We should spare his neighbours' cattle until we know how prions destroy the nervous systemby John Maddox / May 20, 1996 / Leave a comment
Published in May 1996 issue of Prospect Magazine
My neighbours in Wales have spent 20 years creating a well-bred herd of beef cattle which regularly wins the prizes at local and even national shows. Most of the animals would go in a cull of all cattle older than 30 months, yet there has not been a single case of BSE in the herd. Waiting to hear what the government decides to do, the family gets what cheer it can from reports that a trickle of cattle is changing hands at the local auction rings. Disbelief in the danger to human beings from BSE (bovine spongiform encephalopathy) is understandable. It is like radioactivity: how might something that cannot be seen, be fatal to human beings? Pasteur’s germ theory of disease encountered the same scepticism at the outset. So the farmers ask why, if scrapie in sheep is not a hazard to people, beef should be under a cloud because of BSE? The Pattison committee advising the government on BSE is right to insist that the ten cases of the human analogue of the disease, Creutzfeldt-Jakob disease (CJD), which triggered the present crisis are only circumstantial evidence that BSE has caused CJD. That will remain the position however many (or few) new cases appear in the months ahead. In recent years, fears that BSE might cause CJD have been met routinely with the declaration that there is “no scientific evidence” that the infectious agent can jump from one species to another. Now people understand that the claim means simply that there is no evidence-one way or the other. Different parts of animals infected with BSE have been used in feeding trials, which is the basis for banning brain, spinal cord and spleen from the human food chain. The muscular parts of animals that butchers turn into steak fall below the detection level. But they are full of the nerves required to activate muscles, and may still be dangerous. But how dangerous? The crisis in the beef industry would be less serious if the government had been in the habit of saying that there may be a risk from eating beef, but that at the worst it can be only small. The worst case would be that one’s chance of getting CJD from eating beef is proportional to the number of infectious molecules ingested in the process and that infectious molecules are uniformly distributed in nervous tissue. My guess is that the risk now from eating sirloin steak, if there is one at all, is less than one thousandth of that from eating beef in general (before the government cracked down on abattoir procedures). A competent histologist (not me) could make a much better estimate. People would like to know what it is instead of being told that there is “no scientific evidence…” None of that implies that the crisis in farming would be over if only the government acknowledged that there is a risk and offered some numbers by way of an estimate. That might have worked in November 1995; now it is too late. At this stage, people would fear that official estimates of risk are vitiated by the frailty of the assumptions on which they are based. Not without reason. BSE and CJD are “prion diseases,” which means that the infectious agent is a protein, not a genetically autonomous agent such as a virus or a bacterium. Stanley Prusinier of the University of California, a leading research university in the US, claimed as much in 1982. Prusinier’s evidence was largely negative: enzymes which readily chew up DNA and its sister molecule, RNA, leave the infective agent of scrapie intact. It was later found that all mammals and most vertebrates have a gene whose existence determines the production of protein molecules; they appear to sit in and outside of the membranes of nerve cells, but their normal function is not known. We all have prion protein, or “PrP”-as prion molecules are known in the trade. So what goes wrong? As with other genes, mutations (or genetic changes) can arise spontaneously, leading to the production of aberrant molecules. There is even a distressing genetic disease called “fatal familial insomnia” (the name says it all) caused by such a mutation. But neither BSE in cattle nor CJD in people is a genetic disease in the sense of “running in the family.” On the contrary, since the Southwood report on BSE in 1988, it has been assumed that cattle have become ill because their feed has been contaminated by prions from scrapie in sheep. If people have died of CJD from eating beef, prions from BSE cattle must be responsible. The question of how even an aberrant prion molecule can cause such damage to the nervous system has so far been answered mostly by hand-waving. The current suggestion is that aberrant prions induce normal PrP molecules to fold “wrongly,” in turn inducing the cells of the nervous system carrying them to behave aberrantly and, eventually, to die. This version of how the disease develops rules the roost at present, chiefly because there are no more persuasive ideas on offer. A large chunk of the ?4m extra the government plans to spend on prion research could usefully go on checking out this story. Until the mechanisms by which scrapie, BSE and CJD develop are understood, all attempts at controlling them will rest on wish-fulfilment. Culling my neighbour’s prize herd will be symbolic only.