Pursuing prions: A threat past, present and future

Most Canadians no longer pay much attention to the implications of mad cow disease for human health, but Daniel Krewski is watching even more closely than ever.

by Tim Lougheed

It has been some time since the intense mainstream media coverage of bovine spongiform encephalopathy (BSE), the neurological ailment responsible for “mad cow” disease. And most Canadians haven’t missed that coverage. A 2007 national opinion poll sponsored by PrioNet Canada revealed that few of us regarded BSE as a public health threat. Nonetheless many people would acknowledge the potentially serious economic threat it poses to the country’s cattle industry. 

Daniel Krewski is well aware of this finding. Director of the University’s R. Samuel McLaughlin Centre for Public Health Risk Assessment, he is also associate scientific director of PrioNet Canada. 

The research network takes its name from the exotic protein responsible for BSE. Prions—short for proteinacious infectious particles — interfere with the way normal prion proteins arrange themselves in vital tissue. When these complex molecules fold themselves into inappropriate shapes, the result can be devastating. The brain deteriorates from a well-ordered structure to a sponge-like mass, compromising all bodily functions and ultimately leading to death. 

As Krewski explains, the broader impact of BSE can be no less devastating. Following its initial discovery in Canadian livestock in 2003, other countries refused to accept Canada’s beef exports, generating financial losses estimated to be as high as $20 billion. 

Yet, no one in North America has been identified with a domestic case of variant Creutzfeldt-Jakob disease (vCJD), caused when humans acquire BSE. In fact, there have been just a few hundred of these cases all around the world, a number that hardly ranks with the millions killed by malaria, air pollution or even car accidents. 

In spite of vCJD’s low profile, Krewski insists on considering the prospects of this disease. Early last year, PrioNet assembled a group of the world’s leading experts on prions, asking them for some highly technical and detailed information about the ongoing risks to human health. 

“The experience of taking their pulse was a rewarding one,” he says, noting the significant conclusions that emerged. 

While there are species-specific physiological barriers to prevent prions from migrating easily between animals and humans, transmission has happened when people have eaten BSE-contaminated beef. Identifying such transmissions is complicated, however, since vCJD may incubate in the body for as long as 20 years. In the meantime, individuals who have not yet been identified could pass their infection on to others through blood transfusions, for example.

“The risk models that people use to describe transmission from cattle to humans suggest that we haven’t seen the end of vCJD,” says Krewski. 

In addition, the expert panel told him and his colleagues that there could well be other prion-based diseases capable of moving from animals to human populations. One outstanding example is chronic wasting disease (CWD), which remains confi ned to deer and elk in the heart of Midwestern North America. Nevertheless, this limited range could easily expand. 

Krewski asks, “If CWD bears any resemblance to BSE, is it possible that hunters who shoot deer and eat the meat might develop the human version of the disease?” 

A warming climate could also entice these animals further north in Canada, perhaps introducing CWD into the caribou that are a dietary staple there. This possibility leads Krewski to another question: “Can we get out ahead of the risk curve and prevent CWD from becoming the major socio-economic disaster that BSE has been?”

PrioNet was established in 2005 to achieve such a goal. With a team of some 60 researchers, the network has been developing risk management strategies for Canada’s agricultural and health care sectors, as well as investigating the physical behaviour of prions. 

Unlike vCJD, CWD has already revealed itself as a looming health hazard that will cost our society billions of dollars in the coming decades. “We can’t sustain that,” says Krewski. 

PrioNet’s investigations have pointed in some other intriguing directions, including towards a biochemical mechanism that might finally reveal the elusive underpinnings of Alzheimer’s disease. “If this turns out to be correct, imagine the opportunity,” says Krewski, who sees great promise in any link between prions and Alzheimer’s. “Even if we made a small dent, it would be just a fantastic public health contribution.” 

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