With the discovery of mad cow disease in dairy cow in California, two South Korean retailers have suspended sales of U.S. beef, according to the Associated Press.

The U.S. Department of Agriculture announced Tuesday it detected the nation’s fourth-ever case of mad cow disease – the first instance since 2006.

South Korea is the fourth-largest importer of U.S. beef. One of the supermarket chains told the AP they stopped selling U.S. beef because consumers were worried, not because there were any “quality issues” in the meat.

The USDA says the single cow does not pose a threat to the U.S. food supply. It was identified through random testing.

“The beef and dairy in the American food supply is safe and USDA remains confident in the health of U.S. cattle,” said U.S. Agriculture Secretary Tom Vilsack in a statement posted on the USDA’s website. “The system and safeguards in place to protect animal and human health worked as planned to identify this case quickly.”

The last time a cow was confirmed to have the disease was in 2006.

Mad cow disease, or bovine spongiform encephalopathy, attacks a cow’s central nervous system. There is no way to test for the disease in a live animal.

The disease is part of group of “transferable” maladies that renders the afflicted animal or human incapacitated. In humans, the disease is called variant Creutzfeldt-Jakob disease (vCJD).

Symptoms of the disease in a cow include a change in disposition (aggression or nervousness), inability to rise from the floor, uncoordination or wasting. There is no treatment or cure.

If the disease is transmitted to a human, the disease manifests as vCJD and the symptoms resemble those of dementia and Alzheimer’s disease. There can be an incubation period of more than eight years.

Fewer than 200 people worldwide have ever contracted this sub-type of CJD. There have been no known cases in the U.S., according to the National Institute of Health.

According to the Government Accountability Project, food-borne illness sickens more than 76 million people, causing 725,000 hospitalizations and 5,000 deaths in the U.S. every year.

Sarah Damian, a spokeswoman with the GAP, said in a 2010 op-ed that the problem lies with the enormity and highly mechanized nature of our food production process.

She says steroids and anti-biotics pumped into their feed daily can do little to mitigate the problems of overcrowding in food pens – and is probably hazardous to humans.

She cited as an example an Iowa chicken farm owned by Austin DeCoster that houses 80,000 chickens in one house. Overcrowding breeds sick animals, she says. DeCoster testified at a 2010 congressional hearing after the mass recall of eggs from his farm.

DeCoster had a well known history of salmonella poisoning from the eggs on his farm. In 1987, eggs from one of his farms killed nine people. In 2010, thousands were sickened by his eggs.

“Why are people surprised at the result? It’s time to get a clue,” Damian said.

USDA Chief Veterinary Officer John Cliford said in a statement the detection of the afflicted animal “should not affect U.S. trade.”