No one knows how deer and elk in the wild or in captivity pass the disease among themselves, Mr. Herring said. An unknown number of American sheep are infected with scrapie, which, like mad cow disease, is a spongiform encephalopathy. It is not known if scrapie can harm humans who eat infected sheep tissue, but last autumn the fear of such transmission led the Agriculture Department to quarantine and order the destruction of a herd of sheep, exported to Vermont from Belgium. A few of the animals had been found to have developed a spongiform encephalopathy of unknown origin. The sheep's owners, who make cheese from the sheep mild, fought back.
Lawyers continue to argue over the fate of the remaining animals, which show no sign of illness.
Dr. Tom Pringle, a biochemist in Eugene, Ore., and independent researcher on transmissible spongiform encephalopathies, says American officials have not done enough to battle the disease. He says the United States should set up a system to track bovine material coming into this country and increase testing. Out of 900 million cattle, the Agriculture Department tested fewer than 12,000 sick cows for mad cow disease in the last decade. None were found to have the disease. France, with 5.7 million cattle, is now testing 20,000 animals each week and identified 153 infected animals last year.