Exposure of the human population to BSE infectivity over the course of the BSE epidemic in Great Britain and the impact of changes to the Over Thirty Month Rule

The Over Thirty Month Rule (OTMR) was introduced in 1996 as a measure to reduce the exposure of the population to the BSE agent by limiting the age of animals that could be slaughtered for food. This study was carried out as part of the process of reviewing the OTMR that is currently being carried out by the UK Food Standards Agency (FSA). Data on the infectivity of bovine tissues, the development of infectivity through the incubation period and the cattle to human species barrier are reviewed. All the possible routes by which infective material could be included in food for human consumption, including contamination with infected tissues in the abattoir, embolism following slaughter, dorsal root ganglia in meat, mechanically recovered meat and failure of SRM controls are evaluated over the period of the BSE epidemic. It is estimated that the exposure from one fully infected animal slaughtered for food at the present time would be about 27 bovine oral ID50 units, this compares to a peak value of about 1900 early in the epidemic. At present the exposure is primarily due to dorsal root ganglia, but over the course of the epidemic the main contributor to total infectivity is estimated to have been mechanically recovered meat (MRM). However, it is also shown that the potential exposure to infectivity due to MRM in any one meal would have been very small, casting doubt on whether this could have been an effective source of infection. By combining these estimates with estimates of numbers of infected animals by incubation period and year it is estimated that a total of some 54 million bovine oral ID50 units would have been consumed from 1980 to date, reaching a peak of about 11 million units in 1993, but falling rapidly following the introduction of SBO ban and the OTMR. In 2001 it is estimated that the exposure for the whole UK population is only 2.5 bovine oral ID50 units. The study has helped demonstrate that current exposure to BSE infective material in the UK is very low, and would remain very low if the OTMR was amended or abolished.

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