Scrapie (Mad Sheep Disease) May Pose a Risk to Humans

From New Scientist magazine (UK) Online News March 28, 2001

Suspect symptoms
What if you can catch old-fashioned CJD by eating meat from a sheep infected
with scrapie?
Exclusive from New Scientist magazine

Four years ago, Terry Singeltary watched his mother die horribly from a
degenerative brain disease. Doctors told him it was Alzheimer's, but
Singeltary was suspicious. The diagnosis didn't fit her violent symptoms,
and he demanded an autopsy. It showed she had died of sporadic
Creutzfeldt-Jakob disease.

Most doctors believe that sCJD is caused by a prion protein deforming by
chance into a killer. But Singeltary thinks otherwise.

He is one of a number of campaigners who say that some sCJD, like the
variant CJD related to BSE, is caused by eating meat from infected animals.
Their suspicions have focused on sheep carrying scrapie, a BSE-like disease
that is widespread in flocks across Europe and North America.

Now scientists in France have stumbled across new evidence that adds weight
to the campaigners' fears. To their complete surprise, the researchers found
that one strain of scrapie causes the same brain damage in mice as sCJD.

"This means we cannot rule out that at least some sCJD may be caused by some
strains of scrapie," says team member Jean-Philippe Deslys of the French
Atomic Energy Commission's medical research laboratory in
Fontenay-aux-Roses, south-west of Paris.

Hans Kretschmar of the University of Göttingen, who coordinates CJD
surveillance in Germany, is so concerned by the findings that he now wants
to trawl back through past sCJD cases to see if any might have been caused
by eating infected mutton or lamb.

Brain damage

Scrapie has been around for centuries and until now there has been no
evidence that it poses a risk to human health. But if the French finding
means that scrapie can cause sCJD in people, countries around the world may
have overlooked a CJD crisis to rival that caused by BSE.

Deslys and colleagues were originally studying vCJD, not sCJD. They injected
the brains of macaque monkeys with brain from BSE cattle, and from French
and British vCJD patients. The brain damage and clinical symptoms in the
monkeys were the same for all three. Mice injected with the original sets of
brain tissue or with infected monkey brain also developed the same symptoms.

As a control experiment, the team also injected mice with brain tissue from
people and animals with other prion diseases: a French case of sCJD; a
French patient who caught sCJD from human-derived growth hormone; sheep with
a French strain of scrapie; and mice carrying a prion derived from an
American scrapie strain.

As expected, they all affected the brain in a different way from BSE and
vCJD. But while the American strain of scrapie caused different damage from
sCJD, the French strain produced exactly the same pathology.

Multiple strains

"The main evidence that scrapie does not affect humans has been
epidemiology," says Moira Bruce of the neuropathogenesis unit of the
Institute for Animal Health in Edinburgh, who was a member of the same team
as Deslys.

"You see about the same incidence of the disease everywhere, whether or not
there are many sheep, and in countries such as New Zealand with no scrapie,"
she says. In the only previous comparisons of sCJD and scrapie in mice,
Bruce found they were dissimilar.

But there are more than 20 strains of scrapie, and six of sCJD. "You would
not necessarily see a relationship between the two with epidemiology if only
some strains affect only some people," says Deslys. Bruce is cautious about
the mouse results, but agrees they require further investigation. Other
trials of scrapie and sCJD in mice, she says, are in progress.

Deformed proteins

People can have three different genetic variations of the human prion
protein, and each type of protein can fold up two different ways. Kretschmar
has found that these six combinations correspond to six clinical types of
sCJD: each type of normal prion produces a particular pathology when it
spontaneously deforms to produce sCJD.

But if these proteins deform because of infection with a disease-causing
prion, the relationship between pathology and prion type should be
different, as it is in vCJD. "If we look at brain samples from sporadic CJD
cases and find some that do not fit the pattern," says Kretschmar, "that
could mean they were caused by infection."

There are 250 deaths per year from sCJD in the US, and a similar incidence
elsewhere. Singeltary and other US activists think that some of these people
died after eating contaminated meat or "nutritional" pills containing dried
animal brain.

Governments will have a hard time facing activists like Singeltary if it
turns out that some sCJD isn't as spontaneous as doctors have insisted.

Deslys's work on macaques also provides further proof that the human disease
vCJD is caused by BSE. And the experiments showed that vCJD is much more
virulent to primates than BSE, even when injected into the bloodstream
rather than the brain. This, says Deslys, means that there is an even bigger
risk than we thought that vCJD can be passed from one patient to another
through contaminated blood transfusions and surgical instruments.

More at: Proceedings of the National Academy of Sciences (vol 98, p 4142)

Correspondence about this story should be directed to

1900 GMT, 28 March 2001
Debora MacKenzie
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