In its quest for bigger, faster-growing animals, the American livestock industry has turned to feeds boosted with protein derived from other animals for more than 25 years.
But now the possible link between those proteins and an aberrant protein that attacks brain cells in sheep, cattle, mink and man has raised new concern about the use of meat scraps in animal feed.U.S. officials stress that no cattle here have turned up with bovine spongiform encephalopathy (BSE), or "mad cow disease," since an outbreak of BSE in Britain resulted in a 1989 ban on imports and increased surveillance of U.S. herds containing earlier British imports.
Nor has there been any increase in the number of diagnosed U.S. cases of the human form of the disease, Creutzfeldt-Jakob Disease.
"We are confident we're safe," Agriculture Secretary Dan Glickmen said this week, noting that increased spot inspections of suspicious animals at major slaughterhouses were being conducted.
"BSE doesn't pose a threat in the U.S., but as a precaution people should not eat cattle brains or sheep brains," said Pennsylvania State University veterinary virologist Anthony Castro, who adds he has seen no evidence that the disease can be passed to humans.
Yet some scientists and food safety groups are concerned that the type of disease and the path of infection found in Britain might be different in the U.S. and require different protective steps.
In light of the new evidence from Britain that BSE may be transmissible to humans, the U.S. may move ahead with a proposed ban on using rendered products from mature sheep and goats, which has been in limbo since 1994. Critics say such a step would be insufficient to protect animal and human health.
Sheep in both Britain and the U.S. suffer from a version of the brain disease called scrapie, and it is believed that the disease in British cows was passed on when dairy cattle were fed rendered sheep parts as a protein supplement.
U.S. industry officials say cattle are not fed proteins from rendered sheep, although government officials note that there's no way to be certain of this since there's no way to test a rendered product for sheep ingredients.
Proteins from rendered cattle are routinely added to the feed of other cows, however.
"Animal agriculture is a grisly, grisly business," said Scott Williams of the Farm Animal Reform Movement. "You may imagine a contented cow chewing on its cud, but that cow is more likely being fattened on the intestines of its slaughtered comrades."
Veterinary scientist Richard Marsh of the University of Wisconsin first detected a species jump in a neurological disease in 1985 at a mink farm. There, animals had been fed byproducts from "downer cows" - animals which mysteriously fall down and never get up.
When Marsh and colleagues went on to inject cows with brain material from scrapie-diseased sheep, they developed the downed-cow symptoms rather than the more aggressive, rambling behavior of British cows with BSE.
Marsh argues that the USDA surveillance program looking only for cows exhibiting the British symptoms completely ignores the possibility that the disease is different here.
Each year, some 100,000 cows in the U.S. succumb to the downer cow syndrome, and most end up in rendering plants and products ranging from protein supplements to gelatin used to make medicine capsules.
Marsh has convinced some colleagues that the diagnostic net for cows should be cast wider, but so far neither the USDA nor the Food and Drug Administration, which regulates animal feeds, has concluded there is justification either for wider testing or a ban on feeding cow remains to other cows.
The crisis for British beef peaked earlier this month, when a scientific advisory panel declared it could find no credible reason other than contact with BSE-diseased beef for 10 relatively young Britons contracting Creutzfeldt-Jakob Disease. CJD strikes one in a million people, usually an older person.
All spongiform encephalophy diseases appear to progress by some agent attacking and destroying cells in the brain, causing confusion and dementia, creating holes in brain tissue and then physical debilitation and death. There is no known treatment or even a proven test for the disease before death occurs.
One leading theory about the diseases is that they are caused by an infectious agent called a prion, a mutated protein particle that is neither a bacteria nor a virus.
Nor do scientists know for sure how long after infection the diseases become evident. But they believe CJD may require an average of 10 years to incubate while BSE may be dormant for five years or more.
Very little is known about the diseases' transmission. CJD is known to have been spread to people through contaminated injections of human growth hormone, but there's no solid evidence that it can be passed through the digestive system.
Food safety advocates say that if doubts about protein-enhanced beef were to spread in the U.S. as they have it Britain, "you'd probably see the practice of animal cannibalism disappear almost overnight," said Ronnie Cummins, national director for the Pure Food Campaign, an advocacy group that petitioned FDA to end the feeding supplements three years ago.
"The rendering industry in this country is worth about $1.7 billion, while the beef industry itself is worth about $30 billion, so I don't think there's much question which would win out," Cummins said.