Public summary of the meeting on
30th March 2001

The Spongiform Encephalopathy Advisory Committee (SEAC) met in London on 30 March to consider the BSE aspects of various disposal options for cattle carcases in the present emergency arising from Foot and Mouth disease (FMD).

In particular, SEAC's advice was sought on

(a) the assumptions underlying the DNV's risk assessment on burning in pyres, and the Environment Agency's (EA) comments on that risk assessment and on burial;

(b) how the level of risk would be affected by the age of the cattle, and whether they were beef or dairy animals.

DNV Risk Assessment

SEAC considered an independent generic risk assessment on BSE infectivity by DNV Consulting which concluded that a pyre of 100 dairy cattle aged over 5 years old could result in a total of 7 x 10 -4 infectious units being subsequently ingested by those people exposed. (An infectious unit is equivalent to the amount of BSE infected brain material, in grams, that would be required to induce disease in 50% of those each exposed to that amount when ingested by the oral route). DNV noted that the amount of infectivity would be spread over a fairly wide population, and so the risk to an individual from this exposure would be lower. For example, if the infectious units were spread over 1000 people, the individual exposure would be 7 x 10 -7 infectious units per person. In the EA's view this was at the borderline of the widely accepted upper threshold of acceptable excess individual risk of 1 x 10 -6 per annum.

SEAC discussed the main BSE-related assumptions made in the model: these related to the assumed level of BSE infectivity in the cattle population and the infectious dose for cattle and humans. Although SEAC did not consider the assumed parameters to be unreasonable, indeed they were largely ones that it had endorsed in previous meetings, the Committee considered that it would be preferable to base the calculation on a range of plausible values to reflect the uncertainties underlying many of the assumptions, rather than to use point estimates.

SEAC accepted that DNV's assumption that 10% of infectivity remained after pyre burning was a reasonable, if unsupported, value to assume. This meant that the exposure risk from leakage from a burial site into water sources, compared with burying the ash after burning, would be roughly 10 times higher.

In the light of this advice, and taking account of advice from the Environment Agency on hydrogeological aspects, DNV has since produced the following revised estimate of the total infectious units that might be ingested following the disposal of 100 cattle over 5 years old:

Results Summary

                                          Median 95% Range
  2.5% 97.5%
1. Burning Dairy Cattle 9 x 10 -5 5 x 10 -7 2 x 10 -2
2. Burning Beef Cattle 2 x 10 -5 1 x 10 -7 4 x 10 -3
3. Burial Dairy Cattle 6 x 10 -4 3 x 10 -6 1 x 10 -1
4. Burial Beef Cattle 1 x 10 -4 6 x 10 -7 3 x 10 -2

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Variability of level of risk

SEAC had before it an estimate that, assuming 10% maternal transmission of BSE in cattle in the last 6 months of the incubation period and an effective feed ban in August 1996, only 90 of the roughly 7 million animals currently alive that have been born since mid-1996 are BSE-infected. The Committee thought that, based on the number of BSE cases seen so far in animals born after 01 August 1996, (just one, compared to 8 expected in 1999 and 2000 in modelling, assuming 10% maternal transmission) the true incidence of maternal transmission is likely to be lower than 10%. Nevertheless they considered it to be a reasonable working assumption. On this basis, the risk from burning or burying cattle born before August 1996 would be at least 400 times higher than the risk from burning or burying a similar number of cattle born after this date (i.e. under 5 years old).

Based on relative BSE incidence in the year 2000, SEAC also accepted that the risk from burning or burying dairy cattle would be approximately 4½ times more than disposing of the same number of beef cattle.

Other Issues

In discussion SEAC drew attention to the following points. (Additional more detailed points are included in the Annex).

The health and safety issues arising from dispersal of brain tissue during slaughter must be considered and appropriate protective measures taken. The risk arising from an uncovered face (and especially eyes) in close proximity to the slaughter point was of particular concern.

It is unlikely that sufficient lime could be added to buried cattle or ash to have any impact on potential infectivity.

It is important that the potential for animal re-infection by TSEs through contaminated drinking water or pasture land is considered, particularly in the immediate vicinity of a burning or burial site. This applies both to BSE in cattle and to scrapie in sheep.

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SEAC
April 2001

Annex

1. The testing of OTMS cattle identifies animals within about 3 months of clinical symptoms only. The true rate of cattle over 5 years infected with BSE could therefore be higher than the 0.49% assumed although this factor was not quantified.

2. 0.1g is a reasonable, probably conservative, estimate of the infectious dose for cattle. However, a range downwards from 1g, and including values less than 0.1g, should be used.

3. An assumption that 10% of infectivity remains after pyre burning was agreed to be a reasonable if unsupported, value to use. It was noted that the burn achieved in a pyre was likely to be variable and likely to be less effective than controlled incineration. More information about the extent of combustion should be collected.

4. Discussion about the methods of slaughter led to a conclusion that skulls are not left intact. However, no firm view was possible on the extent of brain combustion, an important element when considering the proportion of infectivity which burning would destroy.

5. Environment Agency advice that, on average, the extent of leaching from ash in pits is more than from dilute and disperse landfills in rock, was noted.

6. In considering the dose that might be ingested from contaminated groundwater, it is important to take account of the time over which the dose is transmitted. For some situations (such as Karst geology) this will be a matter of hours. For others, it may extend over tens or even hundreds of years. The human exposure risk would be much reduced in the latter.

7. The risk assessment model when applied on a site-specific basis must be looked at in a local context, including with respect to hydrogeology and the size of the population exposed to a water source. SEAC was not able to comment on specific local factors.