BSE: what have we learned after 18 years?

Author:Sly, Tim
Position:Technical Briefs

At the time of this writing, the cattle industry in Canada is having sleepless nights wondering what will happen next to its multibillion dollar business and the reputation of Canadian beef internationally. Politicians are insisting that other countries examine the science of bovine spongiform encephalopathy (BSE), while at the same time offering their constituencies unscientific reassurances: "What's the fuss? It's only one animal! Well, okay, maybe two." The press is bouncing along with experts, angles, and lots of airtime, while the public is not really certain who or what to believe.

The saga of BSE/variant Creutzfeldt-Jakob disease (BSE/vCJD) started in the United Kingdom in 1985, and after 18 years the pathology remains one about which we know relatively little. But let's try to take an overview.

The best empirical understanding of the epidemiology of BSE/vCJD is a very large "natural" human trial followed in the United Kingdom between roughly 1985 and 1995. Best estimates indicate that approximately 50 million people ate between half a million and one million sub-clinically infected beef animals (and doubtless a few clinical ones) over the 10-year period. The outcome is that 150 people to date have developed vCJD, and while this number can be expected to increase, the onset curve is not exponential, suggesting that the early prediction of hundreds of thousands of human victims was far overstated.

While 150 human victims are of course still 150 too many, the only reasonable conclusion at this stage is that BSE is not highly transmissible to humans, at least through food intake!

The actual scenario is in fact the "worst-case" scenario: When BSE was identified in the mid-1980s, no ruminant-to-ruminant ban was in place, nothing prevented banned tissues (brain, spinal cord, eyes, lymphoid tissue, etc.) from entering the food chain, and meat and bone meal supplement constituted a high proportion (in some cases as much as 25 percent) of an animal's food intake.

Yet the risk appears to have been very small. Assuming equal exposure, the human population risk was about three in 10 million per year, and we can assume that this conjunction of circumstances has never been nor will it ever be as sinister as it was in the United Kingdom during the early 1980s. Since that time, to varying degrees of effectiveness, most countries, including Canada and the United States, have adopted surveillance systems and controls.

But though the risk is very...

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