It was the first time the disease has been confirmed in a U.S.-born cow. The other U.S. case, confirmed in December 2003 in Washington state, was in a dairy cow imported from Canada.
The department's chief veterinarian, Dr. John Clifford, said Wednesday that the new case was identified and linked to the herd in Texas through DNA testing. He said the herd had been quarantined and that none of the infected animal's carcass entered the food or animal feed chain.
"The animal did not enter the human food chain. The safety of our food supply is not in question," Clifford said in a conference call with reporters. He said the government would not identify the cow's owner or the town it came from.
He said that given the cow's age, officials believe it probably was infected before the 1997 ban forbidding the use of cattle parts in cattle feed.
Eating the brain and other nervous tissue of an animal with the brain-wasting ailment is the only way the disease is known to spread. The Agriculture Department and the Food and Drug Administration are trying to trace the history of the herd's feed.
Officials also are trying to identify herd mates born within one year of the infected cow's birth as well as any offspring born within the past two years, Clifford said.
The Agriculture Department confirmed the case last Friday but had to wait for DNA analysis to confirm the cow's origin. Tracing the cow proved difficult because the animal's breed was mislabeled and its tissues got mixed with parts from other cows. It was killed at a pet-food plant in Waco, Texas, but the plant rejected using it.
Pets are not considered at risk from eating cattle remains, which are frequently ground up and added to their food as protein. Cattle remains are also allowed in feed for pigs and chickens.