First, a severe drought in the Southwest cut cattle herd numbers to their lowest level in more than 60 years. Then an intense controversy erupted over a common type of filler known as "pink slime," hurting ground beef sales. The industry was just regaining its footing when the word of the mad cow discovery came Tuesday.

"They say things happen in threes, so hopefully this is the last one," said Buck Wehrbein, who manages a feeding operation in Mead, Neb.

The infected dairy cow, only the fourth ever discovered in the United States, was found as part of an Agriculture Department program that tests about 40,000 cows a year for the fatal brain disease. The animal apparently acquired the infection from a random mutation, not from eating infected cattle feed.

It was the first new case of mad cow disease in the U.S. since 2006 and came just as beef exports finally were recovering from an outbreak in 2003. With billions of dollars at risk, the USDA and other government officials responded quickly, explaining consumers never were at risk because none of the animal's meat was bound for the food supply.

"It looks like that system is working, and for those of us in the business, that's a relief," Wehrbein said.