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Holstein cows feed on a farm in Sunnyside, Washington. After years without an outbreak, a dairy cow in California was found to have mad cow disease. - 

David Brancaccio: For the first time in six years, a cow in the U.S. has tested for so-called mad cow disease. While officials say the dairy cow identified in California at no time entered the food supply -- and scientists say the disease cannot spread through milk -- the price of cattle futures has moved sharply downward. Korea, Japan, and Europe today all confirmed their commitment to keep buying U.S. beef. Things were very different when the first U.S. case of mad cow disease was discovered in 2003.

Marketplace's Nancy Marshall Genzer explains what's changed since then.

Nancy Marshall-Genzer: The first case of U.S. mad cow disease caused our beef exports to drop by $3 billion in 2003. Some countries banned our beef. This time around, the biggest reaction is from South Korea. The government is still allowing imports of U.S. beef. But two grocery store chains have stopped selling it temporarily. They say that's just due to, quote, customer concerns.

Economist Martyn Foreman follows the beef business for Doane Advisory Services. He says U.S. beef is now inspected for mad cow disease, so tainted cows stay out of the food supply. And material from slaughtered cows is no longer allowed in cattle feed. The last case of U.S. mad cow disease was in 2006. And Foreman says markets shrugged it off.

Martyn Foreman: The export market continued on. We didn't really slow things down too much. The beef didn't enter the food supply which is good news.

The U.S. agriculture department says this latest case of mad cow disease doesn't pose any threat to the food supply. And milk from infected cows won't spread mad cow disease.

In Washington, I'm Nancy Marshall-Genzer for Marketplace.