A Wyoming study attempting to understand the mule deer population decline in the southwestern part of the state has found a new killer of fawns — adenovirus hemorrhagic disease, or AHD.
“AHD is a pretty new discovery,” said Samantha Dwinnell, a researcher for the Wyoming Cooperative Fish and Wildlife Research Unit, during a presentation to the Wyoming Game and Fish Commission last week.
Because the discovery is so new, researchers are uncertain how it is affecting the migratory mule deer populations of the Wyoming Range. But of the 52 fawns collared for the study, there were 22 mortalities. Of those, 36 percent of the deaths were attributed to AHD. Twenty-seven percent of the deaths were caused by predators.
The disease’s presence is not limited to Wyoming.
“There have been some outbreaks that have occurred throughout the West and in Iowa,” Dwinnell said.
Mule deer populations have always cycled between years of high populations and lows. But in recent decades many of those populations have nosedived across the West and have been slow to recover. Theories about why have ranged from loss of habitat to climate change and competition with other species, like elk and whitetail deer.
“We find that most of our does are pregnant with two fawns every year, pretty consistently,” Dwinnell said. “But when we look at fall recruitment we find we are losing about half of the fawns over the summer. So the question we want to address is: ‘What’s killing these fawns?’”
Predators are always the first thing that comes to mind when ungulate populations fall. Wolves were blamed for a decline of elk in the southern Bitterroot Valley, but a Montana Fish, Wildlife and Parks study found that mountain lions were a greater threat.
Wyoming biologists thought black bear predation of fawns may have been to blame for mule deer declines, but only two of the fawns killed by predators were attributed to black bears. Coyotes were bigger killers. Evidence that AHD is present points to another possible cause that may have gone undocumented.
“This disease may be more prevalent than we ever knew,” Dwinnell said. “The mortality rate in the young is about 90 percent and about 20 percent in adults.”
In Montana
Blood samples taken from mule deer in Montana have shown exposure to AHD, but no die-off event has ever been documented, according Jennifer Ramsey, a Fish, Wildlife and Parks veterinarian.
“But we’re definitely keeping our eyes open,” she added.
The public or wardens are often the folks who are first to detect sickly animals, Ramsey said, which should be reported to wildlife officials for testing. Testing an animal that was only recently killed or died is important to detect the cause of infection.
“We do rely heavily on the public and field personnel when they are out and about,” Ramsey said.
Dwinnell pointed out that the Wyoming researchers probably wouldn’t have detected AHD’s presence without its intensive study. Mule deer does were captured, collared and had an intrauterine device inserted in their womb to document birth sites. That allowed the scientists to find newborn fawns and collar them. When the collars emitted a mortality signal, the scientists could arrive quickly to determine the cause of death.
“If you’re not closely following mortality in fawns then you aren’t going to see it,” Dwinnell said.
Symptoms
The disease’s symptoms are similar to more well-known infections in deer and antelope like bluetongue, or pneumonia that infects bighorn sheep. Symptoms may include ulcers and abscesses in the mouth and throat, according to the Oregon Department of Fish and Wildlife’s website. “Acute symptoms include rapid or open mouth breathing, foaming or drooling at the mouth, diarrhea (possibly bloody), weakness and copius amounts of fluid in the body cavity.”
AHD is so virulent that it can kill within three to five days of exposure.
The disease was first found in deer in 1993 in Northern California and was blamed for killing thousands of deer in 17 counties. In 2001 it was discovered in blacktail deer in Oregon. In the summer of 2002 AHD was blamed for the deaths of more than 1,000 Oregon deer.
“Disease by itself generally does not determine mule deer abundance,” according to a report from the Western Association of Fish and Wildlife Agencies’ Mule Deer Working Group. Instead, it often takes advantage of animals stressed by other factors like “nutritional or mineral imbalances.
“Rarely do disease outbreaks cause rapid or large-scale mortality in mule deer; most occur as smaller-scale die-offs or reduce productivity,” the report added.
Managing the outbreak of diseases like AHD are difficult, if not impossible, because of wildlife’s free-roaming nature, the WAFWA report said. “Preventing disease introduction into susceptible populations is the most efficient and cost-effective method of disease management,” it added.
Spread of the disease can come from contact with an infected animal, airborne particles or bodily fluids. Consequently, animals that live in close contact, like deer on a winter range or urban deer, may be more susceptible.
There have been no reported cases of humans contracting the virus or getting sick from eating the meat of an animal exposed to AHD, according to the Oregon DFW.
More research
Dwinnell’s research is part of a larger look at a combination of factors affecting the Wyoming Range’s migratory mule deer population. Another study is looking at habitat on the winter range and how that’s been affected by energy development mule deer tend to avoid the areas unless the forage quality is especially attractive. Migration routes and the importance of spring green-up called “surfing the green wave” is also being examined.
“So we’re really hoping to connect the landscape with the animals’ nutrition and how that affects fawn survival,” Dwinnell said.
Another year of research would be helpful in providing answers to some of the questions raised about fawn survival and exposure to AHD, Dwinnell said. That would take another $150,000, she told the commissioners, who sounded willing to consider funding an extension of the work.
“We’re just scratching the surface with our findings,” she said. “Another year of fawn survival data and more analysis to tease apart all of those factors (would be helpful). We should have something pretty comprehensive in two years.”
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