Download the 02/04/04 report as a .doc or .pdf file
from he

http://www.aphis.usda.gov/lpa/issues/bse/bse.html

Quoting from the report:

However, it is probable that other infected
animals have been imported from Canada
and possibly also from Europe. These animals
have not been detected and therefore
infective material has likely been rendered,
fed to cattle, and amplified within the cattle
population, so that cattle in the USA have
also been indigenously infected. Therefore,
animals that have not been identified from
the birth cohort of the index case do not
represent significant additional risk for further
propagation of BSE within the USA. Risk
materials from these unidentified infected
animals must be considered when developing
policies for the prevention of human infection
and infection of cattle through feed.