Scientists fear BSE link to second type of brain disease

"Mad cow" disease may be causing a second epidemic of brain illness in humans which has so far gone unrecognised as being related to bovine spongiform encephalopathy (BSE).

Scientists have found evidence that BSE can cause both the variant form of Creutzfeldt-Jakob disease (vCJD) and the more common "sporadic" form of CJD. Until now, the two were thought to be unrelated.

Experiments on mice have led researchers to believe that the increase in sporadic CJD seen in Britain and other European countries might be due at least in part to the consumption of BSE-contaminated food in the 1980s and early 1990s.

Epidemiologists have dismissed any link between sporadic CJD and BSE, arguing that the doubling of cases of the human disease over the past decade is the result of better diagnosis or "ascertainment".

But Professor John Collinge, head of the Medical Research Council's prion unit at University College London, found that some mice injected with BSE developed vCJD as expected. But others went on to develop a form of brain disease that was indistinguishable from sporadic CJD in humans. "There has been a steady rise in the incidence of sporadic CJD. This has been put down to better ascertainment. What we're speculating is that a proportion of that rise, not all of it but a proportion, is due to BSE," Professor Collinge said.

In Britain, the incidence of sporadic CJD has risen from about 30 cases a year in the early 1990s to between 50 and 60 cases a year. Cases of vCJD – known as "human BSE" – have risen from three cases in 1995 to about 25 cases a year.

Because sporadic CJD had been recognised long before BSE, it was assumed that the recent increase was unrelated to the cattle disease. Now some specialists are questioning this. Adriano Aguzzi, head of neuropathology at Zurich University and a member of Britain's spongiform encephalopathy advisory committee, said a doubling of sporadic CJD in Switzerland could also be caused by BSE.

"We used to have something like 10 cases per year and in the last year we had 20. This year it appears to be even further up so obviously something is happening but we don't quite understand what," he said. "The hypothesis that BSE will only give rise to what we call vCJD ... is on tenuous grounds. We just don't know."

Professor Collinge's study, published today in the journal of the European Molecular Biology Organisation, involved mice that had been genetically engineered to possess the human form of the prion protein, which is known to be involved in triggering BSE and CJD. Some of the mice injected with BSE developed a disease that precisely matched the symptoms of vCJD in humans. Others, however, unexpectedly went on to develop symptoms identical to sporadic CJD.

Professor Collinge said the results questioned some of the assumptions made about the cause of the sporadic disease in humans. "What it should prompt is a re-evaluation," he said. "It might have several causes and one of those might be BSE."