Itasca County encompasses nearly 3,000 square miles. How many ruffed grouse exist within its borders is anyone’s guess — literally. But whatever the number, it’s fair to assume a reasonable percentage have been exposed to Eastern Equine Encephalitis (EEE), a virus that until this fall had never before been detected in a Minnesota wild animal.

In October, hunters in Itasca County found and/or shot three ruffed grouse that didn’t behave, or appear, “normal.’’

One hopped repeatedly when pursued by a dog, instead of flushing and flying away. The other two were shot. But upon inspection they appeared to be in poor health.

All three grouse were submitted to Minnesota Department of Natural Resources (DNR) wildlife officials. Subsequent tests at the University of Minnesota showed two of the birds, and possibly the third, were suffering from EEE, a mosquito-borne virus that most commonly afflicts horses but also can target other species, including people.

Six Michigan residents died this year after contracting the virus.

In Minnesota, DNR wildlife biologists and disease specialists have been on the lookout this fall not for EEE in ruffed grouse but for West Nile virus. Their vigilance follows a disconnect that emerged in the last decade between springtime grouse-population drumming counts and the number of birds hunters see and harvest in the fall.

Historically, drumming counts and fall bird sightings and harvests have correlated as an index that fairly accurately tracks the rise and fall of the grouse’s 10-year population cycle.

The most recent evidence that the link between the spring counts and hunters’ fall ruffed-grouse sightings and harvests is broken occurred in 2017. Hunters that year expected a banner fall season after record- or near-record-high spring drumming counts. Instead hunters had one of their worst seasons.

Hoping to determine whether West Nile virus is killing young Minnesota grouse in summer, the DNR here this fall began the second year of a joint study with Wisconsin and Michigan wildlife officials.

Results from the inquiry’s first year showed about 12 % 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. It remains unknown, therefore, whether grouse actually succumb to West Nile, or instead, once exposed to the virus, can fight it off.

The recent discovery that grouse can become ill, or weakened, after contracting the equine virus raises new questions. Is it possible, for example, that EEE, not West Nile, is responsible for the loss of summertime grouse broods? Or perhaps the two viruses in combination are the culprit?

“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, DNR wildlife health program supervisor. “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.’’

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.

Minnesota’s only known victims of the virus this year, other than the three grouse, were two horses. A vaccine exists against EEE for horses but not for people.

In Michigan, deer, horses, dogs, goats, sheep and zoo animals were stricken this summer with the virus. Ten human cases also were reported.

It’s likely Michigan’s outbreak, and possibly the equine virus’ discovery in sick grouse in Minnesota, is tied to the heavy and persistent rains that prevailed this summer across the Midwest, which likely encouraged mosquito outbreaks.

Yet the types of mosquitoes that typically carry West Nile virus are different from mosquitoes that are “vectors’’ for EEE. This could explain, said Thomas Cooley, a Michigan DNR wildlife pathologist, why his state’s equine virus outbreak was record-breaking this year, yet West Nile cases there were extremely low.

“There are more mosquito species that are vectors for West Nile than for EEE,’’ Carstensen said. “That’s one reason West Nile has been found in every Minnesota county.’’

Future studies of grouse brood survival could be designed to attempt to determine what, if anything, is disproportionately killing ruffed grouse in summertime, Carstensen said. Yet if West Nile or EEE is found to be the culprit, or a combination of the two, it’s unlikely anything could be done about it.

“We’re finding that viruses probably play a role in grouse survival,’’ Carstensen said. “But not every grouse that gets bit by an infected mosquito will die, or even get sick. The larger consideration affecting grouse populations probably will continue to be habitat availability and quality, and weather patterns.’’