BOVINE spongiform encephalopathy, aka "mad cow disease", is almost extinct just 25 years after it was discovered. However, more cases of the human equivalent, variant Creutzfeldt-Jakob disease (vCJD), may be waiting in the wings.
New Scientist's analysis of the latest official BSE figures reveal the death throes of a terrifying disease that scratched beef from the menu for decades and decimated much of the world's beef industry. So far vCJD has claimed 170 human lives, mainly through consumption of BSE-infected beef.
In 2010 just 17 cases of BSE were recorded worldwide, according to figures released by the World Organisation for Animal Health (OIE) in Paris, France. Seven were in the UK, where the disease was first identified in 1986 and which became the centre of infection. Although figures for 2010 are not yet in for Spain and Portugal – two countries that experienced "mini" BSE epidemics between 1999 and 2003 – the numbers are not likely to add greatly to the dwindling global tally (see diagram).
In 1992, at the peak of the BSE epidemic, the UK reported 37,280 cases. Over the decade that followed the disease spread to mainland Europe, Japan and then to the US in 2003.
News of its impending demise has been hailed as a triumph for science, which discovered that BSE was caused by mutated brain proteins or prions, and came up with ways to prevent its spread. "It's a great success story for science, for scientific advice to government, and for the measures governments put in place to stop it spreading," says Chris Higgins of the University of Durham, UK, chairman of the Spongiform Encephalopathy Advisory Committee, which advises the British government on BSE.
The first confirmed cases of the mysterious disease, in 1984, came as a surprise. "This was a new type of disease with a new type of causal agent that was neither a bacterium nor a virus," says Higgins. "We had to discover what it was, how it was passed on and how to stop it."
Within two years, vets working for the UK government established that prions were to blame. They produced their first scientific report into the new disease in 1987. It described how mutated prions turn all normal copies they encounter into rogue versions like themselves, forming plaques in the brain that leave it full of holes.
That discovery helped to identify BSE's modus operandi: it was spreading via cattle feed that contained the ground-up remains of infected cow and sheep brains. Some sheep carried scrapie, a related prion disease. In 1988, the British government responded by banning the inclusion of meat and bonemeal in animal feed. "That broke the cycle of spread," says Alex Thiermann, head of the OIE commission responsible for animal health. The number of cases continued to rise until 1992 because of the prolonged incubation period before infected cows succumbed to the disease. After that the number of cases declined steadily, and may drop to single figures in 2011.
The handling of the BSE crisis was not a complete success story, however. Repeated assurances from the UK government that BSE couldn't spread to humans meant that the ban on feeding cattle to cattle and measures to strip infective material out of the human food chain were routinely flouted – a shortcoming deplored by an official inquiry.
In 1996, seven years after the UK government banned the sale of meat components likely to contain prions, such as brain and nerves, the UK's chief medical officer, Kenneth Calman, announced that a form of BSE had spread to people. Fifteen years later, vCJD has claimed 170 lives worldwide. Four people presumed to have vCJD are still alive.
"The average age at death is 28, compared with 70 for the most common form of CJD," says James Ironside of the CJD Surveillance Unitin Edinburgh, UK.
An abiding mystery, he says, is why so few people succumbed, even though almost the whole of the British population was probably exposed to infected brain and nerve tissue in beef before the disease was discovered.
Higgins says that, in theory, relatively few mutated prions are required to trigger the chain reaction that results in disease. He thinks that most of those exposed to infection stayed healthy simply because it is very difficult for prions of one species to influence those of another. "It turned out that the barriers for transfer from cows to humans were quite high," he says. "But at the start of the epidemic we didn't know this, so it was hard to predict whether there would be hundreds of deaths, or hundreds of thousands."
The biggest worry now is whether there will be further waves of vCJD. This may depend on which two copies of the prion gene you inherit. There are two variants, M and V, and three possible combinations – MM, MV and VV. All infections so far have been in people with the MM variants, who make up 37 per cent of the population. But those with the other combinations may also be at risk, based on experience with a disease similar to BSE called kuru, found in the Fore people of Papua New Guinea and spread by the now discontinued ritual of eating relatives' brains at funerals.
Although most victims of kuru are MM, John Collinge of St Mary's Hospital, London, discovered in 2000 that a group who developed it later in life than the others were all MV. The implication is that the gene variants you inherit dictate the incubation period for prion infection, so vCJD may come in three separate waves.
However, Ironside is encouraged by the relatively low numbers of MM cases, and the non-appearance of any definite "new waves" so far. "The longer it goes on without us seeing these cases, the more likely it is that the number of any new cases will be small," he says.
Crucially, the science of prion disease is much more certain than it was in the mid-1980s. Stricter surveillance of all farm animals, coupled with laws to prevent ruminants being fed to other ruminants, should ensure that something like BSE never gets a chance to spread so widely again. "If it does pop up, we should pick it out early," Higgins says.