Estimating the extent of human BSE: The legacy of mad cow disease

November 28, 2001 The Daily Telegraph (London) by Roger Highfield
Fifteen years ago this month, a new disease was recognised: bovine spongiform encephalopathy - BSE. It has since killed about 180,000 cattle, led to the destruction of millions more and cost the taxpayer billions. Roger Highfield,

Science Editor, examines the resulting epidemic of human

BSE and the risk that the disease may now have spread to sheep

The lowest and most reassuring estimate so far of the full extent of "human BSE" has been published in the past few days, with an Anglo-French team predicting 200 cases overall. However, many uncertainties remain. And this estimate is already proving controversial, with other epidemiologists predicting that thousands or even 100,000 people will succumb to the incurable brain disease.

The current toll of human BSE, or variant Creutzfeldt-Jakob disease, is 111 and predictions of the overall number of deaths vary widely because, although almost a million infected animals were eaten between 1980 and 1996, no one knows how many people became infected nor how long it takes for symptoms to develop.

The new estimate, published in the current issue of the journal Science, differs from earlier work because the researchers used a different mathematical approach, one that focused on the low average age (28 years) of the people with vCJD.

Prof Alain-Jacques Valleron of the Saint-Antoine Hospital in Paris and colleagues at the Western General Hospital, Edinburgh, propose a model in which children are most susceptible to the disease, and then rapidly become resistant after 15.

"We start with the more striking characteristic of this disease: the cases are young, and there is no explanation for this, neither from the epidemiologists, nor from the basic science," said Prof Valleron. His team predicts that the incubation period is approximately 17 years, and that the epidemic is at its peak. From this, the team derives an overall total of 200 cases, far fewer than previously predicted.

Prof Valleron said that the earlier estimates were wide-ranging because of the wide range of incubation times assumed by other teams doing the sums.

Last month, Prof Peter Smith, Dr Jerome Huillard and colleagues at the London School of Hygiene and Tropical Medicine predicted that there would be no more than "several thousand" cases, compared with its prediction of more than 100,000 four years ago, with "hundreds" the most likely.

Dr Huillard said that the various attempts to predict the size and shape of the epidemic agree that the final total number of cases of vCJD could remain relatively low. "Where there is disagreement is in what the maximum epidemic size could be."

However, Prof Roy Anderson, Prof Neil Ferguson and Dr Azra Ghani of Imperial College, London, believe that the new work by Prof Valleron is much too optimistic and that the final toll will be closer to the 100,000 estimate.

Prof Ferguson said they were more pessimistic because, unlike the London School team, they had corrected their figures for human exposure to allow for under-reporting early in the epidemic. And, rerunning Prof Valleron's analysis, "We have come up with much higher upper bounds on vCJD epidemic size using their model and assumptions," said Prof Ferguson.

But Prof Valleron said that his estimate of incubation time in Prof Smith's model produces a figure of 800, and in the Imperial model, between 80 and 630. In either case, he said, the estimates of the disease impact had dropped significantly. [Comment on BSE-L 25 Nov 2001 to 27 Nov 2001 (#2001-292) by Roland Heynkes:

"We all know that the running vCJD epidemic is not the epidemic of the whole British population. Until now the victims are all Met/ Met-homozygot at codon 129 and it is very likely, that there are further factors, that shortened their incubation times. Therefore what we see, is only the first wave of the most susceptible people with the shortest incubation periods. As we don't know this factors and as we don't know how many people actually have this factors, it is of course nonsense to extrapolate from this first wave to the whole population..."
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