The U.S. Department of Agriculture continues its investigation into a case of mad cow disease at a farm near Yakima, Wash. It's the first case of the fatal disease in the United Sates. Officials believe it's an isolated case.
Beef from the plant that processed the infected animal has been recalled, with shipments traced to California, Oregon, Nevada, Washington, Hawaii, Alaska, Montana, Idaho and Guam.
USDA officials maintain beef products are safe, saying if consumers have already eaten recalled beef, they shouldn't worry. Only brain, spinal cord and nervous system tissue -- which is separated from beef at processing plants -- can transmit the disease to humans.
Despite USDA assurances, more than a dozen nations, including top buyers of U.S. beef -- Japan, Mexico and South Korea -- have halted imports.
The infectious agent that causes mad cow disease, also known as BSE (bovine spongiform encephalopathy), can lead to a fatal disease in people. Research suggests humans contract a form of the disease, known as variant Creutzfeldt-Jakob, by eating beef products contaminated with BSE. Cows can get the disease when they're fed ground-up carcasses of cows with BSE. The U.S. government bans feeding animal parts to animals.
Earlier this year, a single contaminated cow was found in Canada, but investigators are still unsure how that animal became infected.