Mad Cow Disease: The History of BSE in BritainBMJ 1995; 311 doi: https://doi.org/10.1136/bmj.311.6996.67a (Published 01 July 1995) Cite this as: BMJ 1995;311:67
- R M Ridley
Richard W Lacey Cypsela Publications, £15.99, pp 200 ISBN 1 899516 00 X
In Mad Cow Disease Richard Lacey presents his view of two aspects of bovine spongiform encephalopathy (BSE)--the science of this intriguing disease and the chronology of the government's response to an unprecedented epidemic.
Lacey accuses the government of acting slowly, of not spending enough money, of not basing its decisions on science, and, more particularly, of being inconsistent about the relationship between BSE and scrapie. On this last point the government had little alternative but to look to scrapie, a disease of almost identical pathogenesis (but which even Lacey believes offers little risk to humans), while remembering that BSE is new and therefore has the potential (but only the potential) to be different. The government has already spent more than £150 million on disease control and research and its speed of response has been constrained by the need to act on a rational and scientific basis in a subject where research is slow and difficult.
Lacey's reasoning appears to show a serious misunderstanding of the pathogenesis of prion diseases. After one incongruous paragraph in which he admits that some cases of human prion disease are genetic in origin, he then presumes that all other cases must have been acquired by infection. Studies in transgenic mice have shown not only that mice with certain mutations in the prion gene will spontaneously develop transmissible spongiform encephalopathy but that mice with multiple copies of the normal prion gene do so as well, showing that the “wild type” protein is also capable of entering spontaneously into the disease process.
Furthermore there is now known to be a genetic contribution to the sporadic cases in humans. What happens in prion disease is that the host prion protein takes on an abnormal (amyloid or polymerised) form. The probability that this will happen in a lifetime is determined by the primary structure of the prion protein (hence the high probability of disease in people with certain mutations in the prion gene) and is accelerated by the presence of some of the abnormal form of prion protein (hence the transmissibility of the disease by contamination and the exponential pace of disease progression) but this process will happen with a very low probability in people with normal prion genes, leading to the worldwide, stable incidence of one case of Creutzfeldt-Jakob Disease (CJD) in 1-2 million of the population. Apparently missing this point, Lacey argues that since there is CJD in Australia but allegedly no scrapie, CJD must always have been caught from some other animal--that is, cows with undiagnosed BSE. But CJD is well documented in India among the Hindus, who do not eat beef.
Lacey next seems to have misunderstood the alleged maternal transmission of prion disease. Transgenerational occurrence of prion disease occurs only in the endemic form of the disease and this occurs in only two species--sheep and humans. Maternal transmission has never been demonstrated from epidemic cases acquired by feeding in any species. The ruminant feed ban in 1988 dramatically reduced the incidence of BSE and the subsequent, more stringent, specified offals ban in 1989 has virtually stopped new infections. Such cases as have occurred in animals born after these dietary restrictions have not selectively been the progeny of cows which subsequently got BSE, and early indications of the geographical distribution of these cases points towards feed contamination as the source. Kuru, the epidemic form of human prion disease, which occurred among the cannibals of Papua New Guinea, did occur mainly in women and their children but this was because the women prepared the bodies for the feast and fed their own children at the same time. Children born after the cessation of cannibalism, including those whose mothers developed kuru, have not themselves become ill. Recent experiments in sheep scrapie have shown that naturally occurring scrapie is determined substantially, or even completely, by polymorphisms in the prion gene, and embryo transfer experiments have shown the probable irrelevance of specifically maternal factors such as contamination of milk or the placenta. Thus Lacey's contention that the BSE epidemic is driven mainly by maternal transmission is without foundation. That he should then go on to suggest that human acquired cases (which incidentally have all arisen from contamination with CJD infected human tissue) will pass the disease to their children is in my view an utterly unsupportable assertion that I know has inadvertently caused unnecessary misery.--R M RIDLEY, MRC comparative cognition team, Department of Experimental Psychology, Cambridge