Mexico became the latest country to bar the import of U.S. beef over fears of mad cow disease (searchas stocks in meat-packing companies and restaurant chains fell quickly Wednesday.

Federal officials sought to calm the nation about the fear of mad cow, which officials announced Tuesday was found for the first time in the United States. The infected cow came from a farm near Yakima (search), Wash., and tested positive for the brain-wasting disease.

"We do not believe there is a risk to human health from this particular situation," Agriculture Secretary Ann Veneman told Fox News.

But foreign countries and investors remained nervous.

In the first hour of trading on Wall Street, the market signaled concern over decisions by several countries to cut off U.S. beef imports, and the potential that the reports might turn consumers away from buying hamburgers and steak.

Early losers included several companies with business heavily reliant on beef consumption, notably McDonald's Corp. and Wendy's International and meat processing giant Tyson Foods.

The decision by Mexico to stop imports of U.S. beef follows similar decisions by Chile, Russia, Japan, South Korea and Taiwan. Japan is the largest overseas market for U.S. beef.

In Brussels, the European Union (search), which already bans much U.S. beef because of fears about growth hormones, said it would not take any additional measures against U.S. beef.

Antonia Mochan, a spokeswoman at the EU's executive Commission, said the United States was already classified as an "at-risk country" as part of the sweeping EU measures adopted following Britain's mad cow crisis, which began in the late 1980s and spread across western Europe. Under those restrictions, imports of specific risk products, such as brains, are banned.

Agriculture Department officials and cattle industry executives tried to allay fears that American beef supplies had become infected, saying the U.S. inspection system was working effectively: The farm where the cow originated has been quarantined and officials were 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.

USDA officials announced early Wednesday that Vern's Moses Lake Meat Co. in Moses Lake, Wash., is voluntarily recalling approximately 10,410 pounds of raw beef that may have been exposed to tissues containing mad cow. They said the beef was produced on Dec. 9 and shipped to several establishments for further processing and is being recalled "out of an abundance of caution" even though it "would not be expected to be infected or have an adverse public health impact."

The department's Food Safety and Inspection Service said it is continuing its investigation to ensure that all the recalled beef is correctly identified and tracked, but gave no further details immediately.

There was no answer at the telephone number listed for Vern's Moses Lake Meat Co., after the recall, which was announced in the wee hours of the morning.

"We remain confident in the safety of our food supply," Veneman said Tuesday in announcing the first suspected U.S. case of mad cow disease.

Veneman also assured Americans that no foul play was suspected.

"This incident is not terrorist-related," she said.

Mad cow disease eats holes in the brains of cattle. It sprang up 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 so far has killed 143 people in Britain and 10 elsewhere, none in the United States.

Veneman said the risk to human health in this U.S. case was "extremely low."

Nonetheless, U.S. 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, Ariz., 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 U.S. needs to be far more proactive in protecting the American food supply," said Michael Hansen, a senior research associate. "We are very concerned that the diseased animal made it into the food supply and that the processing plants could be contaminated."

The disease was found in a Holstein cow, which could not move on its own, from a farm in Mabton, Wash., about 40 miles southeast of Yakima. It tested preliminarily positive on Dec. 9. Parts of the cow that would be infected -- the brain, the spinal cord and the lower part of the small intestine -- were removed before the animal went to a meat processing plant.

Samples from the cow were sent to Britain for confirmation of the preliminary mad cow finding, Veneman said. The results will be known in three to five days, she added. Consumers can get daily updates by reading the department's Web site or by calling 1-866-4USDACO.

Many residents of Mabton (search) -- population 2,045 -- were protective of local cattle owners Tuesday and unwilling to discuss the matter with reporters, who were turned away from businesses and farms.

The apparent discovery of mad cow disease comes at a time when the U.S. beef industry is flourishing, in part because imports from Canada dried up after a single case of the disease was found there last spring.

A USDA Choice sirloin steak sells for more than $6 per pound, compared with about $4 per pound a year ago. The price of pound of ground beef is $2.04, up from $1.84 last year.

"The beef cattle industry has just had a resurgence of growth," said Senate Agriculture Committee Chairman Thad Cochran, R-Miss. "This is going to be a setback."

Some American consumers said Tuesday they weren't ready to find something else for dinner.

"We're beef eaters," said Carrie Whitacre of Omaha, Neb. "Plus we're not going to get beef from Washington state here anytime soon."

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," DeWaal 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."

With an election year approaching, the news concerned some in Congress. Rep. Tim Holden, D-Pa., a member of the House Agriculture Committee, said he expected lawmakers to hold hearings when they return to Washington in late January.

But another member of the Agriculture Committee rushed to support the beef industry.

Rep. Cal Dooley, D-Calif., said, "People I think should not be frightened to have their prime rib on Christmas Eve."

The Associated Press contributed to this report.