Letter: Why we cannot know for sure that BSE-infected beef is harmful t o humans - and why that is no comfort

Sir: The absence of proof that BSE causes CJD is frequently used as an argument for the safety of eating beef products. It is true that no causal link has been established, but I should like to explain what kind of evidence would be required to prove that BSE causes CJD.

An experiment might start with a large number of paired individuals, one of each pair being placed in group A, the other into group B. Each group would be treated identically except that where any bovine products are involved, these would come from BSE-free cattle for group A and from BSE-infected cattle from group B. The incidence of CJD in each group would be monitored over a period of time - 20 years or so. If more members of group B than group A developed CJD, a link would be suggested between BSE and CJD. If none of group A developed the disease, but most of those in group B did, the evidence would be overwhelming and the link established.

Naturally, no such experiments have been done and therefore no proof of a link exists, but this is no kind of reassurance. The recent appearance of BSE, followed by a new form of CJD, strongly suggests a connection between the two. Prion diseases violate the usual rules of transmission of other infections and therefore should be given a wide berth. A deeper understanding of the molecular mechanisms of infection can only arise from research. Indeed, had the farmers' organisations been seriously concerned about the health of their stocks in the past, they could have invested in scientific research themselves to discover the bases of scrapie and BSE transmissions and hence how to eradicate them.

Dr Mark S Bretscher FRS

Laboratory of

Molecular Biology

Medical Research Council

Cambridge