Vietnam and Macau have become the latest countries to ban imports of US beef after America's first case of mad cow disease was confirmed.
The farm where BSE was discovered has been quarantined
They join more than 20 other nations and territories who have already slapped bans on beef from the US.
On Thursday, US officials confirmed that BSE had been discovered in a dairy cow from Washington state.
The animal is believed to have contracted the disease after eating contaminated feed.
The case of BSE, or bovine spongiform encephalopathy, was established beyond doubt after tests by British scientists backed up earlier findings by their US counterparts.
Experts have predicted the news will cost the previously-booming US cattle industry billions of dollars, as countries around the world rush to ban American beef imports.
The list already includes Japan, Mexico and South Korea - the three top importers of US beef.
The European Union has banned most US beef for many years because of growth hormones.
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
The US dollar and Japanese stock markets have fallen since the suspected case was announced, while cattle futures slumped as far as markets allow.
Shares in McDonald's have fallen by about 5% on the New York Stock Exchange, although the burger giant said its supply chain was not linked to the suspected case.
The US Department of Agriculture said it would issue more information about the test results later on Friday.
BSE has been linked to new variant Creutzfeldt-Jakob Disease (vCJD), a human brain-wasting disease.
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 pounds as countries around the world banned British beef.