Testing indicated the presence of the disease in a cow that died on the farm where it lived, said John Clifford, the department's chief veterinarian. The department would not say where the farm was. The cow was at least 12 years old and died of complications during calving, Clifford said.
"It is important to note that this animal poses no threat to the human food supply, because it did not enter the human or animal food chains," Clifford said.
The department is conducting further tests and is sending a brain tissue sample to the internationally recognized laboratory in Weybridge, England, Clifford said.
Two other cases of mad cow disease have been confirmed in the United States. One was confirmed last month in a Texas cow that died in November. The other was in a Canadian-born cow discovered in December 2003 in Washington state.
In the latest case, the cow died on the farm where it lived, and a private veterinarian removed brain tissue for sampling, Clifford said.
However, testing options are limited in this case. Because the farm was remote, the private veterinarian who removed a brain sample used a substance to preserve the tissue. That means that only one type of testing, immunohistochemistry, or IHC, can be done, the official said.
The animal died in April, but the veterinarian forgot to send the sample to USDA until this month, Clifford said.
"While that time lag is not optimal, it has no implications in terms of the risk to human health," he said.
IHC tests returned conflicting results on the Texas cow. Use of the preservative means that the other tests commonly done when mad cow is suspected, initial rapid screening and Western blot, can't be done on this sample, the official said.
Clifford said it's possible to get different results, "depending on the slice of tissue that is tested."
The fatal brain-wasting disease is known medically as bovine spongiform encephalopathy, or BSE. In people, eating tainted meat products has been linked to about 150 deaths from a fatal disorder called variant Creutzfeldt-Jakob disease. Most of the deaths were in the United Kingdom, where there was an outbreak in the 1980s and 1990s.