FRESNO, Calif. (AP) - A California Holstein discovered to have mad cow disease in April was an isolated case and didn't pose a threat to the food supply, a report issued Friday by the U.S. Department of Agriculture said.
A three-month investigation looked into the movements of the infected dairy cow, her offspring and the food eaten by the herd. The investigation turned up no other cases of bovine spongiform encephalopathy.
"The results of this thorough investigation confirmed that at no time was the U.S. food supply or human health at risk, and that the United States' longstanding system of interlocking safeguards against BSE continues to be effective," said John Clifford DVM, USDA's chief veterinary officer.
The 10-year-old dairy cow, only the fourth with the sickness ever discovered in the United States, was found as part of an Agriculture Department program that tests for the fatal brain disease in about 40,000 of the 35 million cows slaughtered each year. It was unable to stand before it was killed and sent April 18 to a rendering plant at a Hanford, Calif., transfer station.
It was one of dozens that underwent random testing at the site, and the positive results set off the federal investigation into the source of the disease.
USDA investigators tracked the cow from the ranch where she was born to a heifer operation where she was raised and bred to activate her mammary glands and to the Tulare County dairy where she spent her life.
Investigators found 282 cattle it identified as birth "cohorts" and attempted to trace the 210 that might have made it into the food system. Ultimately investigators slaughtered one dairy cow that was the Holstein's offspring, but she was found not to be infected. There is no live animal test for BSE.
The California cow with the disease was never destined for the meat market, and some scientists believe it developed "atypical" BSE from a random mutation, something that happens occasionally. Somehow, a protein the body normally harbors folds into an abnormal shape called a prion, setting off a chain reaction of misfolds that eventually kill brain cells.
The strain of bovine spongiform encephalopathy that appeared in the UK in the 1990s and set off a worldwide beef scare was a form caused by cattle eating rendered protein supplements derived from slaughtered cattle, including brains and spinal columns, where the disease is harbored. Scientists know less about the "atypical" strain.
In California, agriculture officials were investigating, among other things, whether feed sources might have played a role in the animal contracting the fatal illness.
It's no longer legal to feed cattle to cattle, but rendered cattle are fed to chickens, and chicken droppings and spilled feed are rendered back into cattle feed.
The Food and Drug Administration investigated a dozen feed suppliers and found that they comply with regulations.
The sick cow was part of a load of 71 cattle delivered that day to the rendering plant's transfer station. The carcasses were quarantined and eventually sealed in plastic vaults and disposed of at a nearby landfill.
(Copyright 2012 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.)