Reuters is reporting that the U.S. Department of Agriculture (USDA) has a possible second case of mad cow disease in the United States. The timing could not have been worse -- and I am not talking about that savory steak I was getting ready to grill after writing today. The Bush administration and beef producers were working hard to get export markets reopened for American beef.

It was two days after Christmas last year that a Holstein cow in the state of Washington was slaughtered and found to have mad cow disease. It was only one steer! Thirty export markets immediately closed to American beef exporters. Tyson Foods (NYSE:TSN), owner of the largest beef producer in the United States, is still talking about the negative impact closed exports markets are having on its current earnings.

Last year, reports of mad cow disease put pressure on beef-sensitive stocks such as Taco Bell parent Yum! Brands (NYSE:YUM), McDonald's (NYSE:MCD), and Outback Steakhouse (NYSE:OSI). Companies devised ways to quickly separate themselves from the problem. For example, Wendy's (NYSE:WEN) reported it did not get beef from the state of Washington. Hamburger chains were quick to note they served chicken sandwiches too.

Today's report is the third time an inconclusive result has caused the USDA to start further testing to determine whether mad cow disease is present. The last inconclusive test turned out to be negative.

Producers of "the other white meat" such as Smithfield Foods (NYSE:SFD) may see a short-term spike in business as the USDA testing continues, but it is unlikely consumers will search out Krispy Kreme Doughnut (NYSE:KKD) products trying to find a beef alternative -- although Krispy could use a boost in business very badly.

In commodity trading, cattle for December delivery were limited down three cents a pound (the one-day maximum price change allowed) -- but have bounced back slightly. So far, Tyson and McDonald's are down only 2%. That muted response is probably attributable to the USDA diligence in responding to last December's case.

Tuesday (my birthday!) I wrote that Tyson Foods Is Looking Cheap. Although 25% above its 52-week low, the company is making good money in chicken (it is No. 1 in that market), pork, and value-added products. For investors looking for opportunities, Tyson is a good place to start your research.

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Fool contributor W.D. Crotty does not own stock in any of the companies mentioned but does love Tyson foods (note the small "f").