LONDON--An epidemic of a fatal neurodegenerative disorder that was linked last fall to "mad cow disease" could be unfolding, warned British scientists in a press conference here today. Although only 15 cases of variant Creutzfeldt-Jakob disease (vCJD) have been confirmed since early 1994, the researchers said, an outbreak numbering "many thousands" of cases is still possible.
A team led by epidemiologists Simon Cousens and Peter Smith of the London School of Hygiene and Tropical Medicine, working with the CJD Surveillance Unit in Edinburgh, Scotland, used mathematical models based on other infectious diseases to predict the numbers of new cases of vCJD. They assumed that vCJD--a disease characterized by tremors, memory loss, and dementia that tends to strike people under 40--results from exposure to products derived from cattle infected with bovine spongiform encephalopathy (BSE), or mad cow disease. From this the group estimated the number of people who would need to have been infected--over many years of likely exposure to tainted cattle products--in order to have yielded the 13 cases whose onset began in 1994 and 1995.
The scientists assumed that the number of people infected each year was proportional to the number of new BSE infections in cattle. They also estimated that after a national ban on tainted cattle offal in late 1989--designed to prevent the most likely infectious organs from entering the food chain--human infections were reduced by 90% to 100%. They also assumed that every infected person eventually develops the disease. Still, several uncertainties were harder to estimate: For instance, the incubation period of vCJD is unknown, so the epidemiologists based their forecasts on incubation periods lasting anywhere from 10 to 25 years.
For each scenario, Cousens and his team predicted the total number of human infections that might have occurred and the expected number of vCJD cases expected to arise between 1996 and 1998. In one scenario, for example, if the number of cases each year were to rise from 20 in 1996, to 30 in 1997, and to 50 in 1998, the outbreak would top out at about 1600 cases. But if the pattern were 25, 50, and 100 cases over the 3 years, the epidemic could reach 12,000 people. The longer the incubation period, the larger the predicted epidemic. The details of their calculations will be published in tomorrow's issue of Nature.
Refining these estimates, says Smith, requires "anything that enables us to detect the [human] infection at an earlier stage." Also helpful, adds Cousens, would be better estimates of when tainted cattle products are most likely to infect humans. Meanwhile, he says, "into the year 2000 and beyond, there may still be enormous uncertainty about ... the size of the epidemic."