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DET NORSKE VERITAS

Det Norske Veritas Limited
Technical Consultancy Services


Palace House
3 Cathedral Street
London SEI 9DE
United Kingdom

Tel: +44 (0) 171 357 6080
Fax: +44 (0) 171 357 0961

Registered in England
No.: 1503799


Assessment of Risk from
Possible BSE Infectivity in
Dorsal Root Ganglia



For the


Ministry of Agriculture, Fisheries and Food

and the

Spongiform Encephalopathy Advisory Committee






Approved by:
             
Philip J Comer
Director of Environmental Services

C7831.
Revision 1
December 1997


Management Summary

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.

Individual risk

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.

Figure 1:Individual Risk of Ingestion of Infectivity

Figure 1:Individual Risk of Ingestion of Infectivity


Contents

1. INTRODUCTION
1.1 Background
1.2 Objective

2. METHOD
2.1 The Pathogenesis Experiment
2.2 Numbers of Infected Cattle Slaughtered
2.3 Infectivity in CNS Tissues
2.4 Fate of Dorsal Root Ganglia

3. RISK ASSESSMENT
3.1 Event Tree
3.2 Risk Evaluation
3.3 Input Data
3.4 Results

4. REFERENCES

Addendum


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