Two UW-Madison researchers believe that their discovery of protein agents responsible for causing chronic wasting disease near mineral licks in Dane and Iowa counties strengthens longstanding theories that gathering spots for deer are hot spots for transmission of the disease.
One of the researchers says people should stop buying mineral, or salt, licks to attract or bait deer because they may be helping to spread the fatal brain disease.
“We don’t want to encourage people to do a lot of those kinds of things. But at this point we can’t quantify what the risk is. That’s the problem,” said Mike Samuel, an emeritus professor of wildlife ecology. He co-authored the study with Joel Pedersen, a soil science professor.
Since its initial discovery in south-central Wisconsin 16 years ago, CWD has spread to 25 counties in the state, according to the Department of Natural Resources. About 6 percent of deer tested last year around the state were infected with the disease, DNR data show.
While scientists have proven that the disease is spread from deer to deer, they have made small inroads trying to prove that it can be passed environmentally. A 2009 study of diseased deer that found disease-causing prions in highly contaminated captive enclosures in Colorado led the UW-Madison researchers to look for the prions in open areas where deer are most likely to get infected, Samuel said.
The UW study showed that detectable levels of the prions were found in soil and water around nine of the 11 mineral licks in a CWD hot zone in Iowa and Dane counties. Nearby, about 50 percent of the bucks have the disease, Samuel said.
Cameras placed at the sites showed the deer eating the soil near the mineral licks, 10 of which were artificial ones used by farmers for livestock.
Deer also were seen urinating and defecating in those areas, too, according to Samuel. “So the prions are getting into the soil, binding to the soil and then they’re being consumed by other deer,” he said.
They also found prions in the water when they conducted tests after a rainstorm, Samuel said.
But the low level of prions detected led to questions of whether the difficulty of separating them from the soil gave them accurate readings, according to Samuel. He said that could explain why they found no prions at two sites.
Also, scientists haven’t proven how many prions are needed to make a deer sick.
“So even if we knew how (many prions were) there, it might be difficult to say that that’s enough to cause an infection, or how many infections, at this point,” Samuel said.
At the very least, Samuel said the study shows that more research is needed on areas where deer congregate.
It also spotlights a dangerous disease that some have tried to sweep under the rug, he said. While no cases of human transmission have been reported, the World Health Organization has recommended that people avoid eating infected meat.
Without an aggressive campaign to slow the spread of the disease, Samuel believes as many as 50 percent of the state’s adult male deer population and 25 to 30 percent of the female population will be killed by the disease before it finally slows down on its own.
Last year, 600 of the 9,882 deer that were analyzed in the state tested positive for the disease — an infection rate of 6.1 percent, according to the DNR. The infection rate was 7.3 percent in 2016 and 9.4 percent in 2015 — the highest since the disease was discovered in the state, DNR data show. The testing period runs April 1 through March 31.
Three of 50 deer tested since April 1 this year have tested positive, according to the DNR.
Don Waller, a UW-Madison professor of botany and environmental studies, said the study suggests that, “we should be looking for hot-spots of CWD prions in the environment and doing all we can to cover them up so animals can’t get to them. We may also want to do more testing in other animal species to see which may be vulnerable to CWD infection.”
Waller, who researches Wisconsin’s deer herds, was not involved in the study.
The study was published in the journal PLOS ONE on Tuesday. One day later, Gov. Scott Walker said he wanted state agencies to get more aggressive in slowing the spread of the disease. He wants regulations developed that would require enhanced deer farm fencing and limited movement of dead deer from CWD- affected counties.
Walker, who is running for a third term, has been criticized by Democrats for not doing enough to stop the spread of CWD.
Samuel criticized the DNR for not keeping residents informed about the science surrounding CWD. “I mean there’s so much misinformation out there about what we know about the disease and what we might try to do about it,” he said. “In my view, the DNR neglected their responsibility to keep the public informed of what the science is telling us.”
DNR spokesman Jim Dick said nobody at the agency was available to comment Thursday evening.
The Associated Press contributed to this report.