Colorado Parks and Wildlife has detected a rising prevalence of chronic wasting disease, a fatal neurological disease, in multiple herds of deer in and around Eagle County.

Chronic wasting disease is caused by exposure to abnormally shaped proteins called prions that can remain on surfaces for years, as well as spread through social contact. Once infected, the prions impact the immune and nervous systems of the deer, causing acute cognitive and physical decline and always resulting in premature death, typically around two years after infection.

Julie Mao is a wildlife biologist at Colorado Parks and Wildlife, and said that symptoms are clearly recognizable in the late stages of the disease, but even early on can cause deer to be more susceptible to premature death.

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“By the end, you’ll see some animals where they’re moving in circles, uncoordinated, drooling, or they’ll have droopy ears. Even their musculature will change,” Mao said. “Many that are mid-stage can be killed by predation. A predator might be able to pick up that there is something wrong with the animal or it’s slower. They’re more susceptible to roadkill as well.”

Male deer are twice as likely as female deer to contract the disease, and the state of Colorado has required mandatory testing of harvested bucks for CWD on five-year cycles since 2000 to track the spread. In 2020, the most recent sampling year, 14% of the 155 bucks sampled in the Sweetwater Creek area tested positive for CWD, one of the highest rates in the state.

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Colorado Parks and Wildlife organizes sample populations into data analysis units, with the Sweetwater Creek area falling under unit D43. Samples from neighboring unit D7, which is located just northwest of D43, had a 15% rate of CWD among its bucks.

These rates far exceed the state goal of 5% or less bucks in the population exhibiting infection. Studies have shown this is the rate at which the deer population is not significantly impacted by the presence of CWD. Once above it, infection rates begin to rise exponentially, populations begin to stagnate or decline and the risk of spread to neighboring herds increases.

Sampling is typically done once every five years, but the state has made 2022 an additional testing season for a number of herds, including D43 and D7. Starting this fall, every deer tag — bucks and does — requires the hunter to submit the head of the deer to Colorado Parks and Wildlife to have it tested for CWD. Information about which areas require testing can be found at the Colorado Parks and Wildlife website.

Mao said that state interest in tracking the disease has been motivated by the results of the first long-term studies of CWD and its impacts, which have led to increased detection and intervention methods. 

“For a long time it was thought that it wouldn’t affect populations, it wouldn’t affect humans if they consumed it, but I think because there is more recent research on both the animal population side of things as well as potential impacts on other primates … there was sort of a renewed concern about CWD,” Mao said.

CWD has only been shown to affect and transmit between deer, elk and moose, but new studies are looking into potential impacts of exposure in other species, including primates. Thus far, no serious impacts have been detected, but the official recommendation on the Colorado Parks and Wildlife website still recommends that people avoid eating or exposing themselves to infected animals.

Hunters are discouraged to shoot, handle or consume deer that are exhibiting symptoms of CWD.

“Disease in humans resulting from CWD exposure has not been reported to date,” the website reads. “However, there may be a small risk from eating meat from infected animals. Consequently, public health officials recommend that people avoid exposure to CWD-infected animals.”

There is no cure for CWD, and the primary method for reducing the disease is population control. Since bucks are twice as likely to carry the disease, Parks and Wildlife is using hunting license quotas to target the more commonly infected deer: older males.

This year, Colorado Parks and Wildlife distributed just over 2,000 buck licenses. Mao said that there are no plans to increase the quota at this time, but if infection rates continue to climb, reducing the buck population is the only method that has proven effective in eradicating CWD in other herds.

“If we fail to address it then more dire consequences can ensue,” Mao said. “Even if we were going to accept that D43 would have a high prevalence rate and we decided to not do anything about it, that’s going to then continue to spread and will affect other herds. It’s not just an isolated incident.”

The results of the 2022 sample will be released in the spring of 2023. To learn more about chronic wasting disease, visit the Colorado Parks and Wildlife website. For questions about mandatory testing, contact Julie Mao at julie.mao@state.co.us.