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Study - Mad cow disease among British at peak
WASHINGTON -- The epidemic of British people infected with a form of mad cow disease may have peaked and the number of deaths from may gradually go down in the coming years, according to a new study.
French researchers say that a new computer model suggests that more than half of the total British cases of new variant Creutzfeld-Jakob caused by eating tainted meat have already occurred and that the peak of the epidemic has passed. There have been a total of 110 people in Britain diagnosed with the disease.
The French researchers, in a study appearing today in the journal Science, said that "the peak of the epidemic will be in 2000-2001 and that the annual number of cases should gradually decrease after this date."
New variant Creutzfeld-Jakob disease, or vCJD, is thought to be caused by eating meat from cattle infected with mad cow disease, known formally as bovine spongiform encephalopaty, or BSE.
About a million cattle in Britain are thought to have been infected with BSE which they caught from feed made from the carcasses of sheep infected with scrapie, an ovine form of the disease.
The diseases are caused by a type of protein, called a prion, that is misshaped. The prion is able to cause other prions to change their shape to the abnormal form. When enough of these infectious prions are present, they cause spongy-like holes to develop in the brain, leading to death.
Experts believe that BSE may have slipped into the British beef supply as early as 1980 and hundreds of thousands of people could have been exposed to the infected meat before November 1989, when the use of sheep carcasses for cattle feed was banned.
Since the ban, medical experts have been trying to estimate how many Brits are apt to come down with vCJD. Some researchers have said that up to 136,000 people may develop the disease over the coming decades.
But the French researchers led by Dr. Alain-Jacques Valleron now estimate that the vCJD epidemic may not be nearly as extensive as the earlier studies had suggested.
Valleron, a scientist at the French National Institute of Health and Medical Research, or INSERM, said that the new analysis is based on a better understanding of the incubation period for vCJD and on evidence that young people are more susceptible to the disease. Using these factors, he said, a computer model suggests that the total number of vCJD cases will most likely be about 205, with an upper limit estimate of no more than 403.
The earlier estimates by researchers in 1997 and 1999, said Valleron, were based on fewer proven cases of the disease and included a higher degree of uncertainty about the incubation period of vCJD.
"Now we have enough cases to make a better estimate of the incubation period," he said.
Valleron also noted that a "striking" characteristic of the epidemic is that most of the cases of vCJD in Britain are young, with an average age at death of 28. He notes that only six patients were older than 50.
From this, the researchers based their prediction on the assumption that the probability of a person becoming infected decreases with age, with the chances of developing the disease dropping significantly after age 15.
In a commentary in Science, Dr. Graham F. Medley of the University of Warwick in Britain, said that there remain many uncertainties in predicting the course of the vCJD epidemic in Britain and that using the age of victims as a key assumption in the French model "can only be unsupported speculation."
Medley said that because researchers must make assumptions in their computer models, "predictions of the vCJD epidemic will continue to be plagued by wide confidence intervals (broad uncertainties.)
While he warns about the uncertainties in analysis by Valleron and others, Medley said it was encouraging that each new prediction seems to drop the estimated number of cases expected from vCJD.