''Mad? I'm f***ing furious." The caption beneath the photograph of a boss-eyed, knock-kneed Friesian captured something of the pantomime humour that first accompanied the concept of mad cows. Yet bovine spongiform encephalopathy (BSE), as mad cow disease is officially termed, was nothing to laugh about. How did it occur? And what is the continuing threat to humans?
Scrapie in sheep was the first documented spongiform encephalopathy. An utterly bizarre agent - the prion - which carries neither DNA nor RNA (a distinct form of genetic material found in some viruses) causes it. This means that the prion protein proliferates and spreads through populations without recourse to its own genes. Faulty forms of the prion become infectious by a remarkable but poorly understood process. They interact with good versions of the prion that are present within all mammals and, bafflingly, transform them into infectious replicas of themselves. In time, these proteinaceous disciples accumulate within the neurons making up the brain. Minuscule holes appear (hence spongiform encephalopathy), causing the progressive dementia and other symptoms of these diseases. There is no cure.
In the late 1980s, a friend of mine working as a pathologist announced that he would eat no more beef. He was convinced that scrapie had jumped a species barrier to cause BSE. If this agent were able to jump species barriers, would humans be next? He was proved correct. Year on year, throughout the 1990s, the numbers of patients with human variant Creutzfeldt-Jakob disease (vCJD) crept eerily up. The encouraging news is that the number of new cases of the human disease has declined for the past two years. Estimates based on the latest data indicate that between 200 and 400 people will eventually die before the disease runs its course. The mad cow epidemic itself struggles on with 50 cows per month still put to the incinerator, a small figure compared with the thousands that were being slaughtered each month at the peak of the epidemic.
Why did BSE occur in Britain? The suggestion that it was simply because farmers had turned cattle into cannibals by making them eat meat and bone meal (MBM) is dismissed by Maxime Schwartz, a former director of the Pasteur Institute in Paris. In this lucid study, he shows that MBM infected with scrapie certainly could have jumped the species barrier when fed in sufficient quantity. But animal products were fed to cattle for at least a century before BSE emerged. The changes in MBM production introduced in the 1970s, which allowed for less intense heat treatment and removal of organic solvents in rendering, may have allowed prions to survive better than before. But on its own, this too is unlikely to have initiated the crisis as other nations adopted the same procedures without triggering BSE. For a while, it seemed that a more exotic explanation was needed. Could, for example, the rumour be true that exceptionally virulent prions from six white tigers that died at Bristol zoo in the 1970s ended up in MBM?
More recently, it has emerged that unique to British farmers was the practice of feeding MBM to calves as well as grown cattle. Young animals, it transpires, are more susceptible to orally transmit-ted prions than older ones (the same applies to humans). The first wave of dead animals would have been converted to MBM and masses of prions fed to cattle of all ages before it was clear that any- thing was wrong. The average time between infection and the onset of symptoms is around three years in cows, which means that huge numbers had been infected by the time a ban on MBM was introduced in 1988.
It is fortunate that the oral route for prion contamination is relatively inefficient in cattle, and even less so in humans - only 1 per cent of cattle that ate heavily infected MBM developed the disease. But direct injection of the agent into the bloodstream had appalling consequences in a separate human epidemic: 140 people injected with human growth hormone prepared from the pituitary glands of cadavers, some of which were themselves infected with poisonous prion variants, developed vCJD. This epidemic, too, is mercifully reaching its end, as a genetically engineered version of human growth hormone has been used since 1985. Had infection through the oral route been more effective, numbers of cases of vCJD could have run into the hundreds of thousands.
Infectious diseases demand our complete respect. New bugs are constantly emerging as a threat to humans. The severe acute respiratory syndrome (Sars) virus is the latest to cause widespread panic. Unheard of 25 years ago, the Aids virus now affects tens of millions of people. It is transmitted by blood contact. What if, like the common cold, the Aids virus had been airborne? In 1969, William Stewart, the then US surgeon general, in effect dismissed infectious diseases as a serious threat to human health. Such optimism today seems naive. Louis Pasteur, who discovered the link between germs and disease in the late 19th century, was more convincing when he predicted that man-kind would never be free from the threat of catastrophic microbial infection.
Michael Barrett is a lecturer at the Institute of Biomedical and Life Sciences at Glasgow University