America's first suspected case of BSE, or mad cow disease, has been discovered on a farm in Washington State.
The impact was evident almost immediately as several countries, including Japan, South Korea, Australia and Taiwan, halted US beef imports just hours after the Agriculture Department announced that a cow at a farm near Yakima, Washington, had tested positive for the brain-wasting disease. Japan is the largest overseas market for US beef.
Agriculture Department officials and cattle industry executives tried to allay fears that American beef supplies had become infected, saying the US inspection system was working effectively.
The farm where the cow originated has been quarantined and officials are tracing the movement of the cow from the farm to the slaughterhouse, and the flow of the meat to three processing plants in Washington state.
"We remain confident in the safety of our food supply," Agriculture Secretary Ann Veneman said, adding that the risk to human health was "extremely low."
Nonetheless, US beef producers worried that they could suffer heavily from a mad cow scare. Restaurants that serve beef also could be affected.
"I think it has the potential to hurt our industry," said Jim Olson, a rancher in Stanfield, Arizona, who owns about 150 cattle.
Consumers Union, the publisher of Consumer Reports magazine, called on the government to test more cows for the disease, formally known as bovine spongiform encephalopathy.
"The US needs to be far more proactive in protecting the American food supply. We are very concerned that the diseased animal made it into the food supply and that the processing plants could be contaminated," said Michael Hansen, a senior research associate.
The disease was found in a Holstein cow which tested preliminarily positive on 9 December. Samples from the cow were sent to Britain for confirmation of the preliminary mad cow finding, Ms Veneman said. The results will be known in three to five days, she added.
The apparent discovery of mad cow disease comes at a time when the US beef industry is flourishing, in part because imports from Canada dried up after a single case of the disease was found there last spring.
"The beef cattle industry has just had a resurgence of growth. This is going to be a setback," said Senate Agriculture Committee Chairman Thad Cochran.
Caroline Smith DeWaal, food safety director of the Center for Science in the Public Interest, said that while whole cuts of meat should be safe, there could be problems with ground meat, which can be mechanically stripped from the bone near an infected part.
"USDA needs to take swift action to insure that the meat that is found in hot dogs, hamburgers and those others doesn't pose a risk," she said.
The beef industry said there was nothing to worry about.
"The infectious agent is only found in the central nervous system tissue," said Patti Brumbach, executive director of the Washington State Beef Commission. "None of that made it into the beef supply. I think once consumers understand that the beef supply is safe, it should be a short-term concern."
BSE eats holes in the brains of cattle. It emerged in Britain in 1986 and spread through countries in Europe and Asia, prompting massive destruction of herds and decimating the European beef industry.
People can contract a form of mad cow disease if they eat infected beef or nerve tissue, and possibly through blood transfusions. The human form of mad cow disease has killed 143 people in Britain and 10 elsewhere, but none in the United States.