Japan, South Korea, Singapore, Malaysia, Taiwan, Thailand and Australia were the first to ban imports of beef, joined soon after by Russia, South Africa, Ukraine, Mexico and Israel.

A single, Holstein cow tested positive for mad cow disease on a farm near Yakima, Wash.

USDA secretary Ann Veneman said Tuesday it was too early to call the test results an isolated incident, although she stressed there is no indication of a larger problem with the U.S. beef supply.

Shares in hamburger giant McDonald's Corp.fell by about 5 percent on the New York stock exchange, although the company said its supply chain was not linked to the suspected "mad cow" disease case.

Washington Gov. Gary Locke said in a statement the results were "presumptive," and that scientific confirmation of the presence of bovine spongiform encephalopathy was due in three to five days from a testing center in England.