ing variant Creutzfeldt-Jakob disease (vCJD, the human variant of the disease, which is acquired through consumption of prion-contaminated meats) from a few cases in the bovine population or through blood transfusion is extremely, almost infinitesimally small.

At present, immunohistochemistry and immunoblot are widely considered in the international community the two gold standards to test for BSE.

The key to prevention is to ensure that high-risk materials from cattle are not incorporated into the feed supply. Enforcement is critical to the success of this approach. When enforcement cannot be guaranteed, a complete ban on feeding of ruminant by-products may be necessary.

In this case, the USDA Food Safety and Inspection Service recalled over 10,000 pounds of meat to prevent human food contamination. The recall involved by-products from 20 BSE-infected cattle, including 2,000 tons of potentially infectious feed, which had already been processed and exported to foreign ports. Over 700 animals were slaughtered during the traceback and traceforward investigation, while the U.S. beef industry continued to see a loss of export markets.

Lessons identified from the BSE experience included the following:

  • The U.S. animal health community realized that BSE can no longer be considered a problem only for other nations.

  • In contrast to traceback required for a highly contagious diseases like FMD and classical swine fever, comprehensive tracing required for a disease like BSE, with such a prolonged incubation period and likely exposure as a young calf, was nearly impossible within the current U.S. system of animal tracking and identification.

from the United Kingdom were freely imported into North America prior to an understanding of the potential of these products to transmit BSE. In addition, U.S. and Canadian restrictions that ban feeding of ruminant by-products to other ruminants were not implemented until 1997, and even then compliance, at least in the United States, may not have been optimal. Thus, the advanced age (6½ years) of the BSE-infected animal in the United States placed her, and her birth cohorts, at risk of exposure to BSE-contaminated ruminant by-products as a calf.

The BSE prion (PrPres) concentrates almost exclusively in nervous tissue in cattle and is found in highest concentrations in the brain, eyeballs, spinal cord, and dorsal root ganglia. In younger animals, the distal ileum, or last segment of the small intestine, can also harbor PrPres. This is the basis for concentrating on control of these so-called specified risk materials (SRMs). Appropriate quality control can ensure that these mate-

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