There appears to be no risk to humans from the dairy cow discovered in California this week to have “mad cow” disease. That’s according to the US Department of Agriculture and the beef industry.

“It is important to reiterate that this animal was never presented for slaughter for human consumption, did not enter food supply channels, and at no time presented any risk to human health,” the USDA said in a statement this week.

But the case involving a 10 year-old Holstein raises questions about how such cattle themselves are fed, which critics say could be part of a dangerous cycle.

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Mad cow, known scientifically as bovine spongiform encephalopathy (BSE), is believed to be carried by animal feed made from cattle brains or spinal cord. Such feed is now banned in the US and other countries, but cases of BSE have continued to appear around the world.

The World Health Organization has called for the exclusion of the riskiest tissues (eyes and intestines as well as brains and spinal cord) from all animal feed to protect against the spread of mad cow disease.

Stanley Prusiner, who received the 1997 Nobel Prize in Medicine for discovering the protein associated with BSE has said the US should ban poultry waste in cattle feed as well.

“Unfortunately, the United States still allows the feeding of some of these potentially risky tissues to people, pigs, pets, poultry, and fish,” warns Michael Greger, director of public health and animal agriculture at The Humane Society of the United States.

“Cattle remains are still fed to chickens, for example, and the poultry litter (floor wastes that include the feces and spilled feed) is fed back to cows,” he writes on his Huffington Post blog. “In this way, prions – the infectious proteins that cause mad cow disease – may continue to be cycled back into cattle feed and complete the cow ‘cannibalism’ circuit blamed for the spread of the disease.”

Part of the problem, according to critics, is that only a tiny fraction of slaughtered cows (40,000 out of 35 million a year) are tested for BSE.

In this week’s instance, the cow (which was to be rendered into products other than meat for human consumption) had been unable to stand – a “downer” cow. This raised suspicions, so the cow was tested for BSE. This showed that the current inspection system works, say beef industry supporters.

According to the USDA, the infected animal discovered this week had “atypical BSE,” which means it most likely did not get the disease from eating infected cattle feed. Still, the USDA is tracking feed sources as a possible cause.

This instance of BSE in US cattle has caused some international ripples.

Indonesia suspended its imports of US beef Thursday, two major South Korean retailers pulled US beef from their stores this week, and South Korea’s agriculture ministry has stepped up inspections of US beef and requested detailed information on the case. Taiwan’s Department of Health is seeking permission from US authorities to visit slaughterhouses in the US, Taiwanese officials said Saturday.

“We will lift the ban as soon as the US can assure us its dairy cows are free of mad cow disease,” Indonesia’s Vice Agriculture Minister Rusman Heriawan told reporters.

“It could be one month or one year,” he said. “It depends on how long it takes to resolve this case.”

BSE is fatal to cows, and eating tainted meat has been linked to a similar disease in humans known as variant Creutzfeldt-Jakob disease (vCJD).

Prior to this week, there were three confirmed cases of BSE in cows in the US – in a Canadian-born cow in 2003 in Washington State, in 2005 in Texas, and in 2006 in Alabama.

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