LONDON — Fears over "mad cow disease" have faded in Britain, where beef sales have rebounded and schools have put hamburger back on the menu.
But scientists still do not know what causes the incurable brain disease or how it is transmitted, and they say the epidemic has not reached its peak.
The scientific name for mad cow disease--so called because infected animals become uncoordinated and stagger--is bovine spongiform encephalopathy.
There are several spongiform encephalopathies. Some strike humans, although rarely. The diseases cause sponge-like holes in the brain and, in the case of humans, can lurk for years without causing symptoms.
The first case of the disease in cattle was discovered in Britain in November, 1986. It had been confirmed in 36,230 British cattle by mid-September, and all were destroyed.
Despite all of the questions surrounding the disease, Britain's Ministry of Agriculture, Fisheries and Food and many scientists contend that beef is safe to eat.
They note that people have been eating sheep, known for 250 years to carry a form of spongiform encephalopathy called scrapie, without becoming infected. They also say that stringent regulations on beef processing imposed since mad cow was detected have reduced the chance of infection from remote to impossible.
What is worrisome, however, is that the disease appears to have jumped species from sheep to cattle--something that had been thought impossible.
The disease is believed to have spread when infected sheep brain, nerve tissue and other offal were ground into cattle feed. The government banned the feeding of offal to cattle in 1988.
There have been no reported cases of humans being infected by eating beef.
But since the infectious agent still has not been identified, Richard W. Lacey, writing a recent issue of the Ecologist, called reassurances about the safety of beef "perilous."
Lacey, a microbiologist at the University of Leeds and an outspoken critic of the safety of British food, has been campaigning for greater public concern.
"The crucial question is: Is British beef safe? And the answer is that it is not," he said in an interview.
"My advice, very emphatically, is that people should not eat British beef, sausages . . . and definitely not steak."
Most people are not listening.
Public concern peaked a year ago when several cases of spongiform encephalopathy in cats, thought to be spread through infected pet food, added to fears that the disease was crossing species boundaries.
A case of mad cow disease in a calf born in March suggested that the disease could be transmitted from mother to calf. If so, it means that many more animals could be infected and that halting the spread would be that much more difficult.
However, beef sales have just about recovered from the 25% plunge they took last year, said David Lewis, spokesman for the Beef and Livestock Commission.
Several British schools that dropped beef from the menu are serving it again for lunch. People avoided beef dishes in restaurants for a while but they have regained their appetites, restaurants report.
"There was a bit of a drop-off when the news broke, but no one seems to be paying much attention anymore," said Graham Middleton, the maitre d' at Joe's Cafe, a chic restaurant in South Kensington.
The United States has blocked the importation of live cattle from Britain, but will permit imports of beef from herds certified as disease-free. Britain's 11 partners in the European Community import beef from herds certified free of mad cow disease for two years.
The Beef and Livestock Commission insists that restrictions on British beef are absurd and that the disease is a worldwide problem. It says Britain ultimately has been hurt by its excellent testing procedures.
No cases of the disease have been reported in the United States. Ireland has reported 43 cases, Switzerland nine and France one.