According to the World Animal Health Organization (OIE) through the end of 2006, 190,129 diagnosed cases of BSE (including domestic and imported animals) have been recorded worldwide. Of these, 97 percent or 184,484 cases occurred in the United Kingdom. BSE cases in the U.K. have declined from a peak of 37,280 cases in 1992 to 114 cases in 2006. View maps of BSE cases to examine the geographical distribution of this disease.

The first case of BSE in the United States was announced Dec. 23, 2003, and was found in a dairy cow imported from Canada. Two cases of BSE in cattle born and raised in the United States also have been reported, one in 2005 and another in 2006, both with an unusual form of BSE that is different from the typical form. All cases were detected through the U.S. Department of Agriculture (USDA) BSE surveillance program.

It is important to recognize that diagnosed clinical cases represent only the visible epidemic. A diagnosis can only be made through analysis of brain tissue and the abnormal prion protein typically is not present in brain tissue of sub-clinically BSE-infected cattle younger than 30 months of age. As an example, it is estimated that about 4 million cattle were infected with BSE during the British epidemic but only about 184,000 were reported clinical cases. An estimated 3.3 million cattle entered the British human food supply and 510,000 died on the farm as casualty animals or fallen stock.1 Because the majority of cattle were slaughtered for human consumption at 18-24 months, before they could develop clinical disease, there were infected animals that were neither detected nor detectable by test methods used in the past 20 years (see Detection of Cases). Although the scale of BSE epidemics in other countries is massively lower than in the U.K., the same principle applies.

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