Wildlife Society lecturers focus on CWD in state mule deer herds

Victoria O’Brien

Last week, the Wyoming Chapter of the Wildlife Society held its annual conference in Cody, Wyo., inviting members, researchers, and academics to convene for three days of lectures on new and ongoing research projects relating to Wyoming’s wildlife and varied ecosystems. Among those presentations given were a series of talks on Chronic Wasting Disease (CWD), a neurological disorder that affects members of the cervid family (deer, elk, moose, et al.) that is caused by misfolded, or mutated, prions. CWD is incurable and, in all cases, fatal.
On Wednesday, Tucker Russell, a Master’s student at the University of Wyoming, presented preliminary research on the migratory patterns of mule deer in the Wind River Basin.
“Understanding the drivers of infectious diseases is difficult because of complex host behaviors; for example, the movement of infected and susceptible hosts can influence the transmission of pathogens both directly and indirectly,” explained Russell. “[And while] looking at the movement behavior is important, so is looking at their habitat use. In some systems, pathogens may persist for long periods of time and remain infectious in the environment.”
The misfolded prions that cause CWD, which is transmitted through feces, urine, and saliva, can survive independent of a host in soil and at shared grazing sites, feeding stations and mineral licks, exposing more animals to the disease.
The overlap between movement and habitat use of CWD positive and negative mule deer is what Russell is studying in the Wind River Basin, the only area of Wyoming in which more than 50% of its mule deer population has tested positive for CWD.
Studying 105 unique individuals, Russell and his team zeroed in on a group of 12 deer and utilized GPS collar data to separate them into three distinct migratory groups: long-distance migrants (travelled more than 100km), short-distance migrants (travelled fewer than 100km), and resident deer (did not travel). Using samples collected before and after death, Russell found one CWD-positive deer in both the long-distance and short-distance groups, and seven out of eight positive deer in the resident group.
Their preliminary findings showed CWD-positive deer use agricultural lands more than CWD-negative deer, especially in the summer. One individual spent more than 50% of his time in agricultural fields as compared to the negative deer, who preferred wetlands. While the mechanism driving the positive and negative deer in this way remains unclear, the early data from this select group has already begun to reveal detail about the difference in movement behavior and habitat-use between CWD-positive and -negative deer. Russell hopes the research may eventually be applied to identify habitats with high transmission rates so that access to these areas may be limited or groups such as the resident deer may be targeted more directly for harvest, or culling.
On Thursday, Dr. Wynne Moss of the U.S. Geological Survey (USGS) presented preliminary research into “The Effects of Hunting to Control CWD in Wyoming Mule Deer Herds.” Traditionally, infected animals are culled using a hunt or harvest method. Moss noted at the outset of her presentation that “in a recent survey, three-quarters of U.S. and Canadian agencies said they did something to increase hunting opportunities after CWD was detected [in their area].” Similarly, Moss noted another recently published study, which showed there is a relationship between high harvest rates and slower increases of CWD positivity.
Working with Wyoming Game and Fish, Dr. Moss and her team selected herds with over 20 years of CWD-positive deer and varying histories of low and high harvest pressure. The results showed herds that had higher harvest levels had “much, much lower CWD prevalence.” They also applied relative harvest pressure — or turning hunting up or down — to their analysis and found that three years of higher pressure led to lower CWD prevalence in herds thereafter.
Moss’s team ran 12 different models and did not find correlation between licenses sold and males harvested, but did show that the proportion of males harvested correlated to lower CWD prevalence.
“Proportion of males is a nice variable to come out as being so influential because this is what’s actually determining the pressure experienced by a population,” explained Moss. “What this means is that the herd is declining, so you don’t necessarily need to sell more licenses as long as you’re keeping the proportion consistent. That seems to affect prevalence.”
Although there have been no documented human cases of CWD, the CDC and state wildlife departments nationwide regularly issue best practices for hunters. They encourage testing game, wearing disposable latex gloves while field dressing, and keeping special blades for antler and bone cutting. Following use, field knives should be sterilized with a 50% bleach solution, which will kill any prions. Agencies discourage the handling of the brain, spinal, and lymphatic tissues without proper protection.