Jared Jansen remembers traveling into the field to fix fence on his father’s ranch southwest of Lavina and coming across 20 to 30 dead deer spread out in a coulee.
That was a decade ago. Since then, he said, he and his father, Mike, have seen up to 100 dead deer at a time along the Musselshell River.
Disease — likely bluetongue or its near-cousin, epizootic hemorrhagic disease — had killed the deer. Death is usually quick, within about three days of the animal exhibiting signs of infection. The continual die-offs have whittled the once hardy deer herds down to a handful of survivors.
“You have to really hunt to find a whitetail now,” said Jansen, 26. “I’ve only seen three does this year.
“I used to see a good 40 to 50 head of whitetail over here,” south of the Musselshell River, he added. “It used to be when I was haying along the river, early in the morning, I’d see 200 to 500 head in the meadows.”
The names sound like something out of a science fiction thriller: epizootic hemorrhagic disease, sylvatic plague, bluetongue, brucellosis, chytrid, chronic wasting disease, West Nile virus. Yet the all-too-real afflictions threaten to reduce the populations of wild mammals, birds and reptiles across Montana, Wyoming and other regions of the United States if left unchecked.
“There is a general consensus among scientists that we are seeing more disease,” said Jonathan Sleeman, director of the U.S. Geological Survey’s National Wildlife Health Center in Madison, Wis.
“If you look at the history of wildlife management agencies in general, they are paying more attention to wildlife diseases and realizing how important they can be,” said Jennifer Ramsey, a Fish, Wildlife and Parks veterinarian in Bozeman.
The consequences of such losses extend beyond the immediate concerns of hunters and wildlife watchers who may find fewer animals on the landscape.
The loss or decline of some species could mean decreased income for hunting retailers and for ranchers who could lose access to federal grazing lands if sage grouse numbers drop and protection of their remaining habitat requires the exclusion of cattle. Some wildlife diseases, such as West Nile virus, can also sicken and kill people who are infected.
“The ecological repercussion of the loss of species, and the ecosystem services they provide, are huge,” Sleeman said. “One study estimated that bats alone provide $2 billion to $50 billion to the agricultural economy in pest control.”
If bat numbers decline substantially, farmers may have to shoulder more of that cost by increasing the use of pesticides, Sleeman said. And that could bring unknown environmental costs.
In recent years, outbreaks of some diseases have increased in frequency and spread to regions previously unaffected. Areas that had seen infections are experiencing more outbreaks and higher mortality.
An example is the rash of epizootic hemorrhagic disease this fall in northwestern Montana, where an estimated 400 white-tailed deer died.
“That was the first case of EHD documented west of the (Continental) Divide,” Ramsey said. “We’ve had it in Eastern Montana year after year after year.”
So what was different about this summer that allowed the disease to migrate farther west?
Sleeman said the biting midge that spreads EHD thrives in warmer weather and drought, which the Missoula area was experiencing this summer. Climate change could be responsible.
“There’s an increase in hemorrhagic diseases where we haven’t seen them before, in the northern latitudes where it’s been too cool for the midge to survive, and the outbreaks that do occur are of greater severity,” he said. “We actually did a study of hemorrhagic disease in Virginia and found a close correlation between outbreaks, temperature and precipitation.”
Warmer winters and summers help the midges in several ways. There is a higher incidence of larvae survival. The midges like to breed in mudflats left when water recedes from ponds and lakes in dry periods. The midges are more active when it is warmer. And it seems that the virus replicates to a greater degree in warm weather, meaning that when a midge bites an animal, it gets a stronger dose of the virus.
EHD typically ends when there’s a hard frost that kills the midges.
“A lot of times, the whitetail deer population bounces back pretty well, so there’s no big push to control the vectors of the disease” by spraying pesticides, Ramsey said.
Under the radar
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Ramsey noted that while there are Montana counties that perform pest control to reduce the numbers of insects like mosquitoes, which can spread West Nile virus, most of the spraying is directed to preserving human or livestock health, not wildlife.
“There’s never really been any discussion between those folks and us to spray for this specific insect,” she said, but she didn’t rule it out if an outbreak was severe enough.
In Wyoming, developers of some coalbed methane wells are required to spray their ponds that hold discharged water to control mosquitoes. One study showed that sage grouse, a threatened species, were dying at higher rates from West Nile virus near such ponds. Mosquitoes spread the disease.
The development of coalbed methane wells also involved other factors that could be reducing the number of sage grouse, including more vehicle traffic and the construction of power lines on which hawks could sit to search out sage grouse as prey.
Such factors point to another, multipronged assault on wildlife populations — human beings.
“The drivers of an increase in wildlife diseases is mostly related to human development, encroachment, the spilling over of disease because wildlife are more concentrated,” Sleeman said. “That’s why we think we’re seeing more outbreaks of avian cholera. There’s also the increasing movement of people across borders. A lot of the disease threats have international origins.”
West Nile virus and white nose syndrome are both believed to have traveled to the United States from Europe. White nose syndrome is blamed in the deaths of 5.7 million U.S. bats and has led to the closure of many caves on public lands to protect the animals. It's believed that cave explorers spread the disease with unwashed boots.
So far, scientists have found only a few ways to combat the spread of wildlife diseases. In preventing the spread of brucellosis from elk to cattle, wildlife managers have been focusing on keeping the animals separated in the spring when cow elk give birth to calves. The disease can be spread through infected birth tissues. Scientists are studying the use of birth control in bison to prevent the spread of brucellosis.
In the case of chronic wasting disease, entire herds of infected deer have been killed to try to stop its spread, which is believed to come from contact between infected animals or from ground infected by diseased animals.
Sleeman said that a study in Wisconsin showed that controlling the spread of CWD worked better if animals on the outlying edges of the outbreak were killed instead of those at the heart of the infection.
“It’s like a controlled burn on the edges of a fire,” he said.
One of the most promising techniques recently developed is an orally administered vaccine against sylvatic plague, which infects prairie dogs that are the main food source for endangered black-footed ferrets. Until a colony of ferrets was discovered in Wyoming in 1981, they were believed to be extinct.
In the past, researchers have dusted prairie dog colonies to kill the fleas that spread the disease. The dusting was time-consuming and not always effective. But Sleeman said that the development of a vaccine that can be given orally to prairie dogs on a bait is showing promise.
“This is a technique that should be looked at in other areas,” he said. “It’s shown great promise in Western states.”
Warriors in the fight
With so many diseases afflicting such a wide variety of animals, you would think that field workers like FWP’s Ramsey, as well as lab researchers like Sleeman, would feel overwhelmed.
“I’m an optimist,” Sleeman said. “We do have several tools to manage these issues.”
He noted that scientists now have more detailed models of diseases, enabling them to exploit weak points in thhe life cycle with treatments. There is also work on diagnostic tools to enable earlier detection of pathogens, as well as ways to test live animals for disease rather than waiting to necropsy dead ones.
But compared with support for researchers studying human and domestic livestock diseases, funding for state and national agencies involved in understanding and controlling wildlife diseases is substantially smaller. Conservation groups, realizing the threats to species they value, have stepped in to help out.
For Montana, concern about the decline of moose is one of the most recent focuses for state researchers. A study is being conducted in northwestern Montana to examine the possible causes to see if moose numbers can be revived.
“I think everyone is a little concerned about moose,” Ramsey said. “We know there are these parasites, but we don’t know if it’s playing a huge role” in their decline.
She noted, however, that even if the moose’s decline in Montana can be narrowed down to a particular health issue — such as parasites — treating a moose, or any wildlife, is difficult even if a cure is known and available.
“We can’t just round them up in a corral and vaccinate them,” she said.