Search continues for Canadian cows in infected herd

Three weeks after the discovery of the United States' first known case of mad cow disease, officials have found only 14 of the 81 cows that came into the country from Canada with the infected Holstein.

The diseased cow, found at a farm in Mabton, Washington, was traced to the farm of Wayne and Shirley Forsberg in Alberta, Canada.

The US Department of Agriculture is working on tracing the rest of the herd that entered the US.

"It's a paper trail," said a department spokesman. "We look at import documents, health certificates, farmer sales and shipping orders."

Finding all the cows is the key to ensuring mad cow disease does not spread to humans. It also helps to reduce the number of cows that have to be destroyed as a precaution.