Ridley R M, Baker H F
School of Clinical Veterinary Medicine, Cambridge, England.
Vet Q. 1999 Jun;21(3):86-92. doi: 10.1080/01652176.1999.9695000.
The epidemic of bovine spongiform encephalopathy (BSE) has been the most expensive disaster ever to have befallen farming in the UK. It is believed to have led to a new form of spongiform encephalopathy in humans and as yet there is no way of knowing how many people will die of this disease. In order to curtail the BSE epidemic major decisions had to be made, often on the basis of inadequate scientific data. These data may have been derived from experiments using small sample numbers. Here we review some examples of where this has happened, sometimes with a beneficial outcome and sometimes with a misleading outcome. The identification of BSE as a new disease depended on precise neuropathological observation of a small number of cases rather than the obvious occurrence of large numbers of sick animals. Similarly, the recognition that BSE may have led to disease in humans was based on the neuropathological and clinical picture of new variant Creutzfeldt-Jakob disease (CJD) rather than on an increase in the number of cases of CJD in the UK. Early in the BSE epidemic the possibility that disease could be maternally transmitted from cow to calf was raised, mainly because of a belief that such transmission occurs in scrapie disease of sheep. But, we argue, the evidence for maternal transmission of scrapie, collected in the 1960s, was based on small numbers and is inadequate. Subsequent research has shown a very substantial genetic component in scrapie and epidemiological data show no excess risk in the offspring of affected ewes relative to the risk in the offspring of affected rams. An experiment to determine whether maternal transmission occurs in BSE was flawed and was unable to distinguish between maternal transmission and genetic susceptibility to environmental contamination. An assessment of the risk of BSE to humans depends on determining the levels of infectivity in tissues and transmissibility across species. Data on both of these are deficient, so it is not possible to predict how many people in the UK or elsewhere will become affected with new variant CJD in the next fifty years. The assessment of whether BSE could be transmitted to sheep and whether sheep therefore pose a risk to humans is hampered by a serious lack of evidence about the epidemiology of scrapie in the UK and elsewhere. The UK has paid a heavy price for the BSE epidemic but lessons should be learned from the experience. Every country should have a Specified Offals Ban even if it has no cases of BSE because, by the time it has, it will be too late. Furthermore, the occasional case of BSE should not be regarded as insignificant since it may be the harbinger of an epidemic in the making.
牛海绵状脑病（BSE）疫情是英国农业遭遇的最昂贵灾难。据信它已导致人类出现一种新的海绵状脑病，目前尚无法知晓会有多少人死于这种疾病。为了遏制BSE疫情，不得不做出重大决策，而这些决策往往是基于不充分的科学数据。这些数据可能来自样本数量较少的实验。在此，我们回顾一些此类情况的例子，有时会产生有益结果，有时则会产生误导性结果。将BSE识别为一种新疾病依赖于对少数病例进行精确的神经病理学观察，而非大量患病动物的明显出现。同样，认识到BSE可能已导致人类患病是基于新型变异型克雅氏病（CJD）的神经病理学和临床症状，而非英国CJD病例数量的增加。在BSE疫情早期，有人提出疾病可能通过母体从母牛传染给小牛，主要是因为人们认为这种传播在绵羊的羊瘙痒病中会发生。但是，我们认为，20世纪60年代收集的关于羊瘙痒病母体传播的证据基于少量样本，并不充分。后续研究表明羊瘙痒病有很强的遗传因素，而且流行病学数据显示，受感染母羊后代的风险相对于受感染公羊后代的风险并无增加。一项确定BSE是否存在母体传播的实验存在缺陷，无法区分母体传播和对环境污染的遗传易感性。对BSE对人类风险的评估取决于确定组织中的感染性水平和跨物种传播能力。这两方面的数据都很缺乏，所以无法预测在未来五十年里英国或其他地方会有多少人感染新型变异型CJD。由于严重缺乏关于英国及其他地方羊瘙痒病流行病学的证据，对BSE是否能传染给绵羊以及绵羊是否因此对人类构成风险的评估受到阻碍。英国为BSE疫情付出了沉重代价，但应从这一经历中吸取教训。每个国家都应实施特定牛杂禁令，即便本国没有BSE病例，因为等到出现病例时就为时已晚了。此外，偶尔出现的BSE病例不应被视为无关紧要，因为它可能是正在形成的疫情的先兆。
