Chronic Wasting Disease
Chronic wasting disease (CWD) or "mad deer disease" is known to affect species of the family Cervidae: mule deer, white-tailed deer, black-tailed deer and Rocky Mountain elk. CWD usually affects animals between three and five years old, and is estimated to have a minimum incubation period of 17 months. CWD is always fatal; most infected animals die a few weeks to several months after first exhibiting symptoms. Symptoms include weight loss (hence the phrase "wasting disease"), listlessness, decreased interaction with other animals, excessive drinking and salivation, pacing, hyper-excitability and/ or acting "sleepy" or sluggish.
CWD is a member of the transmissible spongiform encephalopathy (TSE) neurological disease family, which also includes bovine spongiform encephalopathy (BSE or mad cow disease) in cattle, and the different forms of Creutzfeldt-Jakob Disease (CJD) in humans.
CWD was first reported in Colorado in 1967. It has since spread to Wyoming, Montana, Oklahoma, Nebraska, Kansas, Wisconsin, South Dakota, New Mexico, Minnesota, and the Canadian provinces of Saskatchewan and Toronto. Game farms and deer "baiting" practices, which bring animals into close contact, play a major role in spreading CWD. CWD is believed to be passed between animals relatively easily; perhaps through shared feed or exposure to waste. "Maternal transmission may occur," reports the U.S. Department of Agriculture, but "it appears to be relatively unimportant in maintaining epidemics."
Unlike BSE, there is no hard evidence that CWD can be transmitted to humans. After three deer hunters younger than 30 (from Utah, Oklahoma and Maine) died of CJD in the 1990s, their young age caused the U.S. Centers for Disease Control and Prevention to look for a possible CWD link. As The Progressive magazine reported: "The agency did kill and test some deer where the victims of the disease had hunted. All the animals tested negative. There was evidence, though, that all the hunters were exposed to elk from Colorado or Wyoming, possibly from areas where Chronic Wasting Disease is prevalent. However, it was impossible for center investigators to know if those particular elk were infected."
In September 2000, the European Molecular Biology Organization published a study showing that the malformed, disease-causing deer prion proteins that cause CWD could, at least in a test tube, cause human prion proteins to convert from their normal to disease-causing forms. Two years later, study author Byron Caughey told one U.S. newspaper that "while the risk of people contracting infection from a Chronic Wasting Disease deer is probably low, 'it's not a risk I'd want to take.'"
In 2001, a 25 year old man whose grandfather often hunted deer and shared venison with the family died of CJD. Post-mortem examinations resulted in a diagnosis of Gerstmann-Straussler-Scheinker syndrome (GSS). Howver, because of the man's young age, "it remains unknown whether the possible exposure of the case-patient to CWD-infected venison potentially contributed to the early onset of his prion disease."
A June 2004 review in the scientific journal Emerging Infectious Diseases identified other unusual CJD cases, but noted that no firm links to CWD had been found. Although the current information suggests "that the risk, if any, of transmission of CWD to humans is low," wrote the scientists, "the transmission of BSE to humans and the resulting vCJD indicate that, provided sufficient exposure, the species barrier may not completely protect humans from animal prion diseases." Moreover, they noted, "because CWD has occurred in a limited geographic area for decades, an adequate number of people may not have been exposed to the CWD agent to result in a clinically recognizable human disease."
The USDA's Animal and Plant Health Inspection Service has cautioned "at this time there is no test that can be used reliably on individual animals to determine whether that animal is free from CWD." Contrary to many deer hunters' expectations, a "negative" test result may not mean that the animal under question is CWD-free, due to the disease's long incubation time. APHIS further warned, "no test has been shown to be sensitive enough to support use as a food safety test" and "relatively little is known about the distribution of the CWD agent, so an animal whose brain and nervous system tissue tests negative might actually be carrying the infective agent in other tissues."
State CWD "management" plans usually involve killing free-ranging deer and elk in affected areas, testing animals for CWD and imposing restrictions on game farms, animal baiting and the interstate movement of CWD-exposed deer and elk. But state budget crises have limited such efforts; in May 2004, the Fort Collins Weekly reported that "the Colorado Wildlife Commission has decided to eliminate its mandatory CWD testing requirements in favor of a paid voluntary system," even though the state Department of Wildlife "admits, in an August 2003 memo, that eliminating the mandatory testing requirements will likely result in a 50 percent reduction in identifying animals that test positive for the deadly disease. That would result in a corresponding reduction in the agency's ability to manage and control its spread."
- Mad cow disease - contains list of links to CWD/mad cow-related entries
- Ermias D. Belay, Ryan A. Maddox, Elizabeth S. Williams, Michael W. Miller, Pierluigi Gambetti, and Lawrence B. Schonberger, "Chronic Wasting Disease and Potential Transmission to Humans," Emerging Infectious Diseases, June 2004.
- Brian McCombie, "Who is to Blame for Mad Deer?," The Progressive, August 2002.
- "Mad Deer" website, with extensive news reports