Modelling studies on bovine spongiform encephalopathy occurrence to assist in the review of the over 30 months rule in Great Britain.

The objective of this study was to contribute to a risk assessment to review the over 30 months (OTM) scheme in cattle, whereby all cattle aged over 30 months slaughtered in the UK are removed from the human food chain. We use back-calculation methods to estimate the impact of changes to the OTM rule, by using passive and active surveillance data collected between 1 July 2001 and 30 June 2002. There are two types of change considered: increasing the age limit and allowing animals born after a certain date into the food chain. Results indicate that under the OTM rule less than 1 animal in the last 12 months of the incubation period would enter the food chain in 2003. The birth date changes considered and small changes to the upper age limit would increase this number by a relatively small amount.