Twenty-nine percent of Wisconsin ruffed grouse in a 2018 study had been exposed to West Nile Virus, according to a report released Tuesday by the Department of Natural Resources.

The results represent testing for the virus in 235 grouse submitted last year.

Based on analysis of blood taken from the birds, 68 (or 29%) of the Wisconsin grouse had antibodies to WNV, including 44 birds (19%) listed as "confirmed" and 24 (10%) as "likely."

The results were part of a regional push in 2018 to obtain data on WNV in ruffed grouse, which showed signs of an unexpected and unexplained population decline in late 2017 and 2018.

Michigan and Minnesota also participated in the study. The WNV antibodies were found in 13% (28 of 213) of Michigan grouse and 12% (34 of 273) of Minnesota grouse.

The testing was conducted at the Southeastern Cooperative Wildlife Disease Center in Athens, Georgia.

The study provides a rear-view look at a potential cause for the regional drop in grouse numbers.

However, it does not provide an estimate of how many grouse might have died from the disease.

West Nile Virus, also found in humans and horses among other animals, is known to be fatal to birds.

The disease was historically documented in Africa, Europe, the Middle East and West Asia; it is primarily transmitted through the bite of infected mosquitoes. Most people infected with the virus do not show symptoms.

The disease was documented in 1999 in North America for the first time, in New York, and quickly spread across the continent.

It was first detected in Wisconsin in 2001.

More than 300 bird species are known to be susceptible to the virus; blue jays and American crows have been found to be especially vulnerable.

Many in the hunting and conservation community have been looking for additional information on the impact of the disease on ruffed grouse, a treasured native game bird that thrives in young forest habitats.

The Pennsylvania Game Commission arguably has done the most work on WNV in ruffed grouse, including a study on the virus' impact on wild grouse eggs and chicks.

"WNV clearly kills ruffed grouse and as many as 80% of grouse exposed to the virus are killed outright or might have reduced survival," wrote Lisa Williams, game birds biologist for the commission, in a September 2016 article in Game News.

Researchers in Michigan documented the virus in ruffed grouse for the first time in 2017.

The percentage of Wisconsin ruffed grouse found with WNV antibodies in 2018 appears to be higher than any other such study.

A previous study of grouse killed by hunters in Pennsylvania showed 13% were positive for the antibodies, similar to the 2018 data on Michigan and Wisconsin birds.

The 2018 study was the first of its kind to do more than a cursory look at WNV in ruffed grouse in Wisconsin.

Before 2018, only eight grouse were tested in Wisconsin (seven from 2002-04, one in 2008); all were negative.

State wildlife officials said it was too early to make conclusions from the data.

The study will continue with samples collected in 2019 and 2020 in all three states.

"The ultimate question is: Is there a population level impact?" said Kent Van Horn, Wisconsin DNR bird and habitat manager. " We can't answer that question based on these data."

Van Horn added that while the study will continue, it's already known that high quality habitat helps grouse and other wildlife sustain their populations in response to stressors.

"Our science tells us the best thing we can do for grouse is maintain the health of the young forest," Van Horn said.

Out of concern for the ruffed grouse population, the Natural Resources Board shortened the 2018 and 2019 Wisconsin grouse hunting seasons by about three weeks.

The 173,347 grouse taken last year by hunters was the lowest in the 35-year history of the Wisconsin small game survey.

A DNR survey conducted last year of ruffed grouse hunters found 67% supported a shorter season, Van Horn said.

Among the good news from the study is that nearly one-third of Wisconsin grouse sampled had been exposed to the virus and survived at least until fall.

And another: WNV activity in Wisconsin was high in 2017, with 51 human cases, declined to 33 in 2018 and this year only two (one confirmed and one suspected) have been reported, according to the Wisconsin Department of Health Services.

If infection rates in grouse mirror those seen in humans, the still-unspecified impact of the virus on the Wisconsin grouse population could be substantially lower this year.