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Based on World Organization of Animal Health (OIE) standards for BSE surveillance, the reported national prevalence rates of BSE in North American cattle, particularly in animals born in the United States, is very low, and therefore, difficult to measure accurately. In September 2007, the USDA published updated results of the two statistical models used by Harvard University investigators to estimate the prevalence of BSE in Canada. The results incorporated the 11 Canadian-born animals with BSE that had been reported at that time. A key advantage of these models is that they provide statistical confidence limits that measure some of the uncertainty associated with expected estimates. This model estimated that the true prevalence of BSE in Canada has been 90% likely to be between 18-fold and 48-fold higher than the previously published best estimate of the prevalence of BSE in the United States (3.0 to 8.0 cases per million in Canada compared to a best estimate of 0.167 cases per million in the United States). The previously published best estimate of Canada's BSE prevalence in 2006 using the BSurveE model was 23-fold higher than that of the United States and is the estimate of the BSE prevalence in Canada that continues to be used in the Harvard Risk Assessments' "worst case" analyses when evaluating the risk of imported Canadian cattle causing BSE to spread among U.S. animals.
United States. Centers for Disease Control and Prevention. "BSE (Bovine Spongiform Encephalopathy, or Mad Cow Disease)." Feb. 19, 2015. <http://www.cdc.gov/ncidod/dvrd/bse/>.