A third herd of cattle has been placed under quarantine in the US state of Washington following the discovery of the country's first case of mad cow disease.
The US Agriculture Department's chief veterinarian Dr Ron DeHaven said at least some of the cows from the herds would have to be slaughtered.
He said 11 of the 81 cows believed to have been shipped from Canada with the infected animal had been accounted for.
At least 26 countries have placed restrictions on the import of US beef.
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DNA tests are being carried out to prove conclusively whether the infected cow - slaughtered on December 9th - came from a herd in Alberta, Canada.