A growing number of countries are banning imports of American beef after the US announced its first suspected case of "mad cow" disease.
The diseased cow is undergoing tests
The bans came within hours of news about the single suspect cow, in the north-western state of Washington.
The move was led by Japan, which bought $800m of US beef last year, and Canada later followed with a partial ban.
Officials are trying to trace the cow's birth herd and find out where it may have been infected.
Mexico, Brazil, Russia, Ukraine, and a number of Asian counties from Singapore to South Korea were among those who moved at once to bar imports of US beef.
Japan's decision is a major blow because it accounts for one-third of the total US exports.
BOVINE SPONGIFORM ENCEPHALOPATHY
First surfaced mid-1980s
Can pass to humans through infected beef products
Human form of disease called vCJD
vCJD has killed 137 people, mainly in the UK
Canada's Agriculture Minister, Bob Speller, said the ban did not affect cattle destined for immediate slaughter, boneless beef from
cattle under 30 months of age and dairy products.
Shares in hamburger giant McDonald's Corp fell by about 5% on the New York stock exchange, although the company said its supply chain was not linked to the suspected "mad cow" disease case.
Details of the case were revealed by US Agriculture Secretary Ann Veneman, who said a Holstein cow had tested positive on 9 December - but she insisted the country's beef was safe.
Tissue samples from the suspected cow are being studied in the UK, which suffered a devastating outbreak of mad cow disease, or BSE (Bovine Spongiform Encephalopathy), in the mid-1990s.
The results will be known in a few days' time.
The US Agriculture Department has been trying to identify the birth herd of the four-year-old cow.
Chief veterinarian Ron DeHaven said that because BSE was usually transmitted through contaminated feed and had an incubation period of up to five years, it was "important to focus on the feed where she was born".
The list of countries imposing bans or temporary suspensions of beef imports has steadily grown.
COUNTRIES THAT HAVE BANNED US BEEF
Canada - partial ban
BBC business correspondent Mark Gregory says the loss of Asian markets in particular is a huge blow to the US beef industry, which is worth $38bn a year.
Our reporter says that a collapse of US domestic demand for beef would be the industry's ultimate nightmare, since 90% of US beef is consumed at home.
BSE has been linked to new variant Creutzfeldt-Jakob Disease (vCJD), a human brain-wasting disease.
The farm near Yakima, Washington, where the cow was found, has been quarantined.
"We remain confident in the safety of our food supply," Ms Veneman told reporters - adding that she planned to serve beef on Christmas Day.
The dairy cow had been sick or injured and was never destined for the public food supply, Ms Veneman said.
TOP IMPORTERS OF US BEEF
4. South Korea
5. Hong Kong
Source: US Meat Export Federation
First diagnosed in Britain in 1986, BSE affected 178,000 British cattle and resulted in the eventual destruction of 3.7 million animals.
It cost British farming billions of dollars as countries around the world banned British beef.
Correspondents say the US beef business has been booming - partly due to the popularity of the protein-rich Atkins diet.
The European Union said it was keeping a close eye on the situation, but it has anyway banned most US beef for many years because of growth hormones in the meat.