Paul A. Smith, Milwaukee Journal Sentinel Published 5:10 p.m. CT May 2, 2018
Scientists detected the prion that causes chronic wasting disease in soil and water at mineral lick sites in south-central Wisconsin, according to work published Wednesday by University of Wisconsin researchers.
It’s the first such finding in environmental samples taken from spots where deer gather.
Although the results were not surprising in an area known to harbor CWD, they document a breakthrough in analytical methodology to detect and monitor the presence of the prion and have significant disease management implications for wildlife and agriculture officials.
“Detection of prions in environmental reservoirs represents an important first step in understanding environmental transmission of CWD as well as the potential for cross-species transmission,” said Joel Pedersen, lead author of the study.
Pedersen is professor of Soil Science, Chemistry, and Civil and Environmental Engineering at Wisconsin and also works in the school’s Molecular and Environmental Toxicology Center.
The research, titled, “Mineral licks as environmental reservoirs of chronic wasting disease prions,” was published in PLOS ONE, a peer-reviewed, on-line scientific journal.
The scientists used a technique called protein misfolding cyclic amplification (PMCA) to test for the presence of CWD prions in soil and water taken from mineral licks in south-central Wisconsin.
Eleven lick sites – 10 human-made and one natural – were sampled from 2012-’15. At that time, CWD prevalence ranged from 6 to 19% in adult wild deer in the study area.
Subsequent testing found the CWD prion at nine of 11 sites.
In addition, the PMCA test detected the CWD prion in deer feces collected in the study area.
The methodology had previously been used to detect prions in water, saliva, blood and other types of tissue samples. But the UW researchers work was the first time PMCA was used to test for CWD in soil.
Importantly, no false positives were produced in any of the negative control samples, the researchers said.
The results suggest contamination of mineral licks in the CWD outbreak zone is widespread, Pedersen said, and that the sites may serve as reservoirs of CWD prions that contribute to disease transmission to susceptible animals.
The licks were naturally contaminated with prions and used by free-ranging deer, livestock, and non-cervid wildlife species, according to the researchers.
Such licks in Wisconsin are typically depressions created by hunters and farmers to draw animals to a site and add minerals to the animals’ diets.
The spots are known to congregate animals and have been suspected to increase CWD transmission.
In a 2007 publication, U.S. Department of Agriculture employees used trail cameras to document animal visits to mineral licks in Rocky Mountain National Park in Colorado. The images often showed multiple elk and deer within the same lick at the same time.
Currently, little is known about the relative importance of direct contact and environmental routes of CWD transmission in free-ranging cervids.
It is hypothesized that saliva deposited by a CWD-positive animal in a mineral lick is the most likely mode of disease transmission at the site.
Frequent visitation by infected cervids could allow mineral licks to become potential “hot spots” for indirect transmission of CWD, Pedersen said.
Potentially compounding the risks posed by such sites, certain substrates such as clay soil have been shown to increase the infectivity of prions.
The UW research represents an important new “tool in the tool box” for wildlife, animal health and agriculture officials, said Tom Hauge, retired director of wildlife management for the Wisconsin Department of Natural Resources.
“We’ve known facilities with CWD-positive animals remain infective for years even after the original deer have been removed,” Hauge said. “So there’s been the question of when a facility is ‘cleaned up.’ This work could help answer that question.”
In addition, Hauge said the work helps validate “best practices” such as bans on deer baiting and feeding to reduce the spread of CWD in wild herds.
“Having an analytical technique to detect the presence of CWD prions is valuable to establish an environmental vector as well as support steps agencies should be taking to address the disease,” Hauge said.
The current method does not quantify the concentration of CWD prions; rather, it indicates absence or presence.
Pedersen said future research should be directed at quantifying CWD prion concentrations at mineral licks and other areas where cervids congregate and determining the persistence of prion infectivity at the sites.
“Longer term, we also hope to better understand how direct and indirect sources of transmission play into CWD outbreaks,” Pedersen said. “We’re hopeful our recently published work will help advance work in this area.”