John Clifford, deputy administrator of USDA veterinary services, said officials learned of the "inconclusive" test result late Friday. The carcass is being sent to USDA National Veterinary Laboratory in Ames, Iowa, for additional tests. Results are expected in 4 to 7 days.
Officials sought to downplay the potential gravity of the preliminary test, which they said wasn't unexpected given the test's sensitivity. The United States' beef trading partners had been notified and the company was given earlier notice, Clifford said.
"The inconclusive result does not mean we have found another case of BSE in this country," Clifford said. "Inconclusive results are a normal component of most screening tests, which are designed to be extremely sensitive so they will detect any sample that could possibly be positive."
"The USDA remains confident in the safety of the food supply," Clifford said.
Mad cow disease — known also as bovine spongiform encephalopathy, or BSE — eats holes in the brains of cattle. It sprang up in Britain in 1986 and spread through countries in Europe and Asia, prompting massive destruction of herds and decimating the European beef industry.
Clifford declined to identify the animal or its location until testing is complete, noting that it's "very likely" final testing could turn up negative.
"The animal in question didn't enter the food chain," he said. "If positive, we'll provide additional information on the animal and origins."
The Agriculture Department said the "inconclusive" reading was the first received by officials since they expanded national testing for the disease this month. More than 7,000 animals so far have been tested as part of the program that seeks to check about 220,000 animals over the next year to 18 months.
A form of mad cow disease can be contracted by humans if they eat infected beef or nerve tissue, and possibly through blood transfusions. Variant Creutzfeldt-Jakob disease, the human form of mad cow disease, so far has killed 100 people in Britain and elsewhere, including a Florida woman this week who was believed to have contracted the disease in England.
If the animal tests positive, it would be the second case of mad cow discovered in the United States. In December, a single Holstein on a Washington state farm tested positive, prompting some countries including Japan and South Korea to temporarily ban imports of U.S. beef.
The government last year conducted mad cow tests on tissues from 20,543 animals, virtually all of them cattle that could not stand or walk and had to be dragged to slaughter. After the nation's first case of the disease in December, the Agriculture Department initially doubled the number of animals to be tested this year to 40,000.
With many foreign governments still reluctant to ease bans on U.S. beef, the testing program was expanded at a cost of $70 million to include as many as 220,000 slaughtered animals, following recommendations from an international scientific review panel. About 35 million head of cattle are slaughtered each year in the United States.