杏吧原创

WHEN the British government announced in 1996 that BSE had passed from cows into people, even the nation鈥檚 famously cynical journalists were shocked. When a lone public health expert announced last week that in all likelihood it hadn鈥檛, shock simply didn鈥檛 come into it.

True, some, including the relatives of the dead, were hurt and angry. But most people, journalists included, were just plain baffled by the claim that variant Creutzfeldt-Jakob disease (vCJD) might after all not be the human form of BSE.

It鈥檚 not hard to see why. While mainstream BSE researchers swiftly rejected the claim, the paper making it was published in the British Medical Journal-one of the world鈥檚 most prestigious journals. That suggested it had been vetted by independent scientists and must at the very least be logical and plausible. But is it?

George Venters, the public health expert at the centre of the new controversy, certainly thinks so. A consultant for Lanarkshire Health Authority in Hamilton, Scotland, Venters argues that vCJD, though similar to BSE, is more likely to be an old human disease that鈥檚 only surfaced because doctors started to look out for signs of BSE in humans in the early 1990s. If he鈥檚 right, governments across the world have introduced a host of costly control measures that are not strictly necessary.

Venters鈥檚 argument rests not on any new experimental findings but on a series of questions he raises about the quality of the evidence linking vCJD to contaminated beef. Each of his arguments is contested by BSE experts. But our inquiries reveal that while many are perhaps unconvincing, some highlight genuine gaps in scientific knowledge.

One attack focuses on the undramatic growth of the vCJD epidemic. During the early years of BSE, the number of infected cattle rose exponentially. If people got vCJD from eating these infected animals, says Venters, you鈥檇 expect the number of human victims being diagnosed per year to rise just as sharply in the first few years of the human epidemic. That hasn鈥檛 happened. Also, if the disease was in food, it should affect all age groups in the population whereas so far nearly all the victims have been under 30.

Others dispute these claims. 鈥淭he curve for vCJD is consistent with exposure in the late 1980s,鈥� insists Roy Anderson, an epidemiologist at Imperial College, London, who has monitored BSE and vCJD from the outset. And there are reasons why there might be more young victims. Youngsters may have eaten more suspect food such as burgers or they may be more biologically susceptible to the prion agent that causes BSE.

It鈥檚 possible too that older people will develop the disease as part of a 鈥渟econd wave鈥�. All victims so far had a particular combination of prion genes called 鈥淢M鈥�, which is found in 38 per cent of the population. People with other combinations might also succumb, but incubate the disease for longer.

Venters also claims that crucial studies in mice designed to prove that BSE and vCJD are one and the same are flawed. Again, our investigations found the true picture was more complicated than his paper suggests.

In 1999, scientists in California injected mice with brain material from people with vCJD and compared them to mice injected with BSE. Both groups of animals appeared to develop the same disease, suggesting that BSE and vCJD are the same illness.

Venters insists the experiment is flawed because the mice were genetically engineered. Their own 鈥減rion gene鈥� had been replaced with the equivalent cattle gene. 鈥淚t鈥檚 the wrong experiment-we don鈥檛 feed human brain to cattle,鈥� says Venters. What鈥檚 more, mice carrying the human version of the gene-arguably better models of human susceptibility-resisted infection with BSE or vCJD.

The researchers who performed the experiment say Venters is missing the point. It proves that 鈥淏SE and vCJD are identical when passaged through the same genetic environment鈥�, says Mike Scott of the University of California in San Francisco.

Venters鈥檚 paper also overlooks an important earlier experiment in mice. In 1997, Moira Bruce and her team in Edinburgh showed that ordinary mice injected either with BSE or with vCJD, develop the same disease. Crucially, the disease differs markedly from what they get if injected with any other form of prion disease, such as scrapie or human sporadic CJD.

鈥淲e know that vCJD is more BSE-like than the others, in terms of several parameters,鈥� says Anderson. These include the spongy appearance of the brain, the unusual biochemistry of the BSE and vCJD prions, and the close similarities in the time it takes to cause disease in animals.

Still, nobody has yet succeeded in infecting mice carrying the human version of the prion gene with BSE. Nor, as Venters points out, has anyone yet proved that you can catch prion diseases from cooked meat or offal.

Take scrapie, a disease similar to BSE that鈥檚 also caused by prions. Despite its existence in sheep for at least 250 years, scrapie has never spread to humans via meat. And according to Venters, you wouldn鈥檛 expect it to. The prion protein that causes such diseases is incredibly hardy, but, he says, there鈥檚 no direct evidence it can survive cooking, digestion and the human immune system.

Ministers and scientists themselves once used such arguments to reassure consumers that beef was safe to eat. Now most experts believe prions can jump from ruminants into people. Plenty of experiments have shown that animals including primates go down with BSE after eating BSE-infected food, says Anderson. Cooking doesn鈥檛 matter, most experts claim, because high temperatures do not destroy the BSE prion.

If vCJD is not BSE, then what is it? A disease that was already with us if you believe Venters. Indeed, he claims that the first ever case of conventional, or 鈥渟poradic鈥�, CJD, reported in 1920, might actually have been vCJD.

One of the key factors setting conventional CJD apart from vCJD is supposed to be the age at which disease strikes. Most vCJD victims are younger than 30, most conventional CJD victims older than 60. But this first documented victim of conventional CJD was just 23 years old. She was as young as many of the 107 victims of vCJD so far. Maybe she had the same disease.

And maybe, says Venters, that disease was not in fact classical CJD at all, but Kuru, a similar type of brain-wasting disease that famously affected thousands of the Fore tribe of Papua New Guinea in the 1950s.

Papua New Guinea suffered a major outbreak because of cannibalistic funeral rites in which bereaved family members ate or handled the brains of their dead relatives. Venters is not suggesting anything like this happened in Britain. His point is simply that even before the BSE epidemic there was a prion agent out there in human populations capable of causing a brain condition similar to vCJD in the young as well as the old. 鈥淵et the possibility of them being the same disease was never raised,鈥� he says.

The reason for that is quite simple, says Peter Smith, who chairs the government鈥檚 Spongiform Encephalopathy Advisory Committee: vCJD and Kuru do quite different things to people鈥檚 brains. 鈥淜uru looks like classical sporadic CJD pathologically, not like vCJD.鈥�

On the issue of age, Venters鈥檚 arguments also run into trouble. Historically there have only been four or five recorded cases of conventional CJD in young people globally. More than 100 young victims since 1996 in England alone is therefore hard to explain.

But maybe not impossible. Astonishingly, we now know that before BSE happened doctors missed nearly two-thirds of all classical CJD cases. During the 1980s doctors were diagnosing 20 to 30 cases a year. By 1997, after Britain had set up its CJD Surveillance Unit in Edinburgh, they were more vigilant and picking up three times that number, even though the actual incidence would not have increased in that time.

So before BSE, scores of CJD deaths in Britain were actually being recorded as something else. This more than anything else makes Venters believe cases in youngsters may have been missed too. 鈥淚t鈥檚 time to open this to debate and re-examination,鈥� he concludes.