Quoting from the 02/04/04 report on BSE in the U.S.:

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

The subcommittee recommends that
future surveillance programmes should be
targeted to the population with highest risk
of exposure to the BSE agent. At some
point in the past, targeting based on the
location of cattle imported from Europe and
other BSE risk countries, their points of
slaughter and rendering and subsequent
consumption in cattle feed was theoretically
possible. However, with the passage of time
since the importations and the amplification
of the agent within North America, this
approach is no longer appropriate.