The United States has relaxed its ban on the importation of Canadian meat cuts from cattle less than 30 months of age. Mexico, too, is reopening its borders to Canadian beef.

The principal argument Canada has used to reassure its trading partners is that only one older cow, and none of the 2,700 animals with which that cow came into contact, tested positive for mad-cow disease. Meanwhile, the Canadian Food Inspection Agency has detailed measures it is taking to prevent mad-cow disease, or bovine spongiform encephalopathy, "from entering Canada." Surveillance and testing of animals has been increased.

But it's folly to try to prevent mad-cow from entering Canada, for the simple reason that some BSE occurs spontaneously in nature. The longer animals live, the more likelihood there is that one or two will develop a spontaneous case of BSE. And the more tests that are run on older animals, the more likely it is that another case will eventually be detected.

How should Canada prepare for the likelihood that increased testing will turn up another case of BSE?

I am a transplant surgeon, and in the course of my research I've had to familiarize myself with BSE and the rest of the family of brain-wasting diseases known to affect many animals, including sheep, deer, cows and humans. The human form, Creutzfeldt-Jakob disease, or CJD, mainly affects older people between the ages of 60 and 70. However, in the 1950s, a new form of the disease, called Kuru, was discovered in New Guinea, in a region in which cannibalism was practised; it was found to have an incubation period of as little as two years. Unlike CJD, this transmissible form could appear in the young.

In the 1980s, an epidemic of BSE appeared among British cattle. A British government inquiry found that the likely source was a single case of sporadic -- that is, naturally occurring -- BSE. Unfortunately, the affected animal was sent to a rendering plant. Material from this animal found its way into the cattle feed supply. Then BSE-infected meat turned up in butcher shops across Britain and transmitted the disease to humans. In 1996, the medical journal Lancet reported a Kuru-like illness among 10 unusually young Britons. Investigators found abnormal prion proteins in the brains of humans with this illness, now called variant CJD, and in cows with BSE.

The pathologic properties of abnormal prions lie in their shape and their ability to convert normal prions to that shape. The resulting web of abnormal protein destroys the brain, creating spaces as in a sponge. The disease was dubbed Transmissible Spongiform Encephalopathy.

Sporadic TSE results from spontaneous mutation of the prion. In humans, this form of the disease tends to occur once in every million people. Clusters of sporadic TSE have appeared in humans in Slovakia and among animals in Iceland. Even at the height of the British epidemic, cases of sporadic CJD outnumbered variant CJD -- that is, the new, transmitted form -- by 10 to one.

The Canadian cattle herd numbers 4.7 million. In a human population this size, one might expect to find between four and five cases of sporadic TSE every year. But few cattle make it to old age. They are usually 3 years old or younger when they are sent to the abattoir.

Still, to find a single older cow with sporadic TSE every few years is statistically likely. Indeed, to find such a case is proof that the system does work. In other branches of medicine, if lab technicians use a test and thousands of the results are negative, they won't presume that the disease they are testing for doesn't exist; rather, they will begin to lose confidence in the test itself.

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But matters are not being interpreted that way. Agriculture Minister Lyle Vanclief has pointed out that Canada has been so severely punished for finding one case of BSE that it will discourage other countries from testing and reporting. The U.S. cattle herd is 10 times the size of Canada's, and yet no sporadic TSE has been detected. Should the Americans be looking harder? Will they want to?

Explaining the safety of Canadian beef practices is not likely to change the minds of consumers in countries such as Japan, which is still banning Canadian beef. In their anxiety, the Japanese may have chosen to take their beef supply from the wrong herd; the Canadian system must work, because it turned up a sporadic case.

What should Canada do? One important step would be to educate the public to understand that there will be an occasional positive test, but that this is not reason for panic -- rather, it is proof that the testing system works.

Another step would be to improve regional testing capabilities. The brain tissue from the tainted animal found in Alberta had to be sent to England for a conclusive analysis. Even with current technology, it is possible to train abattoir technicians to sample, prepare and transport tissue to regional laboratories.

The Canadian Institute of Health Research should sponsor investigations to distinguish sporadic from transmitted spongiform encephalopathy. This will cost money -- but the benefits will include more jobs in Canadian agriculture. It could even help build a world-wide market for TSE-tested Canadian beef.

This is the kind of effort it took to restore Canadians' confidence in the blood supply. Now it is time to turn our efforts into rebuilding trust in the quality of the food we sell, and eat.