The first case of U.S. mad cow disease in six years has been discovered in a dairy cow in central California, though regulators say U.S. trade should not be impacted. 

The U.S. Agricultural Dept. confirmed the case of bovine spongiform encephalopathy (BSE), which marks the fourth discovery in the U.S.

The animal’s carcass is being held under State authority at a rendering facility in California, where it will be destroyed.

While live-cattle futures plunged Tuesday afternoon on initial reports of the discovery, the USDA said U.S. trade should not be impacted.  The agency, which found the case as part of its “targeted surveillance system,” said the cow was never presented for slaughter for human consumption and therefore presented no risk.

The meat “at no time presented a risk to the food supply or human health,” according to a statement released by USDA chief veterinary officer John Clifford. Despite being a dairy cow, he said milk does not transmit BSE.

It also in no way affects the United States’ status by the World Organization for Animal Health as a “controlled risk” country for mad cow disease and should therefore not affect U.S. trade, he said. 

Last year, U.S. beef exports climbed 50% to $2.04 billion, but they are still far from the $3.8 billion in overseas shipments from before the disease was first discovered in the U.S. The first case was discovered in December 2003 in Washington State. Two other cases popped up in 2005 and 2006.

Live-cattle futures on the Chicago Mercantile Exchange closed down by 3 cents, or their one-day limit for price movements, after initial reports of the mad cow discovery surfaced. Shares of fast-food chains McDonald's (MCD), Wendy’s (WEN) and Sonic (SONC) all ticked lower late Tuesday.

The U.S. has longstanding safeguards to protect humans and animals against mad cow disease, including so-called specific risk materials, which are parts identified in animals that are most likely to contain the BSE agent if it is present in the animal.

The Agriculture Dept. bans all nonambulatory cattle, or those that are too sick or weak to stand on their own, from being slaughtered and entering the human food chain. It also has a ban on ruminant feed, which helps stop the spread of the disease in the cattle herd.

"Evidence shows that our systems and safeguards to prevent BSE are working, as are similar actions taken by countries around the world,” Clifford said.

In 2011, there were 29 worldwide cases of mad cow disease, a dramatic year-over-year decline and a 99% drop since the peak in 1992 of 37,311 cases.

Samples from the central California cow were tested at USDA’s National Veterinary Services Laboratories in Ames, Iowa. Results found that the cow was positive for atypical BSE, a very rare form of the disease not generally associated with an animal consuming infected feed.

The USDA said it is sharing the results with international animal health reference labs in Canada and England, which have official World Animal Health reference labs with extensive experience diagnosing this rarer string of the disease.  

The agency will also conduct a detailed investigation in conjunction with California animal and public health officials and the U.S. Food and Drug Administration.  

"BSE is a progressive neurological disease among cattle that is always fatal,” Clifford said.

He noted that affected animals may display nervousness or aggression, abnormal posture, difficulty in coordination, decreased milk production, or loss of body weight despite continued appetite.

Clifford said the USDA remains confident in the health of the national herd and the safety of beef and dairy products.

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