Mad cow disease: Big deal abroad. US? Not so much.

[Editor's note: This story has been updated.]

The discovery of "mad cow disease" in a California dairy cow may set off alarm bells among some American consumers, but the biggest reaction may come from abroad.

On all three previous occasions after the United States made public a "mad cow" infection, many of the biggest importers of US beef closed their borders to the product, at least temporarily. After the first such announcement in late 2003, beef exports plunged so much that it took until last year before beef exports recovered completely. Some experts foresee an inevitable international reaction after Tuesday's announcement of a new case of mad cow disease or bovine spongiform encephalopathy (BSE).

"There's going to be some of that" border-closing, says Scott Hurd, associate professor at the Iowa State University's College of Veterinary Medicine in Ames, Iowa, and former deputy undersecretary of agriculture for food safety at the US Department of Agriculture (USDA)."There are some countries that are looking for any excuse to ban US products."

So far, international reaction has been mixed. The European Commission says it does not intend to impose any specific import measures and key beef importers Mexico, Japan, and South Korea have said they will continue imports. But two major South Korean grocery chains have decided to halt sales of US beef.

Russia has requested more information on the case and could consider temporary restrictions, one official said. In Taiwan, where US beef imports have already proved controversial because of growth additives, the national legislature postponed a scheduled discussion Wednesday on US beef imports.

For US consumers, the impact is likely to be far more muted. For one thing, beef consumption barely budged in the years after the 2003, 2005, and 2006 BSE incidents. For another, the system appears to have worked in the current case, according to initial reports. That would be good news for the beef industry, since more than 90 percent of its production is consumed domestically.

On Tuesday, the USDA announced that a dairy cow in California tested positive for an atypical and rare form of BSE. Officials stressed that the food supply was never threatened. Instead of going to a slaughterhouse, the suspect cow was diverted even before BSE testing to a rendering plant, where unfit cattle typically go to be turned into pet food, tallow, and other byproducts not for human consumption. At the plant, officials took samples of the cow and USDA scientists confirmed the presence of BSE before it could cause a problem.

"It wasn't anywhere near the food chain," says Mr. Hurd. Even if the cow had been used in the rendering plant, its spinal cord, brain, and other tissues considered susceptible of carrying the troublesome BSE proteins would have been removed and disposed of. Those precautions are also routine at slaughterhouses.

On rare occasions, BSE-tainted meat has been linked to disease and even fatalities among humans. It does not infect people through milk, however, according to the World Health Organization.

The world has made dramatic progress in controlling BSE since the number of BSE-infected cattle peaked in 1992 at 37,311 worldwide. Last year, officials recorded only 29 such cases. Besides this month's discovery, there have been three cases in the US since BSE-controls went into effect in 1997:  the 2003 case of a cow in Washington State imported from Canada; a Texas cow in 2005; and an Alabama cow in 2006.     

USDA officials don't appear to know what caused the current case. Typically, the disease is transmitted through infected feed. Largely by banning the use of certain parts of the cow (such as the brain) in cattle feed, many nations have gotten a handle on the problem.

By contrast, the current case in California involves a "very rare form of the disease not generally associated with an animal consuming infected feed," John Clifford, chief veterinary officer of the USDA, said in a statement Tuesday. He promised an investigation.

But in the end, scientists may not be able to pinpoint a cause or, for that matter, guarantee zero occurrences of BSE, Hurd says. "These types of diseases can occur spontaneously."

– Wire service material was used in this report.