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THE G E OMETRY OF
Prion Diseases
decades, so symptoms usually appear later in life.
Unlike scrapie, which has a long history in
sheep, prion diseases were unknown in cattle until
modern agricultural practices resulted in the
addition of increasing amounts of processed
ingredients to cattle feed, including protein
supplements to help build muscle mass and
bone-meal supplements as a source of calcium. All
too often, those supplements came from the
carcasses of other livestock, including cattle. This
practice effectively turned cattle into cannibals
just like the New Guinea people whose ritualistic
cannibalism at funerals helped to spread another
prion disease known as kuru (see Why You
Shouldn’t Eat People, next page).
A prion is an unusual protein that has (at least) two different
stable shapes, or conformations—call them C (for cellular)
and Sc (for scrapie, the disease prions cause in sheep).
Conformation C is the default mode, the normal state for
the benign protein in the body. The alternative Sc conformation
is associated with disease. Both forms comprise the
same sequence of amino-acid building blocks; the only
difference between them is their final shape, analogous to
the difference between ice and liquid water.
If one of the harmless C-type prions encounters its abnormally
shaped Sc-type counterpart, something strange
happens: the protein in conformation C permanently switches
We do not know what caused the first case of
mad cow disease, or BSE. One hypothesis is that
cattle were given feed that included the ground-up
carcasses of sheep that had been infected with
scrapie. Some intriguing, although inconclusive,
evidence suggests that BSE may occur sporadically,
like sCJD. Either way, some cow, probably in the
U.K., developed a prion disease. Its carcass was
probably processed into feed eaten by more cattle,
fueling a cycle of animal infections.
Then people started to die. Two of the earliest
patients had all of the usual symptoms of CJD,
except that one victim was a 16-year-old girl and
the other was an 18-year-old boy; sCJD patients
are typically older than 63.
its shape to that of conformation Sc. Prions in the Sc conformation
essentially act like recruiters, and the switching
process they initiate accelerates because each Sc prion can
convert more than one C prion.
Unfortunately, the Sc conformation is stable and irreversible.
The process may resemble an infection in the way it
progresses, but the total number of protein molecules never
increases—only their shapes change.
The prion disease kuru causes voids (black spots, below left) to form around the neurons
(green spots) in the brain of an infected monkey, as seen through an electron microscope.
In variant Creutzfeldt-Jakob disease, prions collect into amyloid plaques (light object at
center, below right) in the brain of a mouse, as seen through an optical microscope.
T HE B I O L O G Y O F
Why You Shouldn’t Eat People
In the late 1950s, researchers noted a strange new brain
disease among the Fore people in the highlands of New
Guinea. The disease resembled CJD but afflicted younger
people and showed up in suspicious clusters. Years of field
and laboratory work by American pediatrician D. Carleton
Gajdusek, M.D. finally revealed the bizarre cause of the
disease known as kuru: it came from ritual cannibalism—
specifically, from the custom, practiced mostly by women
Autopsies of the young victims’ brains showed
a different pattern of damage from that of sCJD,
leading researchers to label the new disease variant
CJD (vCJD). By February 2009, 164 people in the
U.K. had died of vCJD, and more than 40 more
people died of the disease in nine other countries.
Although no one knows for sure how these
people contracted vCJD, the evidence strongly
suggests it was by eating beef or meat products
from BSE-infected cattle (see Mad Cow Disease,
next page). Since the British epidemic, cows with
BSE have been found in nearly every cattle-raising
nation, including the United States and Canada. In
many of these countries, the problem of contaminated
cattle feed, which may have contributed to
the lethal infections of both cows and humans, has
been addressed by new rules against feeding
mammalian protein to ruminants.
Thankfully, vCJD has so far not brought the
epidemic that some feared would come. Millions
of people ate beef in the U.K. between 1986, when
the BSE epidemic was first recognized, and 1996,
when the first 10 cases of vCJD were announced.
The lack of a broader epidemic suggests that the
infection could indeed be very rare.
Alternatively, the disease may possess a highly
variable latency between prion consumption and
the onset of symptoms. If the latter is true, the
cases reported to date could be the leading edge of
a much larger problema possibility that has
raised concerns over the potential for transmission
through blood or organ donations. In the case of
kuru, after all, an intensive surveillance program
found that the latency between infection and
symptoms could exceed 50 years. Only time will
tell whether the same holds true for vCJD.
and children, of eating the brain of a relative as part of
the funeral ceremony. Gajdusek’s finding made him
a co-winner of the Nobel Prize in Physiology or Medicine
in 1976. But Gajdusek could not name the infectious agent.
We now know that kuru is an example of a foodborne
prion disease, transmitted, in this case, through the
consumption of infected brain matter. Once the ritualistic
cannibalism stopped, so did the spread of kuru.
Crazy Cats and Mad Moose
Unfortunately, BSE is not the only worrisome
prion disease. Another is feline spongiform
encephalopathy, which is a disease of cats that
were fed BSE-infected beefprimarily pet cats
but also wild cats that are kept in zoos. This
outbreak appears to have run its course as BSEcontaminated
beef has become rarer.
A related disorder known as chronic wasting
disease (CWD) affects deer, elk, and moose. It has
spread in recent years across North America. Like
other prion diseases, the origins of CWD are
mysterious. And like scrapie and BSE, symptoms
of CWD include disorientation, wasting, and
inevitable death due to disintegration of the brain.
CWD was first recorded in 1967 among mule
deer that were temporarily held at a wildlife research
facility in northern Colorado as part of a nutritional
study, although the true nature of the perplexing
illness would not be known for another decade. By
then, researchers were noting with alarm that the
vast majority of deer housed at the facility for more
than two years either died or had to be euthanized.
In 1980, the disease appeared at a research station
in Wyoming that had shared deer with the Colorado
facility. A year later, researchers detected the disease
in wild elk living in Rocky Mountain National Park.
The researchers realized that the disease was
somehow propagating among captive animals and
that, once returned to the wild (or perhaps after
having escaped from their pens), those animals
could be creating new focal points for the epidemic.
In response, officials ordered all deer and elk at the
Colorado facility to be slaughtered, the soil to be
turned, and all pens and equipment to be repeatedly
doused with chlorine.
158 VOLUME 1 · HISTORY AND FUNDAMENTALS
MICROBIOLOGY FOR COOKS 159