Within the past week, wildlife officials released the results of a study on the health of Minnesota’s most popular upland game bird, the ruffed grouse. The study concluded that a noteworthy share of ruffed grouse in Minnesota, Michigan and Wisconsin, have been exposed to West Nile virus. The fact that the birds tested had survived and were harvested by hunters is thought to be a positive sign of resistance to the disease, which has caused considerable mortality in some birds—most notably crows and jays.
Scientists say there’s been no evidence that handling birds infected with West Nile virus can lead to the disease to humans. Human infections have been traced solely to the bite of infected mosquitoes. The disease—named for the region in Africa where it was first isolated in 1937—is neurological in nature. Birds are the main host of the virus, as well as its most frequent victims. Infected birds in turn provide an opportunity for other mosquitoes to spread it further.
The most common symptoms in humans are fever and headache, abdominal pain and vomiting; less common is the heart condition known as angina. Most who contract West Nile virus recover fully, but in rare instances those who are infected have died.
Because of their mobility, birds are the ideal vector to spread West Nile virus, which was first identified in the United States in 1999 in New York City. Within three years it was found in 44 states and five Canadian provinces, including the West Coast.
Biologists have been looking for an explanation for a downward trend in ruffed grouse abundance in Minnesota. It has long been almost an article of faith that there is a roughly ten-year cycle of grouse abundance, with alternating peaks and valleys. Explanations for this cycle have ranged from fluctuating availability of important winter food sources, to predation, to sunspots!
Importantly, the peaks of abundance in recent cycles have seemed not as high as during the heyday grouse years of a quarter century and more ago. Adding to the puzzle, there have been years when biologists’ spring population census—the “drumming counts” of breeding males—predicted grouse abundance that for unknown reasons was not borne out in fall hunter harvest.
One possible explanation is that Minnesota grouse may be the victim of mortality between spring and fall. West Nile virus, known to be fatal to some birds, was considered a potential answer. This was supported by a study conducted by the Pennsylvania Game Commission in cooperation with Colorado State University. The Commission has boldly stated that West Nile virus “has been killing Pennsylvania grouse since the early 2000s.”
This conclusion prompted wildlife biologists in the Great Lakes states of Michigan, Wisconsin and Minnesota—which have the highest populations of ruffed grouse in the country—to begin a study in 2018. They asked hunters to gather samples to be tested for the presence of West Nile virus. Instructions and supplies were provided for collecting feathers to determine age and sex, and blood samples and the heart from a just-harvested grouse to be tested for the presence of the virus.
The results now made public found that 29% of birds in the Wisconsin sampling carried West Nile virus, as did 12.5% (34 out of 273) of Minnesota grouse tested, and 13% of Michigan grouse. The evidence is found in antibodies in their blood, revealed in the laboratory. When a human—or a bird—is exposed to an invading organism like influenza or West Nile virus, the immune system creates an “antibody,” an infection fighter that hopefully will help it survive the attack. If it survives, a small amount of that truly unique antibody will remain in the creature—grouse or man—to give it a head start in defending itself should it ever face the same infectious threat.
Besides testing for antibodies in the blood, researchers also examined the hearts collected by hunters to determine whether the virus had reached that organ. None of the 34 Minnesota grouse tested were believed to have the virus present in their heart, but birds harvested in Wisconsin and Michigan did.
A Minnesota Department of Natural Resources representative cited the fact that both adult and young birds had been exposed to West Nile virus and yet had survived to be harvested by hunters, suggesting that these birds had been able to defeat the infectious invader. She also pointed out, however, that this study could not measure the extent—if any—of deaths among other grouse that were not part of the 2018 fall hunter harvest. In Nature, scavenging and decomposition quickly erase the evidence of all but the largest animals’ deaths.
Also unknown is the extent to which a viral attack like West Nile virus—even if not fatal—might weaken a grouse and contribute to its death by other causes, or affect its reproductive capability. Pennsylvania biologists also concluded in their study that high quality ruffed grouse habitat—which could be measured by adequate year-round food supplies, and good thermal cover in winter—might equip grouse to more successfully resist infectious attacks like West Nile virus.
The study is continuing in 2019 with the cooperation of ruffed grouse hunters who obtain testing kits from DNR area wildlife offices. It’s generally believed that humans could not contract West Nile virus by handling harvested grouse, or by consuming properly cooked meat. It’s not bad advice, however, to wear plastic gloves when dressing or butchering wild game of any kind.