Is CWD Continuing to Spread?
Biologists in the Wisconsin Department of Natural Resources have become increasingly worried since 2009 as percentage of sick deer rapidly increased in the state’s 210-square-mile core area for chronic wasting disease.
Test results on deer shot during the 2010 firearms season found nearly 23 percent of the bucks 2.5 years and older tested positive for CWD. Further, the disease rate for does in that age group exceeded 7 percent. Both rates are records.
“No matter how you look at the disease rate, the trend continues to climb,” said Davin Lopez, the Wisconsin DNR’s CWD project leader. “If it follows the pattern that’s been documented in Western states, the rates can climb exponentially once the disease gets rolling.”
Lopez is referring to reports of CWD-infected deer in Wyoming and Colorado. In 1997, Wyoming’s endemic area had an 11 percent infection rate. It hit 15 percent in 1998, slipped to 13 percent in 2000, and then grew 2.6 percent annually until surpassing 35 percent in 2007.
Wyoming’s infected region includes nine “hunt areas,” their version of Wisconsin’s “deer management units.” Hunt area 65 has a 43 percent infection rate.
Colorado’s worst infection is an unhunted herd of mule deer near Boulder in a region called Table Mesa. About 41 percent of its bucks and 20 percent of the does carry CWD. Researchers reported that life expectancy for the herd’s uninfected deer was an additional 5.2 years while for infected deer it was only 1.6 years.
The Table Mesa herd declined about 50 percent the past 20 years. Of the 27 infected deer that died during the 2006-07 study, researchers said 13 succumbed directly from CWD and 11 were killed by mountain lions. “Infected deer were much more likely to be killed by mountain lions than uninfected deer,” the researchers reported.
Mule deer aren’t the only species Wyoming is monitoring. David Edmunds, a graduate student with the University of Wyoming’s department of veterinary sciences, captured, tagged and monitored 175 whitetails on a ranch southwest of Glenrock near Deer Creek between 2003 and 2009. As of late 2009, Edmunds reported finding CWD in 28 percent of the overall sample, including a 34 percent prevalence for does and 22 percent prevalence for bucks.
Within the doe population, about 47 percent of 2-year-olds, 42 percent of 3-year-olds and 46 percent of 4-and-older does carried CWD. In addition, the pregnancy rates were lower and survival rates “significantly lower” for CWD-infected females.
Within the buck population, about 28 percent of 2-year-olds, 50 percent of 3-year-olds and 13 percent of 4-and-older bucks carried CWD. Bucks account for about 80 percent of the overall whitetail harvest by area hunters, which might explain lower CWD-prevalence in males compared to females. Deer hunter harvest might also account for the steep drop in infected 4-and-older bucks, but Edmunds said that’s speculative because of his small sample size (eight). Even so, the deer age structure likely shifted to younger bucks because of the combined effects of CWD and hunting. Overall survival rates for adults show significantly lower survival rates for infected deer.
When plotting the herd’s survival and pregnancy rates in preliminary deer-population models, Edmunds found the deer herd was declining 19 percent annually, which he termed unsustainable. Although this projected decline has not been detected in the field, he notes that Wyoming performs little population modeling on whitetails, and few Wyoming hunters target whitetails.
Success in Illinois?
Those studies might explain why Michael Miller, chief veterinarian for the Colorado Division of Wildlife, no longer believes there’s hope of eradicating CWD. In March 2002 Miller flew to Wisconsin to publicly support the state’s initial “scorched earth” plan. But when he returned to Madison in July 2005 he told the Second International CWD Symposium: “Eradication is a nice idea, but it’s not going to happen. … One thing we have learned the hard way over and over again with wildlife diseases is that prevention is a lot easier than control, and control is about the best you’re going to do in most cases.”
Maybe so, but ongoing hunting and sharpshooting efforts in Illinois might be lowering CWD prevalence in that state’s disease area, which is basically a branch of Wisconsin’s smaller southeastern-centered outbreak. Illinois identified CWD soon after Wisconsin’s 2002 discovery. Its core area is about 7 miles wide, centering on the Boone-Winnebago county line.
Paul Shelton, who directs the Illinois DNR’s CWD effort, said deer hunting pressure is too widely distributed to adequately address the disease. Although deer harvests increased 31 percent from 2002 to 2007, the harvest by hunters stayed flat. Sharpshooters boosted the annual average kill by more than 10 deer per square mile in each area they worked. Shelton reports it took three years of harvests exceeding 25 deer per square mile before aerial surveys detected significantly smaller herds.
He’s not claiming success, but the number of CWD-positive deer dropped from 51 in 2005 to 30 in 2008, even as the state increased its sampling efforts. Overall CWD prevalence rates dropped from 2 percent in 2004 to 1.3 percent in 2008 in the state’s defined-risk area.
“Compared to what is being seen in other states where deer management is lacking, the effects of CWD management in Illinois is a very positive finding,” Shelton told the Third International CWD Symposium in July 2009. “We’re on the right track.”
Shelton’s optimism might play in Peoria, but how about in Plain or Platteville in Wisconsin? It’s fair to say many Wisconsinites have become skeptical of nearly anything they hear on CWD. As Mike Miller said during his Madison visit in 2005: “Public interest and public resolve have waned. This is a much larger fight than a lot of people signed on for.”