Scientists scale back predictions of CJD epidemic

Scientists have ruled out a large-scale epidemic of vCJD, the human brain disease believed to be caused by eating beef infected with bovine spongiform encephalopathy (BSE).

Scientists have ruled out a large-scale epidemic of vCJD, the human brain disease believed to be caused by eating beef infected with bovine spongiform encephalopathy (BSE).

A new analysis of variant Creutzfeldt-Jakob disease (vCJD) suggests that fewer than 6,000 people are likely to be infected. A year ago, a similar analysis suggested that many hundreds of thousands could eventually die of the illness.

Scientists from the Wellcome Trust Centre for the Epidemiology of Infectious Disease in Oxford based their analysis on the assumption that only 40 per cent of the population are genetically susceptible to vCJD and that the average incubation period of the disease is no longer than 60 years.

If the incubation period turns out to be longer than this, the final epidemic could reach as many as 136,000 cases, still considerably fewer than forecasts made last year.

Azra Ghani, a Wellcome research fellow and member of the research team, said the new analysis takes into account more recent data on the number of people known to have contracted vCJD. Previous analyses were based solely on the number of infected cattle thought to have entered the human food chain.

"What we've been able to rule out is the possibility of an epidemic of many millions. It's very difficult to predict any epidemic before it has peaked," Dr Ghani said.

The scientists, led by Professor Roy Anderson, a distinguished epidemiologist and government adviser on BSE and Aids, based their predictions on the calculation that 750,000 cattle infected with BSE entered the food chain between 1980 and about 1996.

So far, 79 cases of vCJD have been reported in Britain, with the death toll increasing by one-third each year. The Oxford scientists conclude that although the number of vCJD cases is increasing, the rate of increase does not make a very large epidemic likely.

They say their estimate of the upper limit of 136,000 cases would fall significantly if new cases over the next few years remain stable. In a paper in the journal Nature, the scientists write: "If the average annual incidence of vCJD over the next three years is fewer than 15 cases, then the maximum total number of cases would fall to approximately 20,000,"

A year ago they estimated that, in a worst-case scenario, one cow incubating BSE could on average have given rise to more than 100 cases of vCJD. Now the computer models suggest that this figure is nearer two. "This suggests a substantial species barrier, given that thousands of people might eat material from a single animal," the scientists say.

The Oxford analysis supports the view that younger people are disproportionately represented in the vCJD statistics as a result of them either being more susceptible to the disease or having eaten larger amounts of infected material.

Although vCJD is still an exceptionally rare disease, scientists are puzzled as to why it seems to be more prevalent in people under 35 years old. The youngest victims to date are two 14-year-olds.

The figures also showed a statistically significant cluster of cases near Leicester. An inquiry has been opened to try to explain the cluster, but scientists say privately that finding a cause may not be possible.