Public Summary of the meeting on
29th November 1999

The Spongiform Encephalopathy Advisory Committee (SEAC) met at the offices of the Ministry of Agriculture Fisheries and Food, Tolworth on 29 November. This was Professor Colin Masters' and Dr Jiri Safar's first meeting following their recent appointment to the Committee. Professor Will Hueston was attending his last meeting before retiring from the Committee. Professor Hueston was thanked for his valuable contribution to the Committee's work during his 6 years of service.

The Committee conducted its regular review of research findings and epidemiological information on BSE and vCJD. The total number of people who had died of vCJD now stood at 48. It remained too soon to make predictions with any confidence about the likely total number of deaths in the coming years.

The Committee noted the continuing work to prevent the potential transmission of vCJD through use of surgical instruments.

The Committee received an update from the Advisory Committee on Dangerous Pathogens (ACDP) / SEAC Joint Working Group (JWG) on the work that it had been taking forward on the safe working with TSE agents. The JWG had also considered the categories of patient risk groups set out in guidance issued by ACDP/SEAC 1. The JWG had recommended that recipients of corneal transplants should be added to the list within the guidance 2 of those patients for whom special precautionary measures would be necessary in a health care setting, to prevent any risk of person to person transmission of TSE infection 2. SEAC endorsed the JWG's recommendation.

Scientists from the Wellcome Trust Centre for Epidemiology of Infectious Diseases (WTCEID) at Oxford University presented their latest estimates of the numbers of cases in the BSE epidemic from 1996 to 2001 (Table 1), the likely numbers of infected animals entering the food chain (Table 2) and the impact of the offspring cull and the Over Thirty Month Scheme (OTMS) on the latter estimates. The revised estimates were based on the most recent data from the BSE epidemic, and take account of the estimated effects of demographic changes and the selective and offspring culls.

The epidemic is still in rapid decline but will have a long tail. The Committee noted that the predictions had been revised upwards from the predictions made in 1996 but had much narrower confidence bands than previous estimates for the same period. However, they still fell within the confidence bands of the earlier analysis. The predictions are higher because the estimated risk from exposure to infected feed in the period 1991-1996 did not decline as rapidly as projected in the earlier analysis. The likely explanation for this is that cattle were exposed to feed which was accidentally cross-contaminated with BSE infected meat and bone meal destined for pigs and poultry.

The estimate of the number of animals under 30 months of age slaughtered for human consumption that were infected with BSE and within 12 months of developing clinical disease was key to the Committee's previous advice on the risk from dorsal root ganglia and bone marrow. The revised estimates were comparable to the figures considered previously and consequently SEAC did not consider it necessary to review their previous advice of November 1998.

The Committee reviewed the Over Thirty Month (OTM) rule, taking account of the latest predictions of the BSE epidemic. The Committee agreed that the OTM rule had been the key measure put in place to protect public health in 1996. The Committee has provided formal advice to Ministers on their interim conclusions after reviewing the OTM rule and a copy is attached. SEAC requested further information to enable them to consider the balance of risk from cattle in the UK compared with the risk from imported beef and beef products.

The Committee had been asked to consider further the issues surrounding the possible use of meat and bone meal derived from pigs for inclusion in poultry feeds and whether the Committee should reconsider its previous advice in the light of a recent Scientific Steering Committee opinion. The Committee considered that a relaxation of the feed ban in the UK to allow the inclusion of pig MBM in poultry feed could not be supported on scientific grounds. The comprehensive feed ban in place in the UK had been a critical factor in breaking the cycle of infection in the BSE epidemic and SEAC's previous position on this issue had not changed. The principal concern remained that the recycling of animal-derived proteins within or between species, via feed, represented a potential risk of the development of TSE infection in livestock.

The Committee considered a proposal for a study of offspring of BSE cases to measure the rate of maternal transmission of BSE in cattle. They agreed that the offspring were valuable animals for study but given the revised predictions of the decline of the BSE epidemic and the long term nature of the experiment, the Committee did not think that the study would yield results within a useful time frame and did not consider it high priority. Members agreed that although maternal transmission would not sustain the BSE epidemic in cattle it was important that further studies exploring this route of transmission were undertaken.

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The next meeting of the Committee will be held on 15 February 2000.

1 Advisory Committee on Dangerous Pathogens/Spongiform Encephalopathy Advisory Committee "Transmissible Spongiform Encephalopathy Agents: Safe Working and the Prevention of Infection" (HMSO, April 1998 - ISBN 0113221665)

2 Table 4, page 28, defines those patients who are potentially at risk of developing CJD or a related disorder. The present list includes: recipients of hormone derived from human pituitary glands, e.g. growth hormone, gonadotrophin; recipients of human dura mata grafts; and people with a family history of CJD.

The following data are provided courtesy of the Wellcome Trust Centre for Epidemiology of Infectious Diseases, University of Oxford. All data assume 10% maternal transmission occurs where calves are born to infected dams within 6 months of clinical onset. Data take account of the estimated effects of the offspring and selective culls and assume that there was no residual feed risk after 1 August 1996.

 

Table 1: Predicted BSE case incidence to 2001 (95% prediction intervals shown in parenthesis) including those cases in animals born after mid 1996 from the revised model, compared with actual confirmed cases to date.  

Year Predicted cases Predicted cases in animals born after mid 1996 (1) Actual confirmed  cases (2)
1996 7576
(7315,7854)		 0 7426
1997 4288
(4113,4509)		 0 4241
1998 3074
(2916,3283)		 0.2
(0,1)		 3096
1999 2578
(2394,2798)		 4
(1,11)		 1720 (3)
2000 1753
(1527,2202)		 15
(8,40)		 -
2001 866
(733,1283)		 23
(13,56)		 -

(1) Note: There are currently no confirmed cases in animals born in 1996.
(2) All cases presented according to year of clinical onset, not by year of restriction or year of confirmation. These figures will therefore differ from some official statistics.
(3) Confirmed cases at 10/12/99. There are approximately another 250 cases slaughtered for which results are pending.


Table 2: Revised model: Predicted number of BSE infected cattle entering the human food chain under 30 months of age in the last year of BSE incubation period (95% prediction intervals shown in parenthesis).

Year

1998
1999
2000
2001
Number of animals
6.2
(2,17)
3.1
(0,7)
1.2
(0,4)
0.8
(0,3)

SEAC
December 1999

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Interim Report to Ministers on the OTM Rule Review by SEAC

The Committee has identified the factors which must be taken into consideration when determining whether any change to the rule on slaughter of cattle over thirty months of age could take place. These factors, set out below, have been reviewed by the Committee in discussions held between November 1998 and November 1999:

  • forecasts of the future incidence of BSE cases and of infected animals entering the food chain;

  • the age distribution of cases;

  • the results of examination of brains of cattle slaughtered in the over thirty month scheme (OTMS);

  • the robustness of the cattle identification systems;

  • the robustness of feed controls;

  • the robustness of specified risk material controls;

  • the relative risk from imported beef and beef products.

Not all of the Committees deliberations are complete but, following study of a recent analysis of the BSE epidemic1, it is now possible to provide the following summary and interim conclusion.

The number of BSE infected cattle which might be slaughtered for human consumption in the future is the key factor which will determine the risk to public health from the BSE agent. This risk is reduced by the controls in place. These are:

  • Compulsory slaughter, destruction and regulated safe disposal of any animal showing signs of clinical TSE disease;

  • the prohibition of the use of mammalian meat and bone meal in any farm animal feed;

  • the removal from the food chain and regulated disposal of specified risk materials (any tissue thought to have a significant risk of carrying TSE infectivity);

  • the slaughter and regulated disposal of animals over the age of thirty months with the exception of animals from the beef assurance scheme (where different age limits apply)

  • the slaughter and regulated disposal of all offspring of BSE affected dams born after 1 August 1996.

Current predictions show that the epidemic is still in rapid decline but will have a long tail. The estimated risk from contaminated cattle feed does not appear to have declined as rapidly in the period 1991-1996 as had been estimated previously. However, the estimated risk to humans from animals currently entering the human food chain remains very low. The number of animals entering the food chain which are infected and within 12 months of developing clinical disease is predicted to be between 0 and 4 in the year 2000. As set out in previous advice2 the Committee believes that, given the controls currently in place to remove specified risk materials and continuing high compliance levels, the risk to public health from these animals is extremely small.

One of the key findings from the latest analysis of the BSE epidemic is that the over thirty month rule has had a major impact on reducing the risk to public health by significantly limiting the number of infected animals which might otherwise enter the food chain. From its introduction in April 1996 to the current time, it has reduced the volume of infected cattle in the last year of the BSE incubation period entering the human food supply by over 99%, and is projected to continue at this level of effectiveness for at least the next 2 years. In part this effectiveness is due to the lower incidence of infection in younger animals, and in part to the fact that the great majority of infected animals only show clinical signs of BSE at 4 years of age or later. This means that it is highly unlikely that an animal under thirty months of age would be in the last 12 months of its incubation period.

On this basis, we conclude that modifying the rule to increase the age of animals permitted to be slaughtered for human consumption would be premature as this may re-introduce an unacceptable level of risk at this time. Furthermore, we recommend that the controls set out above should continue to be rigorously enforced to ensure that these risk reduction measures remain effective even though the overall risk is declining with the epidemic.

This represents our interim conclusions. We will continue to keep the over thirty month rule under review in the light of further estimates of the progress of the epidemic, data from surveys of the brains of OTMS cattle, and reports on the monitoring of control measures. We have also asked for further information to enable us to consider the balance of risk from cattle in the UK compared with that from imported beef and beef products.

SEAC
December 1999