Scientists aren’t sure of success in fighting disease in deer, elk
Chronic wasting disease rates appear to be falling in the most infected areas of Colorado and Wisconsin and rising in Wyoming and Nebraska, while the distribution of the disease is spreading in all four states.
But that picture, drawn after two years of CWD surveys in those states where the disease of deer and elk is well established, is clouded by the fact that infection rates are rising in certain pockets even when declining across broader areas, making it hard for wildlife biologists to draw any firm conclusions.
In Colorado, for example, the rate of infection in the disease’s “established area” in the state’s northeastern corner has fallen from 5.2 percent in 2002 to 4.7 percent in 2003, according to tests on thousands of animals killed by hunters, government biologists or vehicles.
But that region also includes smaller study areas where infection rates have climbed over the years. In one pocket of western Larimer County, for instance, the rate of infection climbed from 4 percent to 7 percent since initial surveys dating to 1997.
The conflicting data give little indication whether CWD has retreated in the face of strong – and controversial – measures to combat it, including special hunts, issuing more hunting licenses, extended hunting seasons and shooting campaigns by government biologists.
The idea behind those tactics is to cut deer and elk populations to limit the spread of the disease and curtail infection rates. Two trends suggest the approach might be working, even as biologists who favor culling the herds say it’s too soon to know whether their approach is right.
- In Colorado, where wildlife managers have worked to thin so-called hot spots of infection and given hunters more opportunity to kill deer and elk, infection rates are down in the established area. But in southeastern Wyoming, where wildlife managers have taken a hands-off approach to the disease, infection rates appear to be up as much as 50 percent, although that is only a rough estimate because Wyoming is still compiling numbers.
- In Wisconsin, where biologists sought to completely eliminate deer and the disease from a 411-square mile region, they failed to do either, but rates of the infection dropped slightly – from 1.6 percent to 0.9 percent from 2002 to 2003.
Even so, in the Nebraska panhandle, where the disease first turned up in captive deer in 1997, the infection rate rose despite the fact the state’s Game and Parks Commission has loosened hunting limits dramatically in an effort to stop its spread.
Officials in New Mexico, Utah, South Dakota and Illinois, the other states known to have CWD in the wild, also are tracking the disease’s spread, though most of that data isn’t as comprehensive as that collected in the other states.
Colorado wildlife officials are hesitant to draw any conclusions from the infection rate data, saying they need many more years of data to understand how the disease behaves in the face of culling deer and elk populations. Officials in other states echo that sentiment.
“In no way are we suggesting that we have data to support that our management is definitely resulting in a decline in (CWD) prevalence,” said Todd Malmsbury, spokesman for the Colorado Division of Wildlife. “We hope that’s happening; it’s certainly our goal. But we are not saying that.”
Wisconsin Department of Natural Resources spokesman Bob Manwell said, “Everyone agrees on our end that it’s going to be five, six or seven years out before we can judge the true effect of the (eradication) policy.”
If anything, Colorado officials are increasingly concerned about some of the numbers, particularly pockets of high infection, where samples of small herds show rates of infection rates from 20 to 50 percent. Those sample sets consist of only about a dozen deer, however, and might be too small to have any statistical significance.
Even so, rising infection rates in those pockets and some wider areas worry division officials, since they’ve targeted some of those herds for culling in hopes infection rates will drop.
States coordinate strategies
Colorado scientists first identified symptoms of CWD in the 1960s in research pens near Fort Collins. The first known case in the wild was documented in 1981, when elk were found with the disease in Rocky Mountain National Park.
But it wasn’t until the late 1990s that Colorado and Wyoming started tracking infection rates. And it wasn’t until 2002 that they, along with Nebraska and Wisconsin, began systematically testing thousands of animals shot by hunters in an effort to get better data on its distribution.
That was the same year the states – with the exception of Wyoming – started loosening hunting limits and employing government sharpshooters to cut deer and elk populations. Wyoming agreed to take a hands-off approach, acting as a control to compare how the disease behaved when left alone versus how it fared against more combative strategies.
The generally aggressive stance among some states can be traced to the link between CWD in deer and elk and mad cow disease, a related illness that affected hundreds of thousands of cattle in Great Britain in the 1980s and, as of Dec. 1, has since killed 153 humans in Britain and elsewhere who’ve consumed infected beef.
While there’s no known case of CWD infecting a human, it – like mad cow disease – is a transmissible spongiform encephalopathy, a brain-wasting illness that eventually kills its hosts. Fears that CWD, like mad cow, could possibly affect people who eat infected meat led to concerns that local hunting economies would be devastated by worried sportsmen going elsewhere.
Wildlife managers also fear the disease’s impact on animals. Computer models developed by the Colorado Division of Wildlife and academics at the University of Wisconsin suggested the disease could eventually wipe out deer and elk herds if left to spread unchecked.
But critics, particularly in Colorado and Wisconsin, complain that wildlife managers are advocating the killing of dozens, even hundreds, of healthy animals to get to the small handful that actually carry the brain-wasting illness.
And researchers in Illinois recently issued a paper critical of the computer model Colorado has used to justify culling deer to manage CWD, saying one of its fundamental assumptions is flawed.
But wildlife managers in Colorado, Wisconsin and Nebraska, the states with the most aggressive policy to curb CWD, argue that the strategy is an important way to slow down the disease and eventually lower infection rates.
“We believe that by removing these hot spots of infection, we remove the likelihood that the hot spot will serve as a reservoir for the disease to move outward,” Malmsbury said. “We still believe that left unchecked, CWD will slowly spread.”
Disease creeps south
While it’s not clear if Colorado’s approach is working, it does appear the disease is spreading geographically – even if the rate of infection is declining.
In Colorado, the disease was discovered in four new game management units last year. While the spread didn’t represent dramatic leaps to new regions of the state, it suggests gradual movement.
“None of these are surprises because we’ve already found CWD in animals in adjacent (game management) units,” said DOW veterinarian Mike Miller, one of the country’s top CWD researchers, in a statement late last year.
But even as the Division of Wildlife downplays the spread, the disease is marching onward. One new infected game management unit, 521, is southeast of Grand Junction, and shows the disease is inching southward, out of its traditional northeastern and – more recently – northwestern Colorado confines.
The disease spread in at least three other states as well. In Wisconsin, CWD has grown to cover a wide swath of the state’s southern counties, including along its border with Illinois. In all, the disease appeared in three additional counties this year and a University of Wisconsin-Madison researcher believes the disease is moving about three to six miles a year in that state, according to the Milwaukee Journal Sentinel.
“The (new) areas where we found it were not a huge surprise,” said Manwell of Wisconsin DNR. “Some of these were in areas where we did not get as many samples as we would have liked to get the previous season.”
Faster spread feared
In Nebraska, CWD appears to be moving eastward, though it still hasn’t moved beyond the panhandle. Officials there believe a perceived spread of the disease can be linked to the state boosting its surveillance and finding it in places where they hadn’t looked as hard for it before – a scenario that could probably be applied in several places the disease has been found.
Nevertheless, seeing the disease inch eastward makes Nebraska officials nervous as herds of white-tailed deer grow thicker to the east. Because white-tails are more social, biologists believe they more readily spread CWD.
“The experience in Wisconsin shows the prevalence rate is higher in white-tails,” said Bruce Morrison, Nebraska’s CWD coordinator.
Wyoming found the disease in seven new hunt areas, including one in the north-central part of the state, a leap from CWD’s traditional epicenter to the southeast.
But this was also the first year Wyoming surveyed for the disease statewide, so it’s not clear how long the disease may have existed in the new regions.
Wyoming officials say it’s too early to tell if their hands-off approach to CWD has contributed to its spread. Like other states, they say they need to watch it for a few more years before drawing firm conclusions.
“The thing Colorado can point to is its prevalence is lower than ours. They may not have stopped it, but they may have slowed it,” said Walt Cook, a wildlife veterinarian for the Wyoming Department of Game and Fish.
Even so, Cook doesn’t anticipate Wyoming changing its strategy any time soon.
“So far, the public in Wyoming doesn’t appear to be in favor of doing these large culling operations.”