A USDA scientist who conducts research on atypical BSE also declined to answer whether USDA's new testing regime would continue to find cattle infected with the atypical strain.

First discovered in France and Italy, atypical BSE has become a growing topic at scientific meetings on transmissible spongiform encephalopathies. Several research papers on the issue were presented last month at a meeting in Italy. The struggle is interpreting what atypical BSE can mean, particularly given the rarity of the disease globally.

"To be honest with you, we really don't know," said Linda Detwiler, a former USDA scientist now at the Virginia-Maryland College of Veterinary Medicine. "There's so little (data) and it's really an emerging issue right now." Scientists have found at least two strains of BSE that do not appear to show the same patterns or staining on tests as "classic BSE."

DTN first reported last May that two of the cases in the United States -- one in Texas and one in Alabama -- were atypical. A cow earlier this year in Manitoba also was an atypical case.

The two strains have been broken down into high-molecular and low-molecular strains. The high-molecular strain, which was found in the two U.S. cases, has occurred in older animals ranging from 10 to 16 years of age. When transmitting the disease to mice, the mice take longer incubation periods to show clinical signs of the disease than mice infected with classical BSE. Outside of the U.S. and Canada, the high-molecular strain has been found in six other countries.