ALLAN SISON
Medical Tribune - December 21, 1999
The proteins that cause ``mad cow disease'' have infected humans.
Researchers have found that the strain of
infectious proteins that cause ``mad cow disease'' is the same strain that
causes a new variant form of Creutzfeldt-Jakob disease (nvCJD), a fatal human
disease that causes brain degeneration.
To date, the cases of ``mad cow disease'' and
its human equivalent have all occurred in the United Kingdom; there have been no
reports of ``mad cow disease'' or nvCJD in the United States.
More than 175,000 cattle have died from ``mad
cow disease'' or bovine spongiform encephalopathy (BSE) in the United Kingdom
over the past 10 years. Since 1995, more than 50 people in the United Kingdom
have died from new variant Creutzfeldt-Jakob disease. Until recently, the death
rate from nvCJD had been fairly constant every year. However, a total of nine
new cases were reported during the last quarter of 1998.
Both diseases are caused by infectious agents
called prions and result in destruction of brain tissue. Prions are proteins
that have an altered shape, which makes the protein destructive. Prions also
cause scrapie, a similar condition that affects sheep.
Researchers at the University of California at
San Francisco Institute for Neurodegenerative Diseases engineered mice to
contain genes for normal cow prion protein. By themselves, normal prions, which
have been found in all mammals and birds, are not harmful. Prions become
destructive when their shape is altered either through infection or a genetic
mutation.
The scientists then infected these mice with
destructive prions from diseased cows. A second group of the genetically
engineered mice was then infected with prions from humans with nvCJD. The
researchers found that both groups exhibited the same incubation period the time
between infection and the onset of disease and pattern of brain damage. The
researchers therefore concluded that the two prions were the same.
The researchers also infected a separate group
of mice with prions from sheep with scrapie. The scientists found that the
pattern of brain damage was very different than the damage caused by the prions
from diseased cows and humans.
``These findings argue unequivocally that BSE
and new variant CJD are the same strain of prion,'' said Dr. Stephen DeArmond,
professor of pathology and chief of the division of neuropathology at UCSF and a
senior author of the study.
The study is published in the December 20
issue of Proceedings of the National Academy of Sciences (www.pnas.org).
No one is sure how ``mad cow disease'' came
about, but one theory has been proposed. Cattle may have developed the disease
after being fed meat and bone meal from sheep infected with scrapie. The prions
from the infected sheep meat may have then infected the cattle, causing ``mad
cow disease.'' Since 1997, the U.S. Food and Drug Administration has banned the
use of most mammalian protein for use in animal feeds.
Experts assumed that a phenomenon known as the
species barrier would protect humans from being infected by ``mad cow disease.''
The species barrier contends that prions of one species cannot infect organisms
in a different species. This barrier has apparently been breached.
``It now seems clear that nvCJD arose through
exposure of humans to BSE,'' the researchers concluded.
Proceedings of the National Academy of
Sciences (1999;96:15137-15142)
(The Medical Tribune Web site is at http://www.medtrib.com)
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