HELENA, Mont. (AP) – Five bulls from a Canadian herd that included a cow with mad-cow disease were shipped to Montana six years ago and have since been slaughtered, state officials said Wednesday.

None of the animals showed clinical signs of the disease, said Karen Cooper, spokeswoman for the Montana Department of Livestock. What became of the carcasses after slaughter was unclear.

Ron DeHaven, a spokesman for the U.S. Agriculture Department’s Animal and Plant Health Inspection Service, said the USDA believes it unlikely any of the bulls were infected with the disease, though he did not explain why. He said his agency is trying to find out what happened to the carcasses.

The slaughter would have occurred after the Food and Drug Administration had banned the use of animal parts in livestock feed. Given that fact, DeHaven said it is unlikely the meat would have been fed to other cattle.

He added that the chance of someone getting the human form of the disease from the five bulls is “immeasurably small.”

This is the first indication that the Canadian investigation into the disease known as BSE has reached into the United States.

Two weeks ago, Canadian officials disclosed that a cow in Alberta had been identified as having the disease. The United States immediately banned all imports of Canadian beef and cattle.

Canada has since ordered the slaughter of more than 1,700 cattle in its effort to determine the disease’s source. So far, 800 animals have tested negative, including animals that had been in the same herd as the infected cow.

Canadian authorities said Wednesday some of the bulls from Alberta apparently were subsequently resold in South Dakota and Montana.

Mad cow disease was first diagnosed in Britain in 1986. The human form of BSE is the fatal brain-wasting illness variant Creutzfeldt-Jakob disease. Scientists believe people get it by eating some meat products from infected animals.

AP-ES-06-04-03 1939EDT