DET NORSKE VERITAS
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|Philip J Comer
Director of Environmental Services
Results from the BSE Pathogenesis Experiment being carried out by the Central Veterinary Laboratory have indicated the presence of infectivity in some tissues closely related to the spinal cord, but which would not formally be defined as SBM in the United Kingdom. These tissues are the dorsal root ganglia, which would probably remain attached to the vertebral column after the spinal cord is removed, and may on occasion be included in meat for human consumption. SEAC have requested that a formal risk assessment be carried out to determine the level of risk to people due to the possibility that there could be infectivity in these tissues from cattle under the age of 30 months.
The results of the pathogenesis experiment indicate that significant levels of infectivity are present in CNS tissues, including the dorsal root ganglia, in the three months prior to clinical onset of the disease. At nine months prior to clinical onset no infectivity has been detected.
Estimates have been made of the number of cattle that could have infectivity in CNS tissues at the time of slaughter for human consumption, of the likelihood that the dorsal root ganglia would not be discarded with the bone during boning-out, and that dorsal root ganglia contained in meat sold as bone-in would be consumed. The data have been combined in a simple event tree, and the risk evaluated using Monte Carlo simulation.
Two measures of risk have been determined, both of which are based on the consumption of human oral ID50 units. The first measure is the total consumption per year of human oral ID50 units for all people in the United Kingdom. This is a measure of societal or group risk. The second measure is the individual risk, which is represented by the expected consumption per year by any one individual of human oral ID50 units.
Total Ingestion of infectivity
The median value of the total ingestion of infectivity due to infectivity in dorsal root ganglia of cattle with infectivity in the CNS at less than 30 months of age, has been estimated to be 0.05 ID50 units over the whole UK population in 1997. The 95% range is from zero to 11 ID50 units, and the probability of the total ingestion being less than 1 is 80%.
With the continuing fall in numbers of confirmed BSE cases, the value for 1998 would be less, about 75% of the value in 1997.
The results also show that 24% of this total ingestion of infectivity is due to bone in meat (range 10% - 45%). The remainder is due to the proportion of DRG left in the meat in boning out operations.
The median value of the individual risk of ingestion has been estimated to be 9 x 10-10 ID50 units per person per year. The 95% range is from 5 x 10-12 to 2 x 10-7 ID50 units per person per year, which is some four orders of magnitude. The results are illustrated on a risk perspective scale on Figure 1.