Johanns said the new suspected case involved an older beef animal which was chosen for testing because it was a "downer" animal that could not walk when it arrived at the slaughterhouse.
"This animal was a downer animal and did not get into the food or feed chain.
There just is no risk whatsoever," Johanns told reporters in a hastily called news conference on Friday evening.
The government refused to disclose any information about the suspect animal's origin or where it was slaughtered. "It was getting up in age. It was a beef breed," said John Clifford, chief veterinarian for the Animal and Plant Health Inspection Service.
It is not unusual to have conflicting test results for BSE, cattle experts said.
The USDA said the suspect animal had tested positive for BSE in a rapid, preliminary test in November. When it was retested with a more sophisticated technology, the animal was found free of the disease.
USDA's Inspector General earlier this week asked department scientists to retest the suspect animal, and two others, using yet a third kind of technology known as the "western blot" test. That test showed the beef animal was infected with the brain-wasting ailment, Johanns said.
