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TSEs disrupt the function of nerve cells. In TSEs there are characteristic vacuolar degeneration of neurons in the brain and spinal cord, giving the tissue a spongelike appearance (Fig. 1). TSEs evoke no immune response and are inevitably fatal (2-5).

The incubation period (ie, the time between exposure and the onset of clinical signs) in cattle is very long, rang-

Figure 1. Lesions in the gray matter of the brain of a sheep with scrapie: (A) typical spongiform change in neurons; (B) spongiform change and astrocytic hypertrophy and hyperplasia, (a) Hematoxylin and eosin stain; (b), glial fibrillar acid protein (GFAP) stain. Magnification X500. Source: Courtesy of Dr. Robert Higgins, School of Veterinary Medicine, University of California, Davis.

Figure 1. Lesions in the gray matter of the brain of a sheep with scrapie: (A) typical spongiform change in neurons; (B) spongiform change and astrocytic hypertrophy and hyperplasia, (a) Hematoxylin and eosin stain; (b), glial fibrillar acid protein (GFAP) stain. Magnification X500. Source: Courtesy of Dr. Robert Higgins, School of Veterinary Medicine, University of California, Davis.

ing from two to eight years. Most animals show clinical signs between three and five years of age and demonstrate a slowly progressive disease with nonspecific neurological signs, including incoordination, weakness, tremors, and anxiety. Once signs develop, animals usually die within two weeks to six months.

Epidemiologically, BSE has been considered an "expanded point source epizootic"; that is, it resulted from the consumption of contaminated protein feed supplements (6). Meat and bone meal (MBM) produced from rendered carcasses and offal from dead and butchered livestock has been a common protein supplement used for decades in livestock production. Changes in rendering practices in the late 1970s and early 1980s were considered a risk factor in amplifying the disease through the resulting manufacture of infectious MBM feed supplements (6-10).

About 2000 cattle have been diagnosed with the disease in other countries, some as the result of exportation from the UK of live cattle incubating the disease, and the majority from exportation of contaminated MBM subsequently fed to native cattle (7-9).

More than 170,000 cattle from more than 33,500 British herds (67% of all dairy herds, 16% of all beef herds) have been confirmed with the disease. The low average within-herd incidence, with 35% of the farms experiencing only one case, and 69% with four or fewer cases, is attributed to uneven exposure to the agent in widely spaced batches of feed (6).

In 1996 a possible link was made between BSE in cattle and 10 cases of Creutzfeldt-Jakob disease (CJD) in humans (11). To date 29 humans have been confirmed with v-CJD (12), shown to share identical lesion patterns and chemical characteristics with BSE (13-15). Evidence strongly supports the hypothesis that v-CJD in people and BSE in cattle are the same disease, with transmission to people occurring through the ingestion of infected beef products (16,17).

Perishable foods are especially vulnerable to acute crises in consumer confidence. Following the March 1996 announcement of the possible link between BSE and human disease, British beef consumption fell for more than a month to 25% of its preannouncement level, and the European Union (EU) banned British beef imports. Global markets, including those in countries without BSE, have been impacted by this epizootic.

Beginning in July 1988 the UK responded to the cattle epizootic with a series of control measures. Mandatory programs were enacted requiring euthanasia of affected cattle, prohibiting the use of affected cattle carcasses or specified offal from sheep or cattle in feeds, and eventually banning MBM from all agricultural uses. A selective cull program was instituted targeting herds that had BSE cases and removing nonaffected cattle that might have been shared exposure to the agent. Current projections are for BSE eradication between 2001 and 2005.

Measures to protect human health began in 1989 with the Specified Offal Ban (SBO), prohibiting the use of brain, spinal cord, tonsil, thymus, spleen, and intestinal tissues of cattle origin in foods intended for human consumption. Additional measures, requiring meat from animals over 6 months of age to be deboned prior to sale, and banning most cattle over 30 months of age from use as human food, were enacted.

By-products from slaughtering cattle are used in horticultural applications, pet foods, cosmetics, and pharmaceuticals. Those tissues not meeting specified standards are prohibited from use in manufacturing these products and are now incinerated.

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