Public summary of the meeting on
15th February 2000

The Spongiform Encephalopathy Advisory Committee (SEAC) met at the offices of the Ministry of Agriculture, Fisheries and Food (MAFF), Page Street, London on 15 February. The Committee thanked outgoing member Dr Peter Goodfellow who had retired from the Committee to take up other duties in relation to human genetics.

The Committee conducted its regular review of research findings and epidemiological information on BSE and vCJD. It considered the recent publication in 'Transfusion'1 suggesting that infectivity was detectable after disease onset in the plasma of mice experimentally challenged with a mouse adapted human TSE. It also considered preliminary evidence which appeared to demonstrate that blood plasma, taken from mice with clinical disease caused by a mouse adapted BSE agent and inoculated into the brains of disease-free mice, gave rise to clinical disease in some of the recipient mice. The Committee concluded that priority should be given to carrying out a study on mice infected with a mouse adapted BSE agent at various stages before development of disease. However, the Committee did not recommend any measures in addition to those already in place to reduce any potential risk to public health from human blood and blood products. 

The Committee noted that the pathogenesis of the disease caused by BSE in mice was different to the pathogenesis of BSE in cattle but more similar to that of TSEs in sheep and humans. Given the difference in the pathogenesis of the disease in cattle and this mouse model, and taking account of the results of bioassays of cattle tissues in both mice and cattle, the Committee
concluded that there were no implications for the safety of the food chain from these findings.

The Committee recommended that work to ascertain the sub-clinical prevalence of vCJD in the human population should be taken forward with all possible speed. This might, for example, include large scale surveys testing tonsil and appendix tissue for evidence of infection.

The Committee was informed that the total number of people who had died of vCJD stood at 52. Although the final number of deaths from vCJD during 1999 once all comfirmations were complete was expected to show a fall from the number who had died in 1998, it remained too soon to make predictions with any confidence about the likely number of deaths in the coming years. 

The Committee noted the outcome of the meeting of its Epidemiology Sub-Group on 9 February. On the definitions used for 'confirmed' and 'probable' vCJD, the Committee endorsed the view of the Sub-Group that robust scientific criteria now existed. These definitions would enable live 'probable' cases to be separately identified for the first time. The number of cases meeting these criteria could then be included within the data on vCJD published by the Department of Health.

The Committee conducted a review of key research related to scrapie and BSE in sheep. A representative from the Institute of Animal Science and Health, Lelystad, Netherlands presented some of their recent work on the pathogenesis of scrapie. Progress reports on the work at the Institute of Animal Health and Veterinary Laboratories Agency on the pathogenesis of BSE in sheep and screening brains from sheep with spongiform encephalopathy for the presence of BSE were also presented to the Committee.

Members noted that the preliminary and interim results of the studies of experimental BSE pathogenesis in sheep tend to support the conclusions reached by SEAC in July 1996 and July 1998 that BSE in sheep is likely to behave in a very similar manner to the natural disease of sheep scrapie. The latest work on the pathogenesis of scrapie confirms earlier findings that the disease appears to infect peripheral tissues including the lymph nodes, spleen and parts of the intestine as well as the central nervous system. It also indicates that the spread of the infection is likely to be from the gut via peripheral nerves or via innervations of the lymphatics to the central nervous system.

Agent strain typing is underway on isolates from over 130 brains of sheep with natural scrapie in order to determine if the BSE agent profile exists. Although incomplete (each study takes up to two years) over 30 isolates are advanced enough to suggest that they do not have the characteristics associated with the BSE agent. Members also noted strain typing of isolates from a pool of brains from scrapie affected sheep which had been collected for a rendering experiment between 1990 and 1992. The Committee agreed that further work needed to be done on screening sheep for evidence of infection with the BSE agent but concluded that at present there was no need for further public health controls.

The Committee further noted the existence of an EU proposal on specified risk materials. The proposal would classify individual Member States according to the incidence of BSE in cattle. Members noted that to set out SRM controls for sheep and goats on this basis would not be consistent with the likely routes of exposure of sheep to BSE infection. This is because the risk of exposure would vary according to feeding practices assuming that sheep had in fact been infected from feed. It would also depend on if, and how infection and disease associated with the BSE agent could be propagated in sheep.

Members further agreed that, in the event that the EU proposal to add intestine to the list of specified risk materials from sheep or goats were adopted by Member States, they would not advise against this but would advise that the whole intestine from sheep of all ages should be classified as specified risk material.

The Committee noted a proposal by MAFF to extend the survey of the brains of cattle in the Over Thirty Month Scheme in the next year. They felt it was important to continue this work as it provided an independent check on models of the progress of the BSE epidemic. The performance of diagnostic tests was seen as critical to the outcome of such surveys. To that end the Committee welcomed the intention of the Commission to support a study to evaluate the usefulness of the recently validated tests that can efficiently detect prion protein (PrP) in the central nervous system (CNS) of clinically affected animals, for the detection of PrP in healthy cattle during the incubation period. This study will use CNS tissues from cattle in the study of pathogenesis of experimental BSE in cattle using the tests previously evaluated for sensitivity and specificity compared with conventional diagnostic methods. The study is expected to indicate how long before the onset of clinical signs the test would be reliable to detect PrP which, by inference, would indicate that a positive animal be infected. Members thought this was a key factor which must be taken into account when designing the surveys.

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1Brown et al 1999, Further studies of blood infectivity in an experimental model of transmissible spongiform encephalopathy, with an explanation of why blood components do not transmit Creutzfeldt - Jakob disease in humans. Transfusion Vol. 39, November/December 1169 - 1178.

SEAC
March 2000