A strange, virus-like infection that has killed thousands of cattle in Britain and spread panic throughout the European Community about the safety of British beef may one day turn up among herds in the United States, veterinary experts said last week. There is also concern that some American cows may already be contaminated.
At the same time, health officials have found no cases of the disease that have been transmitted from infected cows to people, and experts say they do not currently believe such infections are likely.
"I don't think there is any danger in consuming British beef," said Clarence J. Gibbs Jr. of the National Institute of Neurological Disorders and Stroke.
Yet the British outbreak, which began in 1986 and has killed more than 14,000 cattle, has led a growing number of countries, including the U.S. and the U.S.S.R, to block imports of British beef and beef products.
The bans have been sparked by consumer fears that "mad cow disease" might pose a hazard to people eating contaminated meat.
The cow disorder, called bovine spongiform encephalopathy or BSE, apparently is spread by a "slow virus" similar to the one that causes the human brain illnesses kuru and Creutzfeldt-Jakob disease (CJD). In both the cow disease and the human disorders, the infection leads to a slow, lethal degeneration of the brain, leaving behind holes in the brain tissue that give it the appearance of a sponge. Symptoms of the disease usually occur long after exposure to the infectious agent.
D. Carlton Gajdusek, a 1976 Nobel Prize winner who works at the National Institute of Neurological Disorders and Stroke, discovered kuru among a Papua New Guinea tribe in 1959. He showed that the infectious agent spread among women and children who ceremonially consumed the brains of dead relatives as they prepared the bodies for burial.
Researchers later learned that kuru and Creutzfeldt-Jakob, a rare disorder that strikes some 250 Americans a year, were the same disease -- suggesting that Creutzfeld-Jakob was also caused by an infection. In 10 percent of the cases, however, the disease appears to be inherited.
The agent that causes these human diseases is called an "unconventional virus" because it appears to lack any genetic material and grows extremely slowly. So far, the infectious agent has not been identified, but researchers have linked the two human diseases with a similar neurological disorder in sheep, called scrapie, that causes the same kind of brain degeneration and can be spread from sheep to sheep.
The British cow epidemic is believed to have started more than a decade ago whenBritish cattlemen began feeding their cattle a protein supplement made from sheep "renderings" -- the waste products left over from butchered sheep. Some of the sheep used to make renderings had died from scrapie.
No one knows whether the same infectious agent that causes scrapie in sheep also causes kuru and Creuztfeldt-Jakob disease in people -- or whether the scrapie agent also causes mad cow disease in Britain.
The British experience suggests that the agent has moved from sheep to cows, though that has yet to be proved. It is not known whether the agent first had to change to do that. There also were reports that it had jumped from cows to two domestic cats. Whether the cow version of the agent can jump from cows to humans is unknown.
Several years ago, U.S. cattlemen also began using sheep renderings as a protein supplement, and some of it may have been infected with scrapie, said Richard Marsh, a professor of veterinary science at the University of Wisconsin at Madison and a scrapie expert.
No American cattle have been diagnosed with this disorder. Signs of the disease, however, may not turn up for three to eight years because of the long incubation period between the time an animal is infected and the appearance of symptoms. No tests exist that can detect a scrapie infection in a sheep or cow; diagnosis is made after death by examining the animal's brain.
The chances of widespread contamination are much less in the U.S. than in Britain because American cattlemen use smaller amounts of sheep renderings than do British cattlemen, said Gary Cowman, associate director of science and technology with the National Cattlemen's Association in Denver.
What's more, there are fewer sheep in the United States -- 11 million -- compared to 38 million in Britain, and far fewer of the U.S. sheep carry scrapie. All this lowers the risk to American cattle.
Nevertheless, the National Renderers Association and the American Protein Producers Industry, two industry groups, recommended in December, 1989, that feed producers no longer use sheep products in cattle food. "This is a precautionary step that we are taking," said Philip Kimball, president of the renderers group.
The National Cattlemen's Association, as well as the U.S. Department of Agriculture, have established an active education and surveillance program for the first signs of the disease in the United States.
"We are certainly tracking it as a high priority," Cowman said. But the association has stopped short of recommending against the use of renderings in cattle feed.
There is anecdotal evidence that some American cattle already have been infected. In 1985, a Wisconsin mink rancher changed his feed to include bone meal and meat from so-called downer cattle, animals unable to stand because they are dead or dying. Some of the mink receiving this feed developed a brain disorder similar to BSE, Marsh said.
"We inoculated two calves with the mink brains to see if it was transmissible to cattle. It was," he said. "We think we have fairly good circumstantial evidence that mink encephalopathy can come from cattle, and that implies that we have a scrapie-like disease in cattle in the United States that we did not recognize."
Nevertheless, scientists see little health risk to humans.
"We are talking about scrapie disease, which is a natural disease of sheep and goat, that is now causing disease in cattle," said Gibbs of the neurological institute. "Sheep and goat have been going to market for more than 200 years with scrapie, and people have not been afraid of that."
A number of epidemiologic studies in several European countries have looked for a link between scrapie infections in sheep and human disease, but none have been found. Nor is there any correlation between scrapie and the incidence of Creutzfeldt-Jakob disease.
At this point, the major consequences of an outbreak of the brain disease in cattle in the U.S. would be economic. There are 10 times more cattle here than in Britain -- some 118 million head in the U.S. compared to 11 million in the United Kingdom. Some of the economic loss would be due to the loss of cattle.
But a large part of the cost would be due to consumers' fears that the beef was not safe. As Cowan said: "It would be a catastrophe, economically. The real threat is consumer perception."
Even if the beef was deemed safe to eat, consumers might react as they did to announcements of the danger of alar, a chemical used to treat apples.
In Britain, consumers rejected government pronouncements that the beef was safe, and more than 1,000 schools have dropped home-grown meat from their menus.