This NYT story reports that some scientists have found the protein that causes BSE, or mad cow disease, may migrate to other organs and that no part of an infected cow may be safe to eat.

But the experiments are only mouse-based at this time, and one scientist argued the U.S. food supply is safe because animals with obvious signs of illness don't enter the food chain.

An excerpt:

In the mouse experiments, reported in the journal Science, researchers in Switzerland found that prions, proteins that are the infectious agent in mad cow disease, follow immune cells, called lymphocytes, in the body. When mice were given chronic infectious diseases of the liver, kidney and pancreas and then inoculated with prions, the prions made their way to the infected organs.

Dr. Adriano Aguzzi, a neuropathologist at the University Hospital in Zurich, who led the experiments, said this meant that cows and sheep infected with prions could harbor the disease in any inflamed organ.

But Dr. David R. Smith, a veterinarian at the University of Nebraska, said the research did not raise alarms about American beef. For one thing, he said, livestock with obvious signs of systemic infection, like a fever, are not allowed into the food supply. And most American cattle are slaughtered while they are young and at reduced risk of infection.