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The brain eaters:

A medical detective story behind our understanding of Mad Cow
Part 4: Where's the Beef?

Biomedical detectives uncovered the answer. The prion that causes Mad Cow and vC-JD appears to have been lurking for many years in sheep. Now, scrapie had been known in sheep In the nearly two centuries, and in all that time it had never been suspected of affecting humans or any other animal. But in the late 1970s or early 1980s, the scrapie prion seems to have jumped from sheep to cows, in which it caused serious disease. At the height of the cattle epidemic, BSE infected nearly 200,000 cows in Britain.

What's more, as this prion swept through cattle, it apparently evolved in a way that increased its ability to cross yet another species barrier and infect humans. After an incubation period of several years, roughly 143 people in Britain, like Victoria Rimmer, who had eaten beef products from contaminated cows began exhibiting symptoms of vC-JD. The situation was terrifying. Politicians ranted and media raved. When the connection with BSE-infected cattle was made, the price of British beef plummeted. British farmers were ordered to destroy entire herds of cattle to allay fears that the disease would spread.

Now, it's fine to suggest that Kuru, scrapie, BSE, and n-vCJD are caused by prions that can be acquired by eating diseased nervous tissue. But if that is the case, how could a strictly vegetarian animal such as a cow be involved? If you donπt know much about modern cattle raising techniques, the answer may surprise you. Some years before the BSE epidemic began, farmers in Britain began feeding cows a dietary supplement that included body parts of sheep and other cattle. Why did they do this? Because additional protein provided by this supplement encouraged rapid growth -- while disposing of an otherwise troublesome waste product from slaughterhouses.

From an ecological perspective, however, this practice altered cowsπ position in the food chain, transforming them from plant-eaters into carnivores. Because they were eating the meat of other cow, you could even consider them cannibalis! This change in feeding habits made cows susceptible to prion transmission in precisely the same way as members of the Fore tribe. That change first opened a route for the infectious agent of scrapie to cross from sheep into cattle. As the disease began to spread among cows, the use of cow parts in cattle feed further enabled the disease to spread from infected cows to other cows. The final jump to meat-eating humans was aided by two factors. First, certain processed meat products consumed in Great Britain contain (how can we explain this delicately?) certain parts of cowsπ bodies (including nervous tissue) that are not included in American hamburger. Second, because the prion that causes BSE and vC-JD isn't really alive, it turns out to be almost impossible to "kill."

The situation now seems to be under control in Britain. Feeding of rendered beef and sheep tissue to cattle was forbidden in Britain, preventing further spread of the disease to other cattle and to humans. But because the incubation period of both BSE and n-vCJD may be as long as 5-15 years, the disease may have infected a yet-unknown number of people who ate British beef products between the mid 1980s and the mid 1990s.

Continue the Story Part 5: What we know now — and what don't we know
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