A new paper from a Denver neurologist suggests the possibility of humans contracting chronic wasting disease is underplayed and that the “most reasonable assumption” is CWD can be transmitted to some people.

Patrick Bosque, an assistant professor at the University of Colorado School of Medicine, also wrote that scrapie – a CWD-like disease in sheep – can likely be transmitted to humans. Evidence to the contrary is “unconvincing,” he said.

Like many other researchers, Bosque said there is no known instance of a human infected by CWD, which fatally damages the brains of deer and elk via a rogue protein called a prion. But since prion diseases of any kind are so rare in humans, scientists might not be able to identify a CWD case were it to occur, he said.

“It would be obvious by now if humans were highly susceptible to CWD,” Bosque wrote, noting the disease has been present in wild deer and elk in northeastern Colorado for two decades or more without any reported cluster of a human version of the disease.

Bosque’s paper, published in Current Neurology and Neuroscience Reports, sounds a cautionary tone throughout and concludes that “practical measures to limit human exposure to animal prions, particularly CWD-infected deer and elk, should be improved.”

At the same, he said, the relative risk involved appears extremely small. As an example, he compared England’s annual rate of mad cow disease in humans – a disease in the same family as CWD – with the rate of U.S. deaths to far more common food-borne illnesses, such as E. coli. The former is lower than one case in 2 million. The latter, about one in 50,000.

Bosque, who earlier worked under Nobel Prize-winning prion researcher Stanley Prusiner, said that should CWD infect humans, the susceptibility rate would probably be similar to that of mad cow disease – that is, very low. So far, scientists believe 138 people have contracted the human version of mad cow even though millions were likely exposed to beef that might have carried the infectious prions.

Medical experts had an advantage in recognizing mad cow in humans, Bosque said: Its victims were far younger than those who typically contract a rare, sporadic human version of the fatal prion illness called Creutzfeldt-Jakob disease, or CJD. In addition, the disease left a distinctive pattern in the brains of its victims.

But should CWD affect humans, it might not leave the same clues as the human version of mad cow disease did, Bosque writes.

“If, as appears to be the case with (mad cow)-to-human transmissions, only a small portion of the population is susceptible to CWD, then the association of disease with the consumption of venison might not be obvious.”

Bosque raises a similar point regarding scrapie: “It would be possible for a substantial proportion of cases of CJD to be caused by exposure to scrapie prions without this being epidemiologically apparent,” he wrote.

Bosque also questions the oft-cited notion that a so-called species barrier will prevent the transmission of CWD to humans. He calls the term misleading, saying the impregnability of such a barrier appears to depend on prion “strains,” each of which exhibit different characteristics, and on differences in the prion’s amino acid sequences.

“People throw out the term species barrier,” Bosque said in an interview. “It’s a poorly understood concept and usually not absolute.” In his paper he cited the example of goats, some breeds of which are more susceptible to sheep scrapie than are sheep.

Bosque’s paper comes at the height of hunting season in Colorado. Early indications suggest that most hunters aren’t concerned about the disease, as only about one in five appears to be seeking to test their animals for CWD.

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