January 25, 1999
Breeding to beat scrapie
By Environment Correspondent Robert Pigott
Scrapie, the spongiform disease that has afflicted British flocks for
hundreds of years, could soon be bred out of
existence, if an experiment on one farm is adopted across the UK.
Jonathan and Caroll Barber of Crogham Farm, near Wymondham in Norfolk,
have 450 breeding ewes which produced
about 700 lambs in December - each one of them is resistant to scrapie.
The breeding programme has been made possible by genetic tests which can
identify those sheep which are not
susceptible to scrapie. Sheep, like all other mammals, have in their
bodies what is known as prion protein.
When the prion protein takes on an abnormal form in sheep, the animals
succumb to scrapie. It is the same in cattle,
although we know the disease as BSE, and in humans it is called CJD.
Scientists know which genes govern the prion protein, and by examining
those genes it is possible to tell whether individual
sheep will be resistant or susceptible to scrapie.
But the Barbers are not breeding from scrapie-resistant sheep one at a
time. Once they have identified scrapie-resistant
ewes, they give them a course of injections so that they produce more
than the normal number of eggs.
These eggs are then fertilised with semen from rams whose genes are also
resistant to the disease. "The embryos are
tiny," says Jonathan Barber, "you could put the whole of the UK's
national sheep flock, up to 40 million sheep, into a pint
pot".
Thirty or 40 fertile embryos are flushed out onto a glass dish, examined
under a microscope to check they are fertile, and
placed in surrogate mothers which give birth to lambs five months later.
However, supplying farmers and breeders with more and more sheep which
have this particular genetic characteristic,
could carry a risk.
Dr Chris Bostock, Director of Research at the Institute for Animal
Health, says the dangers lie in reducing the range of
genes contained in the national flock.
"At the moment we know that there are many different strains of scrapie.
Some of them produce disease in some breeds,
some produce disease in other breeds. The danger is that we don't know
what drives change in the infectious agent.
"If we create a population of sheep that are homogenous - at the moment
homogeneously resistant to strains of scrapie -
we don't know whether the scrapie agent can change in such a way that it
can now infect this previously resistant
population of sheep"
But, the Barbers say enough of their sheep are resistant to make sure
they keep a wide variety of genes.
"I don't think there is a worry," says Jonathan Barber. "We've done lots
of blood tests in lots of different breeds, and
certainly within our own breed we've found a high proportion of animals
which are not genetically susceptible - so that
means that we have plenty of genetics to work at.
"So I don't think we are going to narrow the gene pool to be any worry
at all."
The veterinary costs, of getting ewes to produce more eggs, fertilising
them, flushing out the embryos and reinserting
them into surrogate mothers add up to about a hundred pounds per sheep.
Caroll Barber thinks it will become more profitable as consumers demand
every conceivable protection from disease. "As
the sheep industry we must be very aware and concerned that we are
selling a meat product and it is beholden on us that
we aren't putting at risk our consumers.
"Scrapie has been in the country for two or three hundred years and we
have never seen a problem pass onto a
consumer.
"But we mustn't be complacent. If we have the wherewithal to eradicate
any slim chance of a problem, then it's
something we must do."
So far humans have stayed free of scrapie, but those other prion
diseases, BSE and CJD, have made consumers more
wary of the unknown. The way the Barbers see it, the need to eradicate
scrapie has never been more urgent, and the
prospects of doing so never as great.
--
Judith Richards, London, Ontario, Canada
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