Just four days after discovering the nation's first case of mad cow disease, the United States has lost nearly all of its beef exports, with more than 20 countries banning imports of American beef.

Gregg Doud, an economist for the National Cattlemen's Beef Association says the country has lost roughly 90 per cent of its export market since the single mad cow case was found last week.

At today's market level, the U.S. stands to lose at least $6 billion US a year, Doud estimates.

Japan, South Korea and Mexico -- three of the biggest buyers of American beef -- all cut off all imports last week when the U.S. government announced it had found a cow in Washington State sick with the brain-wasting illness. An international lab in England confirmed the results Thursday.

The European Union already bans most U.S. beef because of concerns over the use of growth hormones by U.S. beef producers.

Canada only has a partial ban in place. That's because its beef industry was heavily damaged when the U.S. shut down beef imports after a lone infected animal was found on a cattle ranch. As a result, it wants to set an example of how countries in this situation should be treated.

The infected animal had been slaughtered. However, there was no threat to the food supply because the parts of the cow's body which harbour BSE -- the brain, spinal cord and lower part of the small intestine - were removed before processing, officials say.

The infected cow had two calves. Those animals were quarantined Friday. Officials say it isn't likely their mother infected them, but the potential is there and they will likely be slaughtered.

The most likely way for cattle to become infected is by eating feed that contains brain or spinal tissue from an infected animal. Such feed has been illegal in the U.S. since 1997, but the U.S. Food and Drug Administration admits there have been two major violations.

Mad cow, formally known as bovine spongiform encephalopathy, is caused by a damaged protein called a prion that eats holes in a cow's brain. It is related to the human disease, variant Creutzfeldt-Jakob disease.

During a mad cow outbreak in Britain in the 1980s, 143 people died of vCJD, apparently after eating infected meat.