NEW YORK - A handful of young adults in the U.K. have died of a fatal neurological disease that may have been caused by eating beef from cattle infected with "mad cow disease," or bovine spongiform encephalopathy (BSE). Could it happen in the U.S.? It's possible, although steps are being taken to help prevent such an outbreak, according to Dr. Paul Brown, of the National Institute of Neurological Disorders and Stroke in Bethesda, Maryland.
Both countries used ground-up animal carcasses to make cattle feed and nutritional supplements, a step that probably helped a neurological disease in sheep, scrapie, to be passed to cows. In the late 1970s, both the U.K. and the U.S. changed the way it processed sheep carcasses, increasing the likelihood that scrapie could be passed to cattle. And both countries have scrapie-infected sheep -- a disease known for centuries that has never caused illness in humans. The U.S. has had an annual incidence of 30 to 50 cases of scrapie in 8 million sheep and the U.K. has 200 to 300 cases in 42 million sheep.
"Why then have we not seen a BSE epidemic in the United States?" Brown wrote in the current issue of The Journal of the American Medical Association. There may be a couple of reasons, he said. For example, the strains of scrapie in the U.S. may not be capable of causing illness in cattle. And in the U.S. only about 0.6% of rendered animal protein is derived from sheep tissue compared with 14% in the U.K.
"A third explanation is that we do have an epidemic, but have simply not recognized it," he said. Cattle injected with U.S. strains of scrapie do develop a neurological disorder, but do not show the spongy holes in the brain that are characteristic of BSE-cows in Europe. And there have been outbreaks of spongiform encephalopathy in U.S. mink that were fed meat from "downer cows," so-called because they lie down from an unknown illness before dying.
However, analysis of more than 6,000 cows in the U.S. with neurological disorders or considered "downer cows" have not yet turned up a single case of BSE.
"This is reassuring, and will be increasingly persuasive as additional thousands of animals are examined in coming years," Brown wrote.
It could take several years to conclusively determine that the 21 people in the U.K. and 1 in France who died of the neurodegenerative disorder Creutzfeldt-Jakob disease, in fact contracted the illness from eating contaminated beef. In the meantime, the British government has made a number of changes to reduce the possible spread of BSE, including slaughter of sick animals, eliminating bovine brain, spleen and thymus from human food, and banning mechanically recovered meat -- meat recovered from a carcass using high-powered hoses -- from the spinal column.
In the U.S., cattle and cattle products from Great Britain have been banned and tighter controls on sheep have been instituted, including a ban on transferring sheep from scrapie-infected flocks to uninfected flocks.
In June 1997, the Food and Drug Administration banned the use of most mammal carcasses in cattle feed, and at some point in the future, the use of any animal tissue in livestock feed may also be banned.
It's possible that high-risk tissues, such as the cow brain, could be banned as food for humans in the U.S., despite the lack of evidence that it is a problem in this country. And other cow-derived products are under scrutiny, including gelatin, which is in multiple products including drug capsules and cosmetics.
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