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...
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.
Also note the nice little slant to the AP article:
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.
Why is that "only" there? And that "ever"? BECAUSE DON'T WORRY ABOUT IT! The cow was "only" found through random testing!