000840 “Mad Cow” Worries Tapering Off

HealthSCOUT News - The danger of contracting "mad cow disease" -- a deadly illness associated with contaminated beef -- appears to be much less than previously thought.

The number of cases of the disease is rising in England, but the overall number remains quite small, according to two new British studies. That reduces the estimates of how many people eventually might develop cases of what's medically referred to as Creutzfeldt-Jakob disease (CJD), the studies maintain.

A rare human neurodegenerative condition, CJD belongs to a family of illnesses in which the victim's brain suffers progressive spongy degeneration. These illnesses are known as TSEs, or transmissible spongiform encephalopathies.

Initial symptoms of CJD include depression or less often a schizophrenia- like psychosis. This is followed by neurological symptoms, including unsteadiness and involuntary movements. Shortly before death, victims become completely immobile and mute.

Not surprisingly, the appearance of "mad cow disease" has sparked massive public concern about the safety of the beef supply and the risk of contracting the human version of this ultimately fatal disease.

Researchers from London and Edinburgh, Scotland, say that, statistically, the number of "mad cow" deaths a year has nearly quadrupled since 1995. But the actual number of annual cases has risen from three in 1995 to 14 in the first six months of 2000, says study co-author Simon Cousens. And that means the overall number of CJD deaths remains low, he says.

"Cases are occurring at an increasing rate, and that's the first time we've been able to detect that," says Cousens, pointing to an annual increase of roughly 23%. But since there have been fewer than 100 cases identified in the last six years, he says, "the cumulative total remains relatively low." The findings appear in the current issue of The Lancet.

Those numbers should set off no alarm bells, the researchers say.

"The fact that the number of cases is increasing at the moment has nothing whatsoever to do with the current risk of infection," Cousens says. These patients acquired the disease years ago, and the numbers represent infection rates in the past, he says. CJD has a very long incubation period, Cousens says.

Still, no one's certain how many cases will develop in the future, the researchers say.

"We're dealing with a new disease that we've never seen before," Cousens says. "It's impossible to say with any confidence."

But the second study, appearing in the Aug. 10 issue of Nature, hopes to put reasonable limits on such predictions. It sharply revises an estimate made last year when the European Union Scientific Steering Committee speculated that as many as half a million people could have been exposed to the illness from a single infected cow.

Now, the authors of this study maintain that fewer than two cases of CJD result from the consumption of an infected cow.

That number points to what scientists call a species barrier, says Azra Ghani, a University of Oxford research fellow and lead author of the study. "In experimental studies, [a species barrier] tends to lengthen the incubation period," Ghani says.

The Oxford researchers developed a model that assumed that only people with a CJD-susceptible gene structure -- found in about 40% of the population -- faced a risk of contracting the disease. Using current mortality data, they estimated that, rather than facing an epidemic of CJD in the tens of millions, the number of cases is more likely to fall between 63 and 136,000. If the annual number of CJD cases in the next three years falls below 15, they say, the maximum number of cases falls to 20,000.

Also, according to Ghani, in order for the number of cases to rise above 6,000, the incubation period for the disease must be longer than 60 years. If the incubation period were between 10 and 20 years, the number of cases would fall below 6,000, he says.

But because the incubation period is unknown, Ghani says, scientists still can't estimate when the epidemic will peak.

However, using the numbers from first study, published in The Lancet, Ghani says, "that trend upwards isn't fast enough for there to be an incredibly large epidemic."

Even without such an epidemic, Ghani says, more research needs to be done into how the disease works and how it can be treated.

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