Posted on 05/13/2003 9:08:10 AM PDT by WaveThatFlag
A place called the eradication zone, lying about 40 miles west of Madison, Wis., covers some 411 square miles. There thousands of white-tailed deer live--or rather, used to live. Last year the Wisconsin Department of Natural Resources instituted special hunting periods to try to wipe out upward of 18,000 deer. During the fall, dead deer were taken to registration areas, where state employees in protective suits and gloves dragged carcasses from pickup trucks and lifted them onto plastic-covered picnic tables. With hacksaws, they severed the heads, double-bagged them and sent them for testing; the bodies themselves were incinerated.
The Dairy State's massacre is an attempt to keep a fatal ailment known as chronic wasting disease (CWD) from infecting its other 1.6 million deer. The testing enables wildlife officials to ascertain the scope of the epidemic--running at nearly 1.6 percent--and determine whether the culling can slow the spread. Currently no practical live test exists to check whether an apparently healthy, wild animal is actually incubating the sickness; only a brain sample will do. The disease occurs because a pathogen peppers neural tissue full of microscopic holes and gums up the brain with toxic clumps of protein called amyloid plaques. Long confined to a patch of land near the Rocky Mountains, the disease has shown up in 12 states and two Canadian provinces. The sickness passes readily from one deer to another--no deer seem to have a natural resistance. "From everything we've seen," comments Michael W. Miller, a CWD expert with the Colorado Division of Wildlife, "it would persist. It would not go away on its own."
The urgency also reflects concern about the nature of CWD, which belongs to the same family as a better-known scourge: bovine spongiform encephalopathy (BSE), or mad cow disease. Spread by animal-based feed inadvertently containing tissue from sick cows and sheep, BSE emerged in the U.K. in the 1980s and continues to plague that country at a low level. (Nearly two dozen other countries have now also reported cases.) In 1996 scientists realized that BSE can pass to humans who eat infected meat, leading to a fatal condition: variant Creutzfeldt-Jakob disease, or vCJD (distinct from the more common sporadic CJD, which arises spontaneously in one in a million people). Researchers are now trying to figure out whether CWD could infect humans and livestock and thereby create an American version of the U.K.'s mad cow disaster.
Pathological Protein The disease agent common to all these maladies is the prion ("PREE-on"), a term coined in 1982 by Stanley B. Prusiner of the University of California at San Francisco. The prion is a protein that exists in all animals, although the exact amino acid sequence depends on the species. It takes one of two shapes. Folded correctly, it is the normal prion protein (PrP), which is especially abundant in brain cells and may help process copper. Folded incorrectly, the prion protein becomes a pathogenic entity that kills. The malformed protein has the ability to refold copies of normal PrP in its own image, thereby making more of itself.
Prusiner's conception of prions initially met with great skepticism. That a pathogen could replicate and pass on its traits without assistance from nucleic acids (DNA or RNA) violated the orthodoxy of molecular biology. But enough evidence has accumulated to prove that some proteins can in fact copy themselves and that variants of PrP are essential players in spongiform encephalopathies.
That prions lack any DNA or RNA is also the prime reason why they are so tough. Germicidal light, formaldehyde baths and boiling water all promptly disrupt bacterial and viral nucleic acids, yet such treatments have little effect on malformed prions. Researchers have exposed prion-contaminated tissue to a dry heat of 600 degrees Celsius and left it buried for three years, only to find that the material, though greatly weakened, was still infectious. Indeed, physicians have unwittingly passed prion diseases on to patients via surgical instruments and transplanted organs that had undergone standard sterilization procedures. (Prion disinfection requires extended heating or corrosive chemicals such as sodium hydroxide.)
Foothold in the Foothills The resilience of misfolded prions appears to be a key reason why chronic wasting disease has persisted and spread from its presumed starting point near Fort Collins, Colo. There, in 1967, at the state's Foothills Wildlife Research Facility, CWD made its first recorded appearance, in captive mule deer that were being maintained for nutritional studies (mule deer are the most common type in the West). As the name of the disease suggests, affected deer lose weight over the course of weeks or months. They often become thirsty, which drives them to drink large amounts of water and, consequently, to urinate a great deal; they also start slobbering and drooling. They may stop socializing with fellow deer, become listless or hang their heads. Death typically ensues three to four months after symptoms start, although some victims expire within days and others in about a year. The incubation period, during which the animals show no symptoms, ranges from about 20 to 30 months.
The Fort Collins facility became a CWD death trap. Between 1970 and 1981, 90 percent of the deer that stayed more than two years died from the disease or had to be euthanized. In 1980 the scourge emerged outside Colorado, at the Sybille Research Unit in southeastern Wyoming, 120 miles northwest of Fort Collins. The two facilities had exchanged deer for breeding purposes, thus indicating that the disease was infectious--even to a different species: soon the elk at the facilities contracted the disease. (Deer and elk both belong to the cervid family.)
For years, researchers thought CWD resulted from nutritional deficiencies, poisoning, or stress from confinement. But in 1977 Elizabeth S. Williams, studying for her doctorate at Colorado State University, discovered that this view was mistaken. When Williams looked at brain slices from infected animals, she saw that the tissue was full of microscopic holes. "I happened to be taking a course in neuropathology and had studied a lot of brain lesions," she recalls. The holes were unmistakably like scrapie, the sheep sickness that was the first documented spongiform encephalopathy.
CHRONIC WASTING DISEASE fills brain tissue with holes (white areas in micrograph, above). In fact, CWD appears to have originated from scrapie. Richard E. Race of the National Institutes of Health Rocky Mountain Laboratories in Hamilton, Mont., conducted test tube studies that revealed no distinction between the malformed PrP of scrapie sheep and CWD cervids. Consistent with this discovery, Amir Hamir of the U.S. Department of Agriculture's National Animal Disease Center in Ames, Iowa, found no difference in the appearance of brain samples from elk with CWD and elk experimentally infected with scrapie. (BSE also probably arose from scrapie, after cows ate feed derived from infected sheep.)
But unlike BSE in cows (or vCJD in humans), the cervids were not getting ill from their food. CWD behaves more like scrapie, in that the sickness spreads among individuals, although no one really knows how it does. The prions could lurk in the urine. During rutting season, deer bucks lap up the urine of perhaps dozens of does to find out which are in heat. Elk females lick males that have sprayed themselves with urine. Saliva could be a vector, too; in both deer and elk, individuals meet and greet by licking each other's mouths and noses, thus exchanging drool. Ranched elk may swap saliva when they feed in close quarters. It is also possible that animals take in the pathogen while grazing in areas where sick animals have shed prions on the ground in their feces, urine and saliva.
By 1985 veterinarians discovered CWD in free-ranging deer and elk, generally within about 30 miles of the two wildlife facilities. Whether the disease originated in the wild and spread to the captives, or vice versa, is not known. The two populations had plenty of time to mingle. Especially during mating season, wild cervids nosed up to captives through the chain-link fences. Incubating deer could also have escaped or been released.
Both facilities tried hard to eradicate CWD. The Sybille center killed all the deer and elk in the affected area and waited a year to introduce new animals; four years later deer and elk started coming down with CWD. The Fort Collins facility acted more aggressively. Officials first killed off all the resident deer and elk; then they turned several inches of soil and repeatedly sprayed structures and pastures with swimming-pool chlorine, which readily wipes out bacteria and viruses. After waiting a year, they brought in 12 elk calves, but a few years afterward two of those elk contracted CWD.
The disease's persistence has permanently contaminated an area of about 15,000 square miles in northeastern Colorado, southeastern Wyoming and (beginning in 2001) southwestern Nebraska. The incidence of CWD among the cervids in this so-called endemic area averages about 4 to 5 percent but has reached 18 percent in some places. To help keep the disease confined here, the research facilities stopped trading captive animals with each other. In fact, no captive cervids now leave the endemic area alive: "They're only allowed out to come to my necropsy room," wryly remarks Williams, now at the University of Wyoming. More important were the mountains and other natural barriers, which scientists expected would keep CWD from spreading rapidly out of the endemic area. There was, however, an easy way past those natural barriers: along the roads, in a truck.
Out and About Some 11,000 game farms and ranches holding hundreds of thousands of deer and elk dot the U.S. and Canada. Besides harvesting the meat, ranchers can sell the antlers--those from elk are marketed as a supplement in vitamin stores ("velvet antler") and as an aphrodisiac in Asia ("velvet Viagra"). To start such farms, ranchers must buy breeding cervids. Somewhere along the line, businesses must have picked up incubating animals from the endemic area. And the interstate trade of cervids continued the spread, west across the Continental Divide and east across the Mississippi River. (These days most states regulate such trade.) The first farmed cervid to display signs of CWD was an elk that fell ill in 1996 on a ranch in Saskatchewan. By 2001 some 20 ranches reported cases across six states (Colorado, Kansas, Montana, Nebraska, Oklahoma and South Dakota) and one other Canadian province (Alberta). Quick, aggressive measures--namely, killing off the herds--appear to have eliminated the problems on the ranches.
Nevertheless, the transport of incubating cervids may have carried CWD to wild populations in those states and beyond--such as to white-tailed deer in Wisconsin's eradication zone. But precisely when and how mule deer gave it to white-tailed deer, the most common type in the eastern U.S., is unknown and may never be clear. "By the time these problems are discovered," Miller says, "they have probably been sitting there for decades, which makes it difficult to go back and retrace how things came about." Based on epidemiological models and on Wisconsin's roughly 1.6 percent incidence in the eradication zone, Miller thinks CWD had probably been lurking there since the early 1990s.
Wisconsin's approach makes sense to scientists studying prion diseases. "The idea is to find a fairly small focus and get rid of all the animals in the area," in the hopes of preventing CWD from attaining a permanent hold in the region, Williams says. A rapid spread is possible in Wisconsin because the deer population in the state's southwestern corner is dense: Thomas Givnish, an expert on the ecology of diseases at the University of Wisconsin–Madison, notes that it runs about 50 to 100 deer per square mile, or 10 times that of the endemic area around Fort Collins. "The alternative is to do nothing," Williams observes, and then "you know it's going to be established." By the end of March, Wisconsin hunters had bagged 9,287 deer--which will cut the fall population by 25 percent but will not eliminate CWD, notes state wildlife biologist Tom Howard. A few more seasons of liberal hunting may be needed.
Considering the persistence of prions, Wisconsin may have to live with CWD, as Colorado does. "The disease has been here a long time," Miller comments of CWD around northeastern Colorado. "We can't get rid of it here. We try to get infection rates down so that it can't spread." Miller says that Colorado had hoped to purge CWD through culling. But "we discovered we were 10 to 20 years too late. It was already out there; we didn't realize it." That statement may apply to other states that have found CWD among wild deer, including Illinois and New Mexico.
Venison and Beyond No one knows whether CWD can pass to humans. A test tube study mixed CWD prions with normal prion proteins from cervids, humans, sheep and cows. The CWD prions had a hard time converting normal human PrP--less than 7 percent of the protein was changed. The downside is that CWD prions converted human PrP about as efficiently as BSE prions do. And because BSE has infected humans, CWD might pose a similar risk. But because beef is far more popular than venison, CWD doesn't present quite the same public health threat.
To see if CWD has already infected people, the Centers for Disease Control and Prevention investigated the deaths of the three young venison eaters who succumbed to sporadic Creutzfeldt-Jakob disease. All were younger than 30 years, which is exceedingly rare in CJD. In fact, through May 31, 2000, just one other U.S. case of sporadic CJD occurred in this age group since surveillance began in 1979. The first was a 28-year-old cashier, who died in 1997; she had eaten deer and elk as a child, from her father's hunts in Maine. The second was a 30-year-old salesman from Utah who had been hunting regularly since 1985 and who died in 1999. The third was a 27-year-old truck driver from Oklahoma who died in 2000; he had harvested deer at least once a year. Tests of the 1,037 deer and elk taken during the 1999 hunting season from the regions where the victims' meat originated all turned up CWD negative (none of the meat came from the endemic area). The victims' brains showed no unique damage or distinct biochemical signs, as is the case with other prion diseases in humans.
Six other patients (all at least middle-aged) raised suspicions about the CWD risk to humans. Three were outdoorsmen from the Midwest who had participated in wild deer and elk feasts and died in the 1990s. The other cases were reported in April and include two from Washington State who hunted together. Researchers, however, could not find any connection with CWD. And states with CWD have not discovered a higher incidence of Creutzfeldt-Jakob disease.
These observations may seem reassuring, but it is too early to conclude that CWD does not pose a human health hazard. The incubation period of prion diseases may span upward of 40 years, and CWD has been spreading noticeably in only the past 10. The rarity of prion diseases and the low national consumption of deer and elk (compared with beef) make it hard to draw any firm conclusions. Because of the uncertainties with CWD and the fact that animal prion diseases have jumped to humans, the CDC warns against eating food derived from any animal with evidence of a spongiform encephalopathy.
Scientists are still trying to determine if CWD poses a threat to livestock. In an ongoing experiment begun in 1997, Hamir and his colleagues injected brain suspensions from CWD mule deer into the brains of 13 Angus beef calves. Two became ill about two years after inoculation, three others nearly five years after. Hamir began repeating the experiment in November 2002, this time with the brains of CWD white-tailed deer.
Under more natural conditions, bovines have not contracted CWD. Williams has kept cows with infected cervids, and more than five years on, the cows are still healthy. Bovines kept with decomposing CWD carcasses or isolated in pens that once housed CWD animals have also remained free of prion disease. (These reports are good news for pasture-grazing cows, which might find themselves in the company of wild deer.) To see whether CWD might pose a danger when eaten, Williams has begun feeding CWD brain matter to calves. The long incubation of these illnesses, however--BSE incubates for up to eight years--means these experiments must continue for several years.
If U.S. livestock so far seem to be safe from CWD, the same cannot be said of other animals. If an infected deer dies in the forest and nobody is there to see it, plenty of coyotes, bobcats and other carnivores will, and they will gladly scavenge what remains of the wasted carcass. Moreover, during the clinical phase, CWD animals undoubtedly make easier prey. The canine family is evidently immune to prion diseases, but felines can contract them. Transmission studies with mountain lions have begun, and local lions that die for unknown reasons end up on the pathology table, Williams says.
Although many states have uncovered CWD, other states "are looking darn hard," Miller says, but have not found it--among them, Arizona, Kansas, Michigan, Montana, Nevada and New Jersey. Apparently, only pockets of outbreaks exist. Wildlife managers therefore have a fighting chance to keep CWD from gaining a permanent grip throughout the country, so long as control efforts begin promptly. Unfortunately, not all states with CWD are as aggressive as Wisconsin when it comes to surveillance and eradication.
To stop or at least slow the spread of the fatal sickness, extensive culling appears to be the best strategy. One could hope that CWD occurs naturally in deer and that the epidemics will run their course and leave behind CWD-resistant cervids. Some lines of sheep, for instance, are immune to scrapie. But so far all white-tailed and mule deer appear to be uniformly susceptible. "I don't think genetics is going to save us on this," remarks the NIH's Race. Sadly, the only way to save the deer, it seems, is to shoot them.
Is that accurate?
Couldn't agree more. Let's start with Senate Democrats.
New slogan for the Wisconsin DNR, "Always a day late and a brain cell short."
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