The discovery of a case of mad cow disease in a cow in California prompted two major South Korean retailers to immediately suspend sales of U.S. beef. Although it's the first such case in six years, and was found in a cow not intended for human consumption, the news set off fresh worries about the safety of American food. Scott Hurd is Associate Professor at the College of Veterinary Medicine at Iowa State University. He’s also former Deputy Undersecretary for Food Safety for the USDA. He explains what's behind the beef trade wars.