, the game bird’s numbers have gone up and down over a 10-year population cycle, with peak numbers found before and after the decade mark.

But in recent years, after some high spring drumming counts (the way grouse numbers are determined) led to hunters finding the forest floors and trees mostly devoid of grouse in the fall, wildlife officials started looking for an answer.

Two years ago the DNR teamed up with wildlife officials from Wisconsin and Michigan to figure out if West Nile was killing birds.

Results from the inquiry’s first year (hunters submit birds to the DNR for testing) showed about 12 percent of Minnesota grouse submitted by hunters for testing were positive for exposure to West Nile.

None had been exposed to EEE, however, and none appeared ill when killed by hunters.

The hunters who harvested the grouse in Itasca County and brought them to DNR staff in late October noted that the birds were acting strange - they didn’t or couldn’t fly away.

When field dressing the birds, the hunters also noticed reduced muscle mass.

The DNR submitted samples from the birds to the University of Minnesota’s Veterinary Diagnostic Laboratory (VDL) and tests concluded two and possibly a third were infected with EEE virus.

According to a news release from the DNR, the third grouse – suspected of having the virus - also had inflammation in the brain, providing further evidence that it likely also suffered from EEE. All birds tested negative for West Nile virus.

I wonder how many hunters experienced the same thing this fall and didn’t report it or submit their birds for testing. Grouse behave strangely when stressed so recognizing an odd behavior might not be easy to some people, particularly newer or inexperienced hunters.

“It is rare for us to find EEE in Minnesota, but this year we’ve diagnosed the virus in these grouse and a horse,” said the VDL’s Dr. Arno Wuenschmann. “I initially 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 EEE virus was to blame.”

After two years of tests we aren’t any closer to knowing if grouse actually succumb to West Nile, or instead, once exposed the virus, can fight it off.

But we do know that there is a real possibility that mosquitos are having a potential negative impact on the population – whether strictly through EEE or through some combination of that and West Nile.

How far and wide that impact is remains to be seen.


So what is EEE? According to the U.S. Center for Disease Control, EEE virus is a rare cause of brain infections (encephalitis). Only a few cases are reported in the United States each year. Most occur in eastern or Gulf Coast states.

Department of Natural Resources officials say the virus primarily moves between passerine (perching) birds 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 EEE virus in humans, horses, dogs and other types of birds is rare because the mosquito that transmits the virus feeds almost exclusively on passerine birds. But if a mammalian host or other type of bird is bitten and encephalitis develops, the fatality rate can be high.

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.

No human cases of EEE have been reported in Minnesota.

According to the CDC, Horses are susceptible to Eastern equine encephalitis virus (EEEV) infection and many cases are fatal, but horses are not a significant risk factor for human infection because they (like humans) are considered to be “dead-end” hosts for the virus.

In other words, the amount of virus in the bloodstream is usually insufficient to infect other mosquitoes.

The state Board of Animal Health reported that the virus was found in a horse in 2019 and three horses in 2001.

Most of the horse cases occurred near tamarack bogs or hardwood swamps in northern and eastern Minnesota.

There is a vaccine to prevent EEEV infection in horses but there is no approved human vaccine or specific antiviral treatment for EEEV infections. Patients with suspected EEEV disease should be evaluated by a healthcare provider, appropriate serologic and other diagnostic tests ordered, and supportive treatment provided.

The Star Tribune reported that Wisconsin first documented the equine virus in ruffed grouse in the 1950s. Wildlife officials there believe the virus has been present in the state’s birds at least since then, but that it has not restricted the grouse’s overall population.

In the United States, an average of seven human cases of EEE are reported annually.

From 2009 through 2018, Eastern equine encephalitis virus neuroinvasive disease cases have been reported in Alabama (1), Arkansas (1), Connecticut (1), Florida (13), Georgia (6), Louisiana (2), Maine (2), Maryland (1), Massachusetts (10), Michigan (7), Missouri (1), Montana (1), New Hampshire (3), New Jersey (1), New York (8), North Carolina (7), Pennsylvania (1), Rhode Island (1), Vermont (2), Virginia (1), and Wisconsin (2).

Prior to this discovery, the DNR had confirmed that wolves and moose in northeastern Minnesota had been exposed to the virus but never found animals of either species 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 percent of the birds had been exposed to

“We’ll keep testing samples that hunters submit for both viruses,” Carstensen said. “Hunters who harvest sick grouse also can help us by contacting a nearby DNR area wildlife office so they can submit those samples for testing, too.”

Grouse sampling information can be found on the DNR website.


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