Scientists doubt cases of 'human' mad cow missed
Published: Monday, January 12, 2004 at 6:01 a.m.
Last Modified: Sunday, January 11, 2004 at 11:14 p.m.
Clearing up CJD confusion: Key differences
Source: Centers for Disease Control and Prevention; National Prion Disease Pathology Surveillance Center
Could scientists have missed a case or cases of the human version of mad cow disease in the United States? Could it be that some of the 250 to 300 U.S. cases of classic Creutzfeldt-Jakob disease, or CJD, reported each year could be traced back to eating beef from cattle infected with mad cow disease?
Possible, but unlikely, most researchers agree.
"Could there be one missed case out there? Maybe," said Lawrence Schonberger, an epidemiologist at the Centers for Disease Control and Prevention who has studied classic CJD for more than a decade. "In this game, we never say something's impossible."
Unlikely, however, in the view of Richard Moyer, professor of molecular genetics and microbiology in the University of Florida's College of Medicine.
"If you compare human symptomatology for 'mad cow disease' with CJD, the symptoms are different," Moyer said.
Both, however, are invariably fatal.
"There are, of course, deaths from CJD that get listed as 'cause unknown,' " he said, but that can probably be attributed to the fact that the only definitive test for CJD is through a brain biopsy in the living patient or analysis of the brain tissue after death, which would be done as part of an autopsy.
Creutzfeldt-Jakob disease, pronounced "Kroits-felt-yaw-kobe," and its variant, the so-called "mad cow disease," are part of a family of diseases linked to prions, rogue proteins that cause sponge-like holes in the brain.
One, a chronic wasting disease, has spread into deer and elk herds in at least 12 states. Another, called kuru, was discovered in cannibals in Papua New Guinea who had eaten human brain tissue.
Creutzfeldt-Jakob disease was first identified in the 1920s. This quickly fatal human dementia affects about one person per million throughout the world.
In some cases, it can have a genetic link, but in most, the cause remains unknown. In most cases, the victims are older, between 50 and 75.
Some 250 to 300 Americans each year develop this classic form of CJD.
Variant CJD is a different version - perhaps even a different disease - that has been linked to eating tainted beef.
As of Dec. 1, 2003, the Creutzfeldt-Jakob disease surveillance unit in Edinburgh reported a total of 153 definite or probable cases of variant CJD. Six of the victims were still alive.
The Centers for Disease Control and Prevention reports that there has never been a case of variant CJD that did not occur in a country where mad cow disease, or bovine spongiform encephalopathy, or BSE, was present.
Again, there is no known treatment and it is invariably fatal. Most cases are in people under the age of 30. In every case, it appears that the victim became infected after eating a beef product contaminated with BSE.
The only person in the United States confirmed to have the human form of mad cow disease is a young woman, now a 24-year-old Miami resident, who spent the first 13 years of her life living in Britain. She began to develop symptoms at 22, and is now bedridden, with no hope of surviving. At the request of her family, the CDC identifies her only by her first name, Charlene.
Much as the symptoms of BSE develop slowly in cattle, those of variant CJD may develop years or even decades after a person has eaten contaminated meat, scientists believe. The variant was first reported in 1996; no cases with symptoms that began before 1994 have yet turned up. According to the surveillance unit, six cases have been reported in France, one in Ireland, one in Italy, one in Canada and one in the United States. The remaining cases have been confined to Britain.
In both forms of CJD, mutant proteins eat holes in the brain, but under the microscope there are distinct differences in the damaged brain tissue. In tissue from variant CJD victims, the spongy holes surround deposits of plaque that aren't present in the classic form.
Moyer said there are any number of questions of interest to scientists about variant CJD that still remain to be answered.
"There are some things that still don't ring true about the human variant of mad cow disease," he said. "You would expect from an epidemiological point of view that people who work at the slaughterhouse would contract it much more frequently. But there is no evidence of an increased incidence."
Other questions surround the prions that are thought to cause Creutzfeldt-Jakob disease.
Prions are normal proteins with an abnormal shape, and that is what causes the problem, Moyer said, yet no one necessarily knows why they assume that rogue form within the tissues of the brain and spinal cord. Do they also have some positive, supportive function?
While scientists consider the many unknowns surrounding mad cow disease, should the public change its dietary habits?
That's a question individuals will have to answer for themselves, Moyer said.
"Do those unknowns bother you enough to change what you eat?" he asked.
"It doesn't bother me . . . and I think that is the acid test. If you have relatively educated scientists saying it is not a risk to eat beef in this country, would you change your eating habits? Frankly, at this point I don't think I would."
Diane Chun can be reached at 374-5041 or email@example.com.
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