Study: Human barrier to chronic wasting disease substantial

BILLINGS – The infectious agent that causes chronic wasting disease – an always fatal affliction infecting moose, deer and elk – did not infect human neural tissue, a newly published study found.

“Overall, the unsuccessful propagation of CWD in cerebral organoids supports a strong species barrier to transmission of CWD prions to humans,” the study concluded.

The information provides a temporary sigh of relief for many hunters who fear contracting the infection, which causes holes in infected animals’ brain tissue. The infectious agent is prions, misfolded proteins that, unlike bacteria and viruses, are difficult to neutralize and can persist in the environment.

Many states where the infection has been identified in wild cervids – including Montana, Wyoming and Idaho – offer testing of hunter-killed moose, deer and elk to avoid the possibility of the infection jumping to humans. Hunters are also encouraged to dump carcasses only at approved landfills and to take extra precautions in cleaning any cutting boards, knives or grinders they use to process wild game.

Concern over CWD transmission to humans has been highlighted by a similar prion-based infection – bovine spongiform encephalopathy, also known as mad cow disease – that was transmitted from cattle to humans in the United Kingdom in the 1980s and mid-1990s.

“Over the next decade, 178 people in the U.K. who were thought to have eaten BSE-infected beef developed a new form of a human prion disease, variant Creutzfeldt-Jakob Disease, and died,” according to Rocky Mountain Laboratories.

In addition, an April news story reporting that two friends who hunted deer together in an area of Texas with known CWD-infected deer, and who both died of brain diseases, also heightened fears about CWD transmission from eating contaminated venison. The Centers for Disease Control and Prevention reviewed the two cases and said they were caused by Creutzfeldt-Jakob disease, a different prion disease in humans.

Scientists conducting the new research, published in the June edition of the journal “Emerging Infectious Diseases,” included eight Hamilton-area residents working at Rocky Mountain Laboratories, a National Institute of Allergy and Infectious Diseases facility. They included Bradley R. Groveman, Katie Williams, Brent Race, Simote Foliaki, Tina Thomas, Andrew G. Hughson, Ryan O. Walters and Cathryn L. Haigh. Wenquan Zou, of the First Affiliated Hospital of Nanchang University, in Nanchang, China, also contributed to the study.

To conduct the research, the scientists exposed lab-produced human brain tissue, called organoids, to CWD prions collected from infected elk and deer.

“Human cerebral organoids are small spheres of human brain cells ranging in size from a poppy seed to a pea,” RML explained. “Scientists grow organoids in dishes from human skin cells. The organization, structure and electrical signaling of cerebral organoids are similar to brain tissue.”

Despite week-long exposure to infectious prions, none of the tissue showed infection after six months.

“This finding indicates that, even after direct exposure of human central nervous system tissues to CWD prions, a substantial resistance or barrier to the propagation of infection exists,” the researchers concluded.

The research was the first to use human cells to study transmission. Previous research has infected genetically engineered mice and squirrel monkeys, some of which showed signs of CWD infection. Macaques, which are more genetically similar to humans, were also exposed to chronic wasting disease with no signs of infection. The same species were able to be infected with mad cow disease.

However, the scientists hedged by noting there is the possibility some humans could be more genetically predisposed to infection, as was the case with mad cow disease.

The study also said although the brain tissue is the closest the researchers can get to testing the effects of a prion disease on a human, there are limitations because the tissue “does not reproduce all aspects of the human brain” which might make it more susceptible to CWD prions.

“We cannot exclude unknown susceptibility factors that could make a small population more vulnerable to infection, and we have not tested all cervid genotypes against all human genotypes,” the study said. “Likewise, the possibility remains that new strains of CWD with the capacity to cross the species barrier could emerge in the future.”

This most recent study is consistent with earlier research at the Hamilton lab using animal models.

Groveman, one of the lead authors of the study, is a biologist in the Laboratory of Neurological Infections and Immunity at Rocky Mountain Laboratories. His primary research interests involve infectious diseases of animals and humans, particularly involving prions and prion-like proteins.

Correction: An earlier version of this story misstated the status of chronic wasting disease in Washington. CWD has not been found in Washington.