ANN ARBOR, Mich., Mar 24, 2001 (United Press International via COMTEX) -- The British government's top advisor on mad cow disease said Friday he expects his country to have a "small" number of human cases of a related disorder, perhaps only hundreds of affected people, instead of the hundreds of thousands originally predicted.

Mad cow, or bovine spongiform encephalopathy, is a brain-wasting disease, eating holes in the gray matter of infected animals. It is suspected that people who eat BSE-contaminated meat can develop a rare, fatal condition -- a variant of Creutzfeld Jakob disease, another brain-wasting disorder.

"We can't be certain about anything," said Peter Smith, acting director of the Spongiform Encephalopathy Advisory Council. "But I think it is likely to be hundreds or perhaps thousands [of cases], rather than tens of thousands to hundreds of thousands, spread out over decades, possibly longer." Smith spoke at the University of Michigan in Ann Arbor following a panel discussion on the trade implications of mad cow disease.

Smith, head of the Department of Infectious and Tropical Diseases at the London School of Hygiene, said the lower case estimate isn't related to government measures to destroy possibly contaminated cattle and to ban the feeding of animal products to other animals. Instead, the period between exposure to contaminated meat and the appearance of symptoms seems to be shorter than was once believed, Smith said.

"I think the measures have been effective," Smith said. "That is a different question, really. The way [variant CJD] has evolved in people suggests that unless you make these extreme assumptions -- like the incubation period being longer than most of us will live -- then you can't get a large epidemic.

"We have no experience with diseases with incubation periods of up to 60 years." Smith said. "That's not to say there couldn't be one. If we think the incubation period is 10 to 15 years on average, then that means we are actually quite a long way into the epidemic, because most of the exposure was 10 to 15 years ago."

Paul Brown, currently researching mad cow disease as a senior investigator with the Laboratory of Central Nervous Systems Studies at the National Institutes of Health in Bethesda, Md., called lower estimates "very comforting."

"They have come down from the dizzying count of hundreds of thousands of cases, based on incomplete data a couple of years ago," Brown said. "But they are guesses. They don't carry a whole lot of weight. Whatever numbers come in this year will change that. It could go up or down."

Smith said he believes the United States will eventually see this variant of CJD appear.

"I couldn't say ... a lot of people from the United States visit the U.K. If you came to the U.K. between 1990 and 1996 and you ate of lot of U.K. beef, your risk is not zero," he said. "You could be unlucky. If you just happen to have a sizable chuck of brain in your meat, then that may have been sufficient. We know that it doesn't take much brain to affect a cow."

In England, BSE reached epidemic status in cattle from the mid-to-late 1980s, but was not thought to pose a threat to humans until 1995. At that point, three young people showed signs of CJD, which generally affects older individuals. To date, 97 people in the U.K. have been diagnosed with this variation of CJD, and most have died.

Earlier this week, British authorities reported four victims of variant CJD had eaten meat purchased from two butcher shops that processed meat with methods likely to have mixed infected cattle brains with other products.

Thus far the U.S. population has been free of BSE or variant CJD cases, but U.S. government officials this week seized two herds of sheep in Vermont that may have come in contact with disease before they entered the country.

By THERESA DEFINO, UPI Science Writer

Copyright 2001 by United Press International.

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