There’s been a lot of talk lately about the risks involved with chronic wasting disease in deer. Can CWD be transmitted to humans? A recently completed 50-year scientific study indicates that it’s very unlikely. The study also shows no evidence that human deaths due to Creutzfeldt-Jakob disease are linked to endemic areas of chronic wasting disease in Colorado, Wyoming and Nebraska — hotbeds for CWD infections in free-ranging deer herds.
BY JOHN J. OZOGA, D&DH RESEARCH EDITOR
To date, CWD has been found in 21 U.S. states and two Canadian provinces. According to Colorado researchers, led by Samantha MaWhinney, the highest reported occurrence of CWD is in a region spanning parts of Colorado, Wyoming and Nebraska.
The Colorado researchers note the following: “Humans and animals can acquire TSEs by consuming prion-contaminated food. Outbreaks of prion disease include an epidemic of kuru among the cannibalistic Fore tribe of the New Guinea highlands and an epizootic of BSE in the United Kingdom, caused by feeding cattle protein supplements derived from prion-infected cattle offal.”
Although “food-based transmission” of these diseases can occur, a phenomenon known as the “species barrier” decreases transmission efficiency. Studies have shown that this natural barrier reduces human susceptibility to animal prion disease, including CWD. Despite extensive human exposure to scrapie, for example, there is no evidence that CJD in humans has increased.
Conversely, in Britain the consumption of BSE-infected cattle led to an epidemic of a CJD variant in the mid-1990s. Even so, less than 200 cases of the variant disease were identified despite the dietary exposure of millions. Evidence indicates future risks are minimal.
Considering that there have been no completely reliable experimental studies conducted for testing the potential for CWD to cause CJD in humans, the Colorado researchers analyzed historical data for evidence.
The researchers centered their study in seven northern Colorado counties where CWD has occurred in free-ranging deer and elk for more than 25 years. Hunting license records for the area indicate that about 75 percent of the deer and elk hunting licenses purchased from 1995 through 2001 were issued locally. Hence, county residents consume most regionally harvested game, and would have a high potential risk of consuming CWD-infected animals.
Using Colorado death certificate data from 1979 through 2001, the researchers tested whether residence in a CWD-endemic county affected the probability that a death was from CJD. They also examined whether the likelihood of a death from CJD increased over time or that it was misclassified.
Given their analysis, the researchers found that human prion disease is extremely rare, and increased risk due to CWD exposure appears to be subtle or nonexistent. They found no significant difference in the proportion of deaths from CJD in CWD-endemic versus non-CWD endemic counties.
Likewise, no significant increase in CJD death rates were found over time. In fact, they saw a slight decrease in CJD risk over time; 43 percent of the deaths occurred before 1989.
CJD risk also decreased with age of death; 46 percent of deaths occurred in persons 56 to 70 years of age and 40 percent in those older than 70 years.
Overall, CJD death rates were low, less than one per million in both CWD-endemic and non-disease-endemic counties.
The Colorado researchers acknowledge that this study was based solely on historical data and identifying cases of human prion disease remains a challenge. For example, death certificate data undoubtedly underestimate the prevalence of CJD, as the probability of CJD being accurately diagnosed is influenced by changes in diagnostic practices, medical care, etc.
Also, due to the long incubation periods of prion diseases, infected persons might not have had sufficient time for the disease to develop or might have left the state before disease onset.
Active education about CWD has been ongoing in Colorado since 1995. This information campaign includes specific recommendations to minimize exposure for hunters, meat processors and taxidermists. In addition, since 1994, testing has been available for game harvested in CWD-endemic counties, thereby removing a proportion of harvested, CWD-infected deer and elk from the human food chain.
Although CWD has existed in Colorado since the 1960s, this study found no evidence that human deaths due to CJD in CWD-endemic areas have increased. Hence, the researchers conclude that human prion disease resulting from CWD exposure is rare or nonexistent. Even so, given the many uncertainties involved, they recommend that preventative steps to reduce exposure to the CWD agent and other animal prion diseases, such as BSE in cattle and scrapie in sheep, should continue.
In their view, “The possibility that the CWD agent might cause human disease cannot be eliminated.”
For more information on CJD, click here.
— John J. Ozoga is one of North America’s most respected deer research biologists. He holds bachelor and master of science degrees in wildlife management from Michigan State University. He worked at the Cusino Wildlife Research Station for more than 33 years, where he wrote or co-wrote more than 80 technical papers, including some of the most comprehensive scientific studies on white-tailed deer ecology, physiology, reproduction, nutrition, behavior and population dynamics. Ozoga retired and became Deer & Deer Hunting’s freelance research editor in 1994.
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