Deer and elk in Southwest Colorado have lower rates of chronic wasting disease than other parts of the state, according to results from Colorado Parks and Wildlife’s first five years of mandatory testing during hunting season.
Matt Eckert, CPW’s terrestrial programs supervisor, briefed the Colorado Parks and Wildlife Commission about the agency’s chronic wasting disease findings at the commission’s meeting last week in Sterling. Results from CPW’s first five-year rotation of mandatory testing show that the fatal disease infects deer and elk in Southwest Colorado less than other parts of the state.
CPW has yet to explain the lower infection rates in Southwest Colorado, but environmental factors and geography may limit the spread of the disease to the region and between herds.
“To date, we have lower prevalence than most of the rest of the state,” said Jamin Grigg, senior wildlife biologist for CPW’s Southwest region. “Some of that may be because of environmental conditions that are different. Some of it may also just be that it just hasn't really established here yet.”
Chronic wasting disease is a neurological disease caused by misfolded proteins that infects deer, elk and moose. It is transmitted through direct contact between animals and also the soil after infected animals release the proteins through their feces and urine or after they die. It is always fatal, and the proteins can remain infectious in the soil for years.
“It’s a disease that essentially causes holes in the brain and destroys the nervous system of the animal over time,” Grigg said. “Basically, the animal eventually loses cognitive function.”
From 2017 to 2020, CPW conducted mandatory testing for all deer herds across the state. Hunters who harvested deer were required to bring in their kills so that CPW could remove a lymph node or brain stem from the animals and test them for chronic wasting disease.
In 2021, CPW also tested 14 priority elk herds to update infection rates that were last estimated in the mid-2000s.
The results suggest that chronic wasting disease is more prevalent in deer than elk, and the disease is concentrated in the northeast, northwest and southeast parts of the state for deer and northern half of the state for elk.
Northeast Colorado had the greatest concentration of deer herds with infection rates of more than 20%, which is four times CPW’s targeted infection rate of 5% or less. Northwest Colorado also had herds with infection rates of more than 20%, while the southeastern part of the state had herds with rates between 5% and 20%.
All of the deer herds in Southwest Colorado had infection rates of less than 5%. Chronic wasting disease was not present in the Hermosa deer herd in western La Plata and San Juan counties.
The 14 elk herds CPW tested were concentrated along the western edge of the state and included the San Juan Basin herd, which extends east of Durango to the eastern border of Archuleta County and north to parts of San Juan County.
None of the elk herds in the southwestern corner of Colorado were infected with chronic wasting disease.
Grigg identified Southwest Colorado’s soil and geography as potential explanations for lower rates of chronic wasting disease.
“We don’t know all the details yet, but there is some evidence that environmental conditions can impact prevalence rates,” he said. “There’s at least speculation that different types of soils hold chronic wasting disease prions (proteins) longer in the environment than other soil types. There may be some relationship with moisture and temperature as well.”
A number of studies have begun to conclude that soil plays a role in the transmission of chronic wasting disease.
In a 2006 study, researchers with the University of Wisconsin-Madison found that chronic wasting disease proteins attached tightly to a type of mineral found in clay soils and remained infectious.
Another 2018 study by scientists with the University of Alberta in Canada concluded that soils high in organic matter degraded the proteins and reduced their ability to infect animals.
Chronic wasting disease was first documented in captive deer at a research facility in Fort Collins in the late 1960s. An elk from Rocky Mountain National Park was the first wild animal diagnosed with the disease in 1981.
Since then, chronic wasting disease has spread from northeastern Colorado throughout the state, Grigg said.
“Unfortunately, we first detected it in the southwest part of the state several years ago,” he said. “We still have really low prevalence (and) haven’t even detected it in all of our herds yet, but that may be because it’s just recently been detected down here within the last few years.”
CPW’s annual monitoring allows the agency to document the spread and provides the agency with the data it needs to inform its management of deer and elk.
“Overall, the decision to commit to annual mandatory testing has been resoundingly important to understanding the status of this disease in Colorado,” Eckert said in a news release. “It’s helped us in acquiring and communicating reliable infection rate estimates and laying a foundation to assess herd-specific management actions to combat CWD.”
When infection rates register above 5%, wildlife managers can then make important management decisions. CPW managers can reduce herd densities or change male-female ratios – because males have higher rates of chronic wasting disease – to limit the spread of chronic wasting disease.
Based on the deer monitoring data CPW collected from 2017 to 2020, the agency is implementing management strategies in half of the state’s deer herds, Eckert said in the release.
Last year’s elk testing also helps CPW understand the overlap between deer, elk and chronic wasting disease.
“Not only were we interested in generating reliable estimates of chronic wasting disease infection rates in elk, but we also wanted to analyze relationships of chronic wasting disease infection rates among mule deer and elk harvested in the same areas,” Eckert said. “If management actions prescribed in our most infected deer herds successfully maintain or reduce chronic wasting disease, those same actions taken for deer may also affect chronic wasting disease infection rates in elk.”
There have been no reported cases of chronic wasting disease in humans, but the Centers for Disease Control and Prevention, CPW and the Colorado Department of Public Health and Environment recommend that people avoid eating the meat of infected animals.
While CPW’s first five years of testing have been a positive sign for Southwest Colorado’s deer and elk herds, Grigg said the future of chronic wasting disease in the region is still unsettled and could pose a risk to herds.
“We’re hoping we don’t see a huge increase in prevalence rates, but I don’t think we know yet what we’re going to see,” he said. “It’s a mess. Aside from habitat loss, it’s probably the No. 1 threat facing deer populations right now. It’s a huge issue.”