M03041: SRM Controls Review - Update

Study Duration: July 2006 to January 2007

Contractor: DNV

A study was carried to review the system of SRM controls in UK abattoirs. The objective of the original study was to develop a model that could be used to assess the implications of alternative specified risk material(SRM) inspection strategies on the risk of exposure to BSE infectivity. The first stage was to develop a model to assess the exposure to BSE infectivity given the level of SRM controls in place at the time. The study only considered SRM controls for cattle.

This study will update the original model to take account of the fact that over thirty month (OTM) old animals may now be slaughtered for food, provided they were born after August 1996 and have a negative BSE test; to take account of the expected changes that would result from the lifting of the beef export ban by the EC and the resulting harmonisation of SRM controls (including use of head meat). The new study will also consider SRM controls for sheep. This update would then provide the basis for the evaluation of alternative options for change in the way SRM controls are enforced by the Meat Hygiene Service.

Under Thirty Month cattle slaughter
• The amount of BSE infectivity entering the food supply from slaughter of under thirty month old cattle with the current SRM control regime is extremely small, about 0.2 bovine oral ID50 (measure of infectivity) unit per year (range 0.02 to 4) for the whole UK population. Taking account of the cattle to human species barrier this represents an extremely low level of risk.
• About 76% of this exposure is from dorsal root ganglia(DRG) in under thirty month old cattle with the remainder due to contamination of head meat, although this is dependant on the assumptions made regarding slaughterhouse processing. The contribution from SRMs entering the food supply due to failures of SRM controls is negligible.
• The contribution from OTM animals being slaughtered and not identified is extremely low (.0001%), with about 1% from higher risk animals not identified at the ante-mortem inspection.
• In the unlikely event that a late stage infected animal was to be slaughtered and not identified, then it is estimated that the total infectivity entering the food supply would be a median value of 2 bovine oral ID50 units (range 0.1 to 67). More than 99% of the total infectivity present would be removed by SRM controls.
Over Thirty Month cattle slaughter
• The amount of BSE infectivity entering the food supply from cutting plant handling over thirty month old cattle is estimated to be a median value of 1.4 bovine oral ID50 units. This is a factor of 6 greater than the total contribution from the UTM slaughter and a factor of 40 greater per head slaughtered.
• The exposure from OTM cutting plant is due entirely to the risk that some DRG will remain in the meat after the vertebral column is removed. The exposure is higher because the prevalence in OTM cattle is greater than for UTM animals, even taking account of the BSE test. The results also show that the majority of the exposure is from the normal operation and not due to any failures in the system.
• The amount of BSE infectivity entering the food supply from the head meat harvested from over thirty month old cattle is estimated to be a median value of 1.1 bovine oral ID50 units per year. The total exposure from OTM cattle is estimated to be a median value of 2.8 bovine oral ID50 units per year, a factor of 12 greater than for UTM slaughter.
Sheep slaughter
• The results of TSE surveillance on sheep now indicate that the most likely proportion of TSE positive cases in sheep that could potentially be BSE is zero. In this case the SRM controls at sheep abattoirs will have no impact on exposure to BSE infectivity.
• If the maximum estimated prevalence of BSE cases in sheep is used (7 flocks in 2005) then the exposure from BSE in sheep is estimated to be a median value of 120 bovine oral ID50 units per year. This is about 40 times that from the present estimated total exposure from cattle (UTM and OTM slaughter).
• Most (99%) of the estimated exposure is from the lymph nodes and intestine consumed following normal slaughter, and not from failures in the SRM controls.
• The total infectivity that may be consumed from a sheep with clinical BSE is estimated to be 14 bovine oral ID50 per animal, about 32% greater than that from a bovine with clinical BSE.