The experiments will mirror one begun five years ago when brain tissue taken from CWD-infected mule deer was injected into the brains of 13 cattle.

That same slurry of infected brain material was given orally to cattle at the University of Wyoming five years ago. CWD has not been transmitted to any of the cattle that ingested the material orally. But four of the 13 cattle injected with CWD in an Ames, Iowa, laboratory have tested positively for the abnormal protein - called a prion - that destroys the brains of infected animals.

The latest positive test came only two months ago when an animal was found dead after apparently falling during the night. Although a fractured vertebra suggested death was because of the fall, scientists still tested it.

"We were shocked," said Janice Miller, a U.S. Department of Agriculture veterinarian and the lead scientist for the transmissible spongiform encephalopathy (TSE) project. "It had been 59 months since the cattle had been inoculated. We were getting ready to terminate the study. But now we're hanging on to the remaining animals."

The other three animals infected by CWD died after 23, 24 and 28 months. But those animals, like those in an earlier experiment that were injected with scrapies - a TSE that affects sheep - proved unusual.

All of the cattle that received scrapies injections into their brains developed the disease within 18 months. But, surprisingly, the lesions in the brain that are typical with the disease didn't show up. Tests, however, revealed the presence of the prion protein.

The absence of lesions and presence of the prion protein also marked those animals infected with CWD.

The same CWD matter was injected into the brains of eight sheep and several raccoons three years ago. Only one sheep, and no raccoon, has developed CWD.

The injection of CWD into brains is a radical method of transmission, and Miller said the fact that only four cattle and one sheep developed the disease indicates it is resistant to crossing the species barrier.

"It doesn't appear to cross very readily into mice, raccoons, sheep or cattle," Miller said. "You can't say it's impossible because we've demonstrated that it is possible. But transmission by injection into the brain is a method that would not be possible under normal situations."