Mad cow comes to town!
2. Mad-cow history
3. "Abundant caution"
4. Making sense of the
5. The weirdest agent
Bovine spongiform encephalopathy
(BSE): Mad cow disease.
Chronic wasting disease:
A TSE in deer and elk.
Creutzfeldt-Jakob disease (CJD):
Fatal, human TSE, strikes one person in a million.
Prion: A protein that, when
misshapen, can cause other prions to fold wrongly. Causes all TSEs.
Scrapie: A TSE in sheep.
Known since the 1700s; does not infect people.
Transmissible spongiform encephalopathy:
TSE. Infectious brain diseases caused by prions.
Variant CJD (vCJD): The
"human" version of mad cow. Similar to CJD; strikes younger
This 4,000 pound-capacity stainless steel
tank is the heart of a state-of-the-art mobile tissue digester.
The $900,000 digester uses heat, pressure and caustic chemicals
-- sodium hydroxide or potassium hydroxide -- to reduce animal and
microbial tissues to a sterile slurry that can be safely disposed
of in a sanitary sewer. The digester is being used to dispose of
deer carcasses tested in the context of Wisconsin's outbreak
wasting disease. Photo by Jeff
When scientists talk prions, they’re referring
to different varieties of one protein -- PrP -- which
appears widely in all mammals. In its normal form (PrPc, the c standing
for cellular), the protein sits on the outside of cell membranes
and is entirely benign. Presumably, it has a function, but oddly,
despite a lot of work on the subject, scientists have yet to pin
normal and diseased prions have different structures but the same
amino-acid sequence. Normal PrPC is mostly alpha-helix (the tubes)
and some beta-structures (the waved arrows) connected by loops (the
linear sections). The yellow gadget at bottom attaches the protein
to cell membranes. The misshapen protein PrPSc has more of a beta-structure,
which may explain its resistance to -degrading enzymes, and thus
its durability. Image: ©McCusker
and Novakofski, University of Illinois
PrP can misfold into a shape called PrPSc (named
for the disease scrapie), usually through contact with the infectious
agent,that bundles into clumps. The clumps cause plaques in the
brain that are the trademark of the transmissible spongiform encephalopathies,
or TSEs. As the name suggests, an infected brain becomes riddled
with holes, until it resembles a sponge, at least under a microscope.
Tiny holes in the brain's grey matter gives
the brain of BSE-affected cows a spongelike appearance when tissue
sections are examined in the lab. Photo
by Dr. Al Jenny USDA.
In people, the most common prion disease is
Creutzfeldt-Jakob disease, or CJD, which affects about one person
in 1 million. Much rarer are Gerstmann-Straussler-Scheinker syndrome,
fatal familial insomnia, kuru and Alpers Syndrome. All human varieties
are similar, and with the exception of kuru (which is only known
to have occurred among isolated tribes in Papua New Guinea, who
once practiced cannibalism) are distinguishable mostly by age of
Cows get bovine spongiform encephalopathy
(BSE), AKA mad cow disease. Sheep can contract scrapie, the oldest
TSE, and deer and elk can carry chronic wasting disease. Sometimes,
as probably happened with mad cow disease in Europe, the disease
may jump to another species unlucky enough to eat a diseased animal.
infection occurs through diet (eating infectious parts of mad
cow) or medical procedures (including brain surgery with contaminated
equipment), growth hormone injections (which were once extracted
from human pituitary glands, which is located near the brain)
or corneal transplants. As doctors have identified these routes,
medical procedures have changed to eliminate them.
transmission, caused a rare mutation in the gene that codes for
the normal form of the protein, explains about 15 percent of human
prion disease cases.
disease. The prion changes shape for no known reason.
According to the Centers for Disease Control,
213 Americans died of Creutzfeldt-Jakob disease in 2000, including
inherited and spontaneous disease. None of the cases was blamed
on eating meat.
Whatever their original source, prion diseases
seem to undergo a chain reaction in the brain: The distorted form
of the prion (PrPSc) causes native PrP to contort into the diseased
form. Since prions appear largely in central nervous system tissue
-- brain and spinal cord -- prion diseases can spread through eating
infected tissue. Most scientists think bovine spongiform encephalopathy
(BSE, or mad cow disease) causes the human disease variant Creutzfeldt-Jakob
No treatment can cure or control any prion diseases.
Although some drugs can relieve pain and muscle spasms, about 90
percent of patients die within a year.
According to the Centers for Disease Control, the risk of contracting vCJD from
BSE-infected beef remains very low in the United States.
Experts say boneless cuts of beef are the
safest, since they are least likely to contain nerve tissue. Processed
beef products, including hamburger, hot dogs, and sausage, may pose
a higher threat since spinal tissue is more likely to enter the
product. Mechanically recovered meat, which is extracted from bones
at high pressure, is even more risky. Added to ground meat and other
meat products as a kind of meat pulp, mechanically recovered meat
is facing tighter regulation.
Organic beef, reports Consumers
Union, carries the least risk, because organic beef producers
do not use animal by-products.
As lone proteins with no genetic material, prions
aren't considered organisms, so they can't be killed. But proteins
can be destroyed or, in scientific jargon, "denatured," using enzymes,
heat or other means. Prions, however, are notoriously hardy, and
resist normal disinfection techniques like boiling or irradiation.
How to stamp out prions? The list is short,
the methods vicious:
Yum. Plenty of infectious protein
in our bibliography.