Disease of cervids causing local population declines
July 26, 2017
This article is more than 3 years old
An always-fatal neurologic disease is contributing to declines in Western deer and elk herds and raising the possibility of local extinctions.
Bryan Richards, chronic wasting disease project leader within the U.S. Geological Survey's National Wildlife Health Center, has seen clear patterns of increasing spread and prevalence.
"Where it's established, it's spreading and growing," he said. "We have a lot of geographic areas with relatively recent detections, and we really do not have a very effective arsenal of tools with which to manage this disease once it becomes established in a free-ranging herd."
Where it is new, he said, "It looks more or less like one would expect if you drop disease at a focus on the landscape, and you have, over time, deer-to-deer-to-deer transmission."
Dr. Todd E. Cornish, a pathologist and associate professor who studies chronic wasting disease at the University of Wyoming, said two of his recent field studies show the disease has driven population declines among the mule deer and white-tailed deer studied. The article he co-authored on white-tailed deer, published a year ago August in the online scientific journal PLoS ONE (http://jav.ma/CWDdeer), described a 10 percent annual decline among deer in an area with a CWD prevalence of about 33 percent. His mule deer figures are pending publication.
Melia DeVivo, PhD, collared mule deer in southeastern Wyoming from 2010-14 to study CWD's effects, and she reported in her 2015 doctoral dissertation that the disease had caused significant declines in the herd studied.
In scenarios modeled for the dissertation, Dr. DeVivo and her co-authors found that the mule deer herd could be extinct within 41 years, although selection for a known genetic resilience to CWD could preserve a population about one-tenth the size of the original—several hundred deer—through the next 100 years. The minority of mule deer with that trait still acquire fatal infections but tend to live longer than others with CWD.
Where it's established, it's spreading and growing. We have a lot of geographic areas with relatively recent detections, and we really do not have a very effective arsenal of tools with which to manage this disease once it becomes established in a free-ranging herd.
Bryan Richards, chronic wasting disease project leader within the USGS National Wildlife Health Center
"While natural selection may not occur quickly enough to prevent a severe reduction of some cervid populations, the sustainability of a remnant population comprised of mostly less-susceptible genotyped deer remains to be determined," the dissertation states.
Richards said some genotypes in mule deer, white-tailed deer, and elk are categorized as resistant to CWD because, in wild, free-ranging populations, these animals seem to have lower infection rates and prolonged incubation periods. While a typical white-tailed deer could have a CWD incubation period of 18-24 months before clinical signs are seen, for example, a resistant deer could have an incubation period of 50-60 months, he said. But those deer also may shed infectious prions for a longer time.
The misfolded prion proteins responsible for CWD spread through saliva and waste. They are resistant to heat and ultraviolet light, and—like the prions that cause scrapie in sheep—they may remain viable for decades, Richards said. They also escape immune responses, so wildlife managers lack vaccines.
CWD prions are not known to cause infections in humans, but public health advisories recommend against eating meat of infected animals.
Difficult to control
A state natural resource agency needs to detect CWD emergence soon after introduction to have a chance at eliminating it, Richards said. But state governments have reduced surveillance of cervids killed by hunters since the Department of Agriculture reduced its funding to states five years ago, he said.
Donna Karlsons, a spokeswoman for the Department of Agriculture's Animal and Plant Health Inspection Service, said APHIS ended its cooperative agreements supporting state-run CWD surveillance when appropriations to the agency declined in fiscal year 2012. APHIS had spent about $10 million in the preceding two fiscal years.
Congress appropriated $3.5 million to APHIS for all cervid health activities, including CWD, for the fiscal year ending this September, an increase from $3 million annually in the past several years, Karlsons said.
The Minnesota Department of Natural Resources is trying to eliminate the disease in Fillmore County in the state's southeast corner, where a cluster of 11 infections has been found in wild deer since the fall 2016 hunting season. Lou Cornicelli, PhD, wildlife research manager for the Minnesota DNR, said the infections could have emerged from three sources: illegal movement of a deer carcass, migration of a young male deer, or contact between wildlife and deer from a captive cervid farm with an unidentified infection.
Fillmore County alone has 23 of the state's 460 cervid farms, and at least two farms in other counties have CWD infections. Before the outbreak in fall 2016, Minnesota had found one CWD-positive wild deer, in 2010, also in the southeast and likely transmitted from a farm with infected captive elk, Dr. Cornicelli said.
Dr. Michael W. Miller, senior wildlife veterinarian for Colorado Parks and Wildlife, said surveillance is identifying CWD in new areas, most of them adjacent to where the disease is endemic.
"However, our surveillance efforts in recent years lack the intensity to assure the early detection of geographic extensions," he said. "Consequently, some of these 'new' foci were likely established some time ago but detected only recently."
Existing surveillance indicates one Colorado deer herd has had a decline in CWD prevalence since the state used sustained hunting and culling campaigns in the early 2000s, whereas other affected herds have had stable or increased prevalence, Dr. Miller said. Colorado Parks and Wildlife is gathering data to try to identify which hunter-based management strategies are most likely to stabilize or lower CWD prevalence. The state also is implementing required participation by hunters in some areas.
"Developing sustainable approaches for CWD management is a subject of keen interest among several western wildlife management agencies, so hopefully we'll be able to make progress on this in coming years," Dr. Miller said in a message.
Wisconsin's Department of Natural Resources has been dealing with the disease for 15 years, and data show increases in prevalence and distribution there as well, said Tami Ryan, the DNR's Wildlife Health section chief. It is endemic in southern Wisconsin, and newer cases have been found in central Wisconsin and once in the northwest.
The state was aggressive the first five years, intending to eradicate the disease through all conceivable responses: expanded hunting, professional sharpshooting, and trapping and removal, she said. Prevalence declined where CWD-positive animals were removed.
But hunters became discouraged because they saw fewer deer, Ryan said. Public acceptance of those tools waned, and funding declined, so the department shifted to disease monitoring.
Some tools remain, such as special hunting seasons and a program requiring that hunters kill an antlerless deer to be allowed to shoot a buck. The state also works with a citizen advisory team on increasing participation in surveillance.
Dr. DeVivo, who now works for the Washington Department of Fish and Wildlife, said strong management practices, such as culling, are not viable for the Wyoming population she studied. The disease agent likely was circulating in the herd decades before it was identified there in the early 2000s, when prevalence was already at 15 percent, she said.
"The environmental contamination in that area is probably some of the highest that you'll see in any area, just because CWD has been there for so long, and we know that the environment does play a role with transmission," she said.
Herds in the area can have disease prevalence upward of 30 percent in female deer and 50 percent in males.
"Without any of those typical tools that you might employ for other diseases that we just don't have as a resource for this disease, our best tool right now, quite honestly, is continuing to study the disease in this population as well as continuing surveillance of the disease," Dr. DeVivo said.
"And some might find surveillance and research to be doing nothing, but it's doing a whole lot because this is an area where we have so much historic data already. To know exactly what it's doing now and into the future will only help us to get, possibly, somewhere with management."
Richards said his heart sank when he read an article (J Gen Virol 2006;87:3737-3740) that indicates the infectious agent of sheep scrapie can remain viable in an environment for at least 16 years, which he said has implications for the survival of CWD.
That article indicates a farm in Iceland had a scrapie outbreak in 1982, removed sheep from the barn until 1998, and experienced another outbreak in 2000. The authors wrote that epidemiologic investigation established "with near certitude" that the disease had not been introduced from outside the farm.
Dr. Mary Wood, Wyoming state wildlife veterinarian, said her agency will work with the public to find and test new strategies to manage CWD.
"Any management strategy that we try is going to be somewhat experimental in nature," she said.
Wyoming's surveillance indicates CWD is spreading westward, but Dr. Wood notes that the surveillance is imperfect, and CWD has uneven distribution even in herds with high prevalence. In a herd with low prevalence, it can be difficult to detect.
By the time CWD surveillance began in Wyoming's core endemic area, she said, prevalence there was higher than 10 percent.
"We have certainly seen prevalence increase in those areas, and we've seen prevalences at 40 percent in some areas," Dr. Wood said. "And it does appear that, now, we're seeing other areas with an increase in prevalence as you look at the areas that surround our core endemic area."
While prevalence seems to be increasing, Dr. Wood needs larger sample sizes to know for certain. Wyoming, like many states, relies on diagnostic samples collected from cervids killed by hunters, most of them mule deer, followed by elk, white-tailed deer, and moose. Other samples come from roadkill and animals seen with clinical signs.
Answers are elusive
CWD prions are amazing for their ability to spread as a virus would, through saliva, urine, and feces as well as contaminated environments, according to Dr. Edward A. Hoover, a professor and infectious disease researcher at Colorado State University. The Hoover laboratory's CWD research includes assay and vaccine development, although Dr. Hoover said the latter is a difficult prospect because of the lack of a known immune response to prion diseases.
A typical prion protein produced within an animal cell folds in a specific way, and a typical misfolded version is degraded through cellular editing, he said. Prion diseases result from misfolded prions that not only escape the editing mechanisms but also cause a zombie outbreak–like chain reaction among normal proteins within a cell, resulting in generations more of misfolded prions.
In terms of prion disease research, Dr. Hoover said understanding how misfolded prions cause this chain reaction would be like finding a holy grail.
The misfolded proteins escape the cell where they were created, enter other cells, cross mucous membranes, spread to other tissues such as those in the lymphoid system—where they replicate—and spread to the brain, Dr. Hoover said. In the case of CWD, they again spread across mucous membranes to infect other deer.
Yet, he thinks there may be a way to interrupt the disease process or spur an immune response without causing an autoimmune disease. Any such solutions could have implications for human neurologic diseases involving misfolded proteins, including Parkinson disease and Alzheimer disease.
Dr. Hoover also noted that, as far as we know, a species barrier is protecting humans from epidemics with CWD and scrapie, neither of which are known to infect a broad range of species as bovine spongiform encephalopathy can. But he also recommends caution since much about that barrier remains unknown.
Dr. Wood said that, while a vaccine candidate study was wrapping up at Wyoming's state wildlife research center, the results were not promising.
"We don't think that we have the ability, at least with the tools we have now, to eradicate the disease," Dr. Wood said. "So, our goal now is to try and figure out how to manage it to minimize its impact on our free-ranging deer and elk herds."
Developing sustainable approaches for CWD management is a subject of keen interest among several western wildlife management agencies, so hopefully we'll be able to make progress on this in coming years.
Dr. Michael W. Miller, senior wildlife veterinarian, Colorado Parks and Wildlife
Dr. Wood said that, even where the prevalence is 10 to 20 percent, CWD is affecting herds.
"I think it makes them less resilient and less able to bounce back after a harsh winter, less able to grow, less robust," she said. "And, so, even at lower prevalences, even if there's not a true decline in the population due to the disease, it still impacts that population, it still causes added mortality."
Dr. Cornish, of CSU, said applying any action across broad swaths of the CWD-endemic areas of Wyoming or Colorado, where deer, elk, and moose are vulnerable to the disease, would be daunting. He also noted that mule deer in particular are stressed by human development, droughts, and harsh winters.
A Minnesota DNR announcement about the state's disease response included a reminder that feeding wild deer, including providing salt or mineral blocks, is prohibited in the five counties of the state's southeastern tip to reduce deer congregation and disease transmission. Dr. Cornicelli said the state also prohibits scents or other lures that could entice deer to congregate within that disease management area, and it forbids movement of adult deer carcasses from that area, absent negative test results for CWD.
"We also have mandatory CWD testing that gives us the opportunity to define our prevalence, our change in prevalence, and also our spatial distribution of disease," he said.
The Minnesota DNR also is trying to gain funding for research on deer movement patterns.
"Most of us who work in this profession—deal with big game populations and wildlife disease—don't want to live in a world where CWD is on the landscape 100 or 200 years from now," he said. "I think we're starting to see these population-level effects in the Western states that have had the disease for 50 years, we're starting to see these big increases in prevalence in Wisconsin, so they're, ultimately, going to have a population-level effect on deer, and we're doing what we can to hold back that tide."
Dr. Cornicelli expects surveillance will find more CWD infections, but he hopes to see a decline.
"If we can knock it back to seven and then, the next year, it's four, and then it's two, and then we don't find it for three years, then I think we're in good shape," he said. "But, if we find 13 next year scattered 20 miles away, then I'll have a completely different attitude."