Japan, South Korea, Malaysia, Taiwan and Singapore have temporarily halted imports of American beef, a move likely to be followed by other major US customers.
Japan on Wednesday halted the issue of import permits. "That means for now, imports have been banned," said agriculture ministry spokesman Hiroaki Ogura.
South Korea's agriculture ministry followed suit, suspending quarantine clearance for all US beef and beef products.
Singapore also suspended US beef imports as part of "stringent precautions".
"Any country found to have BSE will be immediately suspended from exporting beef to Singapore," the government said.
Malaysia has taken similar action.
"We have suspended the import with immediate effect since it is a health risk to humans and other cows."
"It is a precautionary measure," Hawari Husayn, director-general of the Verterinary Services Department said.
Japan is the number one export market for US beef, accounting for 32% of US exports in 2002, according to the US Meat Export Federation, followed by Mexico, Canada, South Korea and Hong Kong.
Australian shares rise
Australia's largest cattle company saw its share prices zoom on Wednesday after news broke of the suspected case in the US.
The Australian Agricultural Co rose 14 cents or 12.8% to 1.23 Australian dollars (91 US cents).
Australia is one of the world's largest beef producers, with an industry worth about 3.75 billion dollars (US$2.78 billion) in 2003.
"The shares are obviously moving up in line with the rumours coming out of the US about the first case of mad cow disease being diagnosed," Macquarie Equities private client adviser David Halliday said.
Blow to beef industry
The discovery of the brain-wasting disease in Washington state is likely to hit hard the $27billion US cattle industry, while raising serious concerns about food safety.
"A single Holstein cow from Washington state was tested as presumptive positive for BSE or what is widely known as mad cow disease," US Agriculture Secretary Ann Veneman said.
A sample from the diseased "downer" cow - one that is
Mad cow disease had caused huge
losses in Europe in the 1990
too sick to walk - was taken on 9 December and tested positive, Veneman added.
The sample has been flown by US military jet to an animal laboratory in England for additional confirmation. Those results will be out in three to five days.
The farm where the cow was found near Mabton, Washington, has been quarantined and officials are checking with other processing plants where parts of the animal were sent.
Mad cow disease, also called bovine spongiform encephalopathy, has never been found in the United States before.
Scientists believe humans can be infected by eating meat contaminated with diseased brain or spinal column material.
However, Veneman said these materials did not enter the human food chain as they were sent to a rendering plant to be made into pet food or for other uses.
The official urged American consumers not to panic. "I plan to serve beef for my Christmas dinner," she said. "The risk to human health from BSE is extremely low."
The only previous outbreak in North America was a single case of BSE at a farm in Alberta, Canada in May.
Canada said it was awaiting further tests before taking any action.
"It is premature," Canadian Food Inspection Agency veterinarian Brian Evans said. "There will be measures if the case is positive."
Cases of mad cow have been detected since 2001 in several countries, including four in the Czech Republic, one in Greece, one in Israel, seven in Japan, two in Luxembourg, five in Poland, 11 in Slovakia and three in Slovenia.
Mad cow disease has been linked to a fatal brain-wasting disease in humans and caused huge losses for the European beef industry in the 1990s.
Eating BSE-infected beef has been linked with a form of Creutzfeldt-Jakob disease (vCJD) and more than 130 people in Britain have died of vCJD since 1996 and at least four in France.
British farmers had destroyed nearly 3.7 million cattle because of the outbreak.