"As part of our targeted surveillance system, the USDA's Animal and Plant Health Inspection Service confirmed the nation's fourth case of bovine spongiform encephalopathy," John Clifford, USDA chief veterinary officer, said in a statement.
"The carcass of the animal is being held under state authority at a rendering facility in California and will be destroyed. It was never presented for slaughter for human consumption, so at no time presented a risk to the food supply or human health. Additionally, milk does not transmit BSE."
Clifford said the United States has longstanding interlocking safeguards to protect human and animal health against BSE, including a USDA ban on specified risk materials -- parts of the animal that are most likely to contain the BSE agent if it is present in an animal -- from the food supply.
The USDA also bans all non-ambulatory cattle -- cows that fall to the ground while trying to walk -- from entering the human food chain. In addition, the U.S. Food and Drug Administration banned ruminant material such as the brains and spinal cords of cattle 30 months of age and older in cattle feed, Clifford said.
Samples from the animal in question were tested at USDA's National Veterinary Services Laboratories in Ames, Iowa, and confirmed the animal was positive for atypical BSE, a very rare form of the disease not generally associated with an animal consuming infected feed, Clifford said.
"We are sharing our laboratory results with international animal health reference laboratories in Canada and England, which have official World Animal Health reference labs," Clifford said. "In addition, we will be conducting a comprehensive epidemiological investigation in conjunction with California animal and public health officials and the FDA."
ABC News said the four U.S. cases date back to December 2003, with the most recent one prior to the California cow occurring in Alabama in 2006. No humans have been infected in the United States.
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