Scientists lower estimate of mad-cow epidemic

LONDON -- Scientists have dramatically lowered their estimates of how many people are likely to die from the human form of mad cow disease, though experts say much uncertainty remains.

Part of the reason the estimate has dropped is because early predictions of worse-case scenarios did not materialize, forcing a shift in calculations.

Experts previously estimated that anywhere between a few hundred to 100,000 people in Britain could eventually get the fatal brain-wasting illness, variant Creutzfeldt-Jakob disease.

Now, new research published by the Royal Society, Britain's academy of scientists, forecasts that as few as 10 additional people and as many as 7,000 could get the illness by 2080. Variant CJD has killed 132 people so far -- 122 of them in Britain.

Much uncertainty still surrounds the likely course of the epidemic, said James Ironside, director of Britain's national Creutzfeldt-Jakob disease surveillance unit, who was not involved in the research.

Predicting a struggle

Since the disease emerged in England in 1996, the task of predicting how many people are likely to get it and when they will get sick has been a struggle because there have been fewer than 150 cases so far and the incubation period is unknown. Scientists believe it may be longer than 30 years.

"The only good way I think we could get a clear handle on the future number of cases is if there was a test we could apply to an anonymous cross-section of the population to see how many people are incubating the disease -- similar to the way it was done in the early days of HIV," Ironside said.

The latest predictions, made by infectious disease specialists at Imperial College of Science, Technology and Medicine in London, are published this week in Proceedings, Biological Sciences of the Royal Society.

The estimates are based on death trends and recent insight into the biology of the disease.

In a separate analysis, Ironside's national CJD Surveillance Unit reported this week in The Lancet medical journal that death rates from the disease are in decline -- at least for now.

Last year, 17 people died from it in Britain. That compared with 20 deaths in 2001 and 28 in 2000, the unit reported.

The illness occurs when normal proteins found in the brain, known as prions, change shape and prompt healthy prions to do the same. When enough prions have done so, they deposit a plaque on the brain and surround the mark with spongy holes, killing the victim.

Experts believe variant CJD comes from eating products from cows infected with a similar illness, bovine spongiform encephalopathy, also known as BSE or mad cow disease.

Cattle were infected in the late 1980s and early 1990s in Britain after they were fed meat and bone meal from infected animals. Since then, cases have been reported in many other countries, from Europe to Asia. Experts believe the disease was spread through exports of infected animals and meat products.

Three cases of variant CJD -- one each in Ireland, Canada and the United States -- occurred in people who had lived in or visited Britain at the height of the BSE epidemic. The other seven -- six in France and one in Italy -- occurred in people who had not been to Britain.

So far, all the variant CJD victims had a certain version of the gene that makes the normal prion protein. There are three versions of this gene and the variety linked to the disease is present in about 40 percent of the population, according to the Royal Society study.

"There's no way of telling whether the group of patients who got the disease and died are going to be the only ones who get it or whether they are just more susceptible than others and just got it sooner," Ironside said. "If the disease is ever identified in other genetic subgroups, then the whole scenario will have to be revisited and that would mean the number of people at risk would be substantially increased."

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On the Net:

British Department of Health Creutzfeldt-Jakob disease surveillance unit, http://www.doh.gov.uk/cjd/index.htm

The Royal Society, http://www.royalsoc.ac.uk