Thus far, all the articles in this blog site have dealt with conventional microbes (viruses, bacteria, parasites, and fungi) that affect the health of humans. This week we’ll consider an unconventional infectious agent—a prion—that causes a devastating infection: Chronic Wasting Disease (CWD), only present in cervids (deer, moose, and elk). Not a single case has been reported in a human, at least not yet.
Of the 140 or so infections that have emerged in humans over the past half century, about 60% are zoonoses, infections in animals that can be transmitted to humans. The zoonosis that has infectious disease epidemiologists on high alert as it pertains to CWD is another prion-mediated disease, variant Creutzfeldt Jacob disease (vCJD). This uniformly fatal infection confounded most experts by crossing over from beef to humans in 1995. Until then a major epidemic in the U.K. of bovine spongiform encephalopathy (BSE) in cattle (so-called “mad cow disease”) was thought to pose no threat to humans. Since 1995, however, more than 225 cases of vCJD have been reported, most in the U.K. and four cases in the U.S. All of these vCJD victims are thought to have consumed beef during the BSE epidemic.
The idea that prions, misfolded proteins that contain no genetic material, caused scrapie, an encephalopathy (brain disease) in sheep similar to BSE, was considered heresy when proposed in 1982 by the neuroscientist Stanley Pruisner. Vindication came, however, in 1997 when he was awarded the Nobel Prize in Physiology or Medicine for his work. The misguided belief that BSE, a type of transmissible spongiform encephalopathy (TSE), would stay in cattle was based upon an understanding at the time of what is called the species barrier.
The ability of infectious agents, in general, that never have spilled over from another animal species to humans requires substantial changes in the nature of the infectious agent. For example, the prion that causes scrapie has stayed in sheep for many decades if not centuries, despite the consumption of potentially contaminated lamb or mutton.
One aspect of CWD that has many epidemiologists so worried is the rapidity of its spread. First identified in 1967 in captive mule deer in a Colorado research facility, the number of infected animals has increased dramatically. In 2000, CWD was found in five states in the U.S. and in one Canadian province. By 2010, it had spread to to 17 states and two provinces. It’s now in 26 states and three provinces. Worldwide it has been documented in cervids in South Korea, Finland, Norway, and Sweden.
Also worrisome is that unlike the prion that causes BSE, which is found only in tissues of the nervous system, prion infection in cervids involves muscle. This, of course, should be concerning for those who relish eating wild venison or elk. Despite this concern, public health officials estimate that between 7,000 and 15,000 CWD-affected animals were consumed last year.
Given the mounting pandemic of CWD and the lessons learned from vCJD, on July 23, 2019 in the journal mBio a team of experts led by Michael T. Osterholm, director of the University of Minnesota’s Center for Infectious Disease Research and Policy, issued an urgent call for funding of research on CWD by state and federal agencies. A special emphasis was placed on developing new diagnostic tests that would help hunters detect prions rapidly in the field, thus reducing the risk of ingesting contaminated meat.
Public health authorities are mindful of the benefits of deer hunting. Not only is it enjoyed by many people and of considerable economic value (in Minnesota, deer hunting contributes an estimated $1 billion to the state’s economy), but it is an essential population management tool: if you don’t reduce the size of deer populations, the spread of CWD would be even worse.
Some states offer free testing of cervids for prions but the uptake by hunters often is low. In Minnesota, where the deer hunting season is about to commence, testing is now mandated in areas where CWD has been found. This should help with surveillance for CWD. Despite the inadequacies of current testing, public health authorities highly recommend that hunters should never eat deer or elk meat until it is certified prion-free.
Finally, nobody knows if or when a new TSE will show up in humans acquired from an infected cervid. For now, all we can do is hope that the species barrier continues to hold!
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