Colorado Parks and Wildlife announced last week that at least two mule deer had died of bluetongue disease in Alamosa.
Bluetongue virus, a hemorrhagic disease that can devastate white-tailed deer populations and also infect mule deer, elk, bighorn sheep, pronghorn and bison, has been particularly severe in the western United States this year.
The disease leaves deer lethargic, disoriented and unresponsive. They can foam at the mouth, bleed from the nose, and their tongues can swell and turn blue. Death comes quickly, often within one or two days, CPW’s news release said.
It’s one of two hemorrhagic diseases, epizootic hemorrhagic disease being the other, typically found in Colorado between the months of August and October.
“Pretty much every year we get some EHD and bluetongue activity,” said Karen Fox, a wildlife pathologist with CPW. “This year’s been really bad.”
“It’s probably the worst one that I’ve seen, and I’ve been around about 10 years,” Fox said.
Fox estimates that statewide wildlife deaths are triple or quadruple a normal year.
With no catastrophic die-offs like those seen in eastern Washington, where there have been more than 500 reports of dead deer this year, Coloradans have yet to notice.
But wildlife managers are warning that Colorado could see the explosion of these diseases with climate change.
It comes down to how hemorrhagic diseases are transmitted.
Flies or midges bite the deer, infecting them. Flies and midges that carry bluetongue and EHD typically appear around standing water.
“There’s definitely an association between water and the disease,” Fox said. “And that’s because the little gnats they develop ... in mud.”
Lakes and ponds can be sources of disease, Fox said, but even a pool of water in the divot of a hoof print can spread bluetongue and EHD.
The close link between hemorrhagic diseases and water make it more common in the southeastern U.S., Fox said. Yet, counterintuitively, it’s actually drought that worsens outbreaks.
“If you look at the literature, there’s more of an association with drought in this disease than with lots of moisture,” Fox said. “That’s because you have the water and then it recedes and it allows you to get more of this muddy habitat for the gnats to proliferate in.”
“What drought does is it forces animals to congregate,” said Erik Hofmeister, a research virologist with the U.S. Geological Survey.
Drought also brings wildlife together, leading to more infected animals, Hofmeister said.
The Southwest’s most severe drought on record made much of Colorado a perfect environment for the spread of both bluetongue and EHD this fall, a prelude of what scientists say will come.
From January 2020 to August 2021, the Southwest U.S. experienced its lowest precipitation total since 1895, according to a report issued by the National Oceanic and Atmospheric Administration on the 2020-21 Southwestern U.S. Drought Monitor. The region also had its third-highest daily average temperatures since 1895.
The report said the warm temperatures that drive drought would only continue to increase as climate change worsens.
But climate change will exacerbate hemorrhagic disease in other ways, too.
According to CPW’s news release, cases of bluetongue and EHD fade when midges die off with the fall’s first frost. But climate change models show the frost-free season will increase across the West. In the Southwest, the lengthening of the frost-free season is projected to increase twice as much as the rest of the country, according to the Third National Climate Assessment released in 2014.
It’s possible that with a longer frost-free season the midges and flies that transmit bluetongue and EHD could survive longer, spreading more disease.
Climate change is also altering the food that deer and other wildlife rely on, making them more susceptible to disease.
“Some of these dry years we aren’t getting the quantity or quality of vegetation that we used to in the past,” said Brad Weinmeister, a wildlife biologist with CPW. “You tie that back into disease and the animals don’t have as good of food to eat. It’s not as available, it’s not as nutritious. So they’re more stressed, which is going to put them at higher risk of disease.”
With the risk that climate change poses, wildlife managers have taken notice.
“People have specifically talked about these diseases being affected by climate change,” Fox said.
However, the challenge is not trying to stop bluetongue or EHD. The challenge is simply to understand the scope of the problem.
CPW and many other federal and state wildlife management agencies use passive disease surveillance. If a person sees a dead or disoriented deer, they reach out to Fox and CPW who then investigate and test the animal to get a diagnosis.
CPW does not go out and actively conduct studies to understand the prevalence of hemorrhagic disease or the effect that they have on deer populations.
“We’re not getting enough samples to make those inferences,” Fox said. “It’s just kind of anecdotal. It’s more subjective.”
With passive surveillance, scientists lose much of the key information they need to manage outbreaks, Hofmeister said. They know neither the timing nor the location of the disease.
“With passive disease surveillance, you really don’t know the breadth of what’s out there,” Hofmeister said.
Wildlife managers are left blind to make key decisions about wildlife populations affected by disease. And unlike humans, disease in wildlife is much harder to manage.
“There’s steps that can be taken, but it’s not like going to the local pharmacy and getting a flu shot to decrease the number of cases,” Weinmeister said. “... You can’t do that with animals.”
Bluetongue and EHD are just two of the diseases that affect deer and other wildlife in Southwest Colorado. With worsening climate change and passive surveillance, wildlife managers face perhaps their toughest task.
Fox said she and others who study wildlife disease are already beginning to see the changes.
“We’re moving from a phase of prediction to a phase of documentation,” Fox said.
“I think that we’ll start seeing a lot more coming out in the next decade here, where people are looking at those predictions and saying, ‘Yeah, we can detect that something has truly changed.’” Fox said. “And I think this is maybe one of the first things where we’ve had enough data over the years to be looking at it and say, ‘Yeah, it’s changing.’”