LONDON - An outbreak of a fatal neurodegenerative disease that has been linked to last fall " mad cow disease "could be unfolding, warned British scientists in a press conference today. Although only 15 cases of variant Creutzfeldt-Jakob (vCJD) have been confirmed since the beginning of 1994, the researchers said, a numbering epidemic "thousands" of cases is still possible.
A team led by epidemiologists Simon Cousens and Peter Smith, of the London School of Hygiene & Tropical Medicine, in collaboration with the Edinburgh CJD Surveillance Unit, Scotland, used mathematical models based on other infectious diseases to predict the number of new cases of vCJD. They assumed that vCJD - a disease characterized by tremors, memory loss, and dementia tends to strike people under 40 - resulting from exposure to products from cattle infected with bovine spongiform encephalopathy (BSE), or mad cow disease. This group found that the number of people who would need to have been infected - for many years of exposure likely contaminated cattle products - to have transferred the 13 cases whose apparition began in 1994 and 1995 .
scientists have assumed that the number of people infected each year was proportional to the number of new infections to BSE in cattle. They also felt that after a national ban on contaminated beef offal at the end of 1989 - designed to prevent infectious organs most likely to enter the food chain - human infections have been reduced by 0% to 100% . They also assumed that each infected person eventually develops the disease. Yet many uncertainties were more difficult to estimate. For example, the incubation period of vCJD is unknown, so epidemiologists based their forecasts on incubation periods lasting from 10 to 25 years
For each scenario, Cousens and his team predicted the total number of human infections that could occur and the expected number of cases of vCJD should ask themselves between 1996 and 1998 in a scenario, for example, if the number of cases each year had to pass 20 in 1996, 30 in 1997 and 50 in 1998, the epidemic would peak at about 10 cases. But if the pattern was 25, 50 and 100 cases over the three years, the epidemic could reach 12,000. Over the incubation period, the more the predicted epidemic. The details of their calculations will be published in tomorrow's issue of Nature .
Refining these estimates, Smith said, requires "all that we can detect the [human] infection at an early stage." It is also useful, Cousens added, would be a better estimate of when contaminated beef products are most likely to infect humans. Meanwhile, he said, "in 00 and beyond, it can still be huge uncertainty ... the size of the epidemic."
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