|  New
research at the University of Leeds has overturned
existing advice to farmers that has been maintaining
the disease toxoplasma in the nation’s
sheep flocks for years.
Toxoplasma is a disease humans catch from
sheep and cats that causes human abortions
and birth defects with greater frequency than
rubella.
In
a study of a pedigree Charolais flock and
commercial flock, Dr Judith Smith (left) and
her colleague Dr Geoff Hide from the University
of Salford found that some sheep families
had abortion rates as high as 36%, while other
families showed no abortions at all.
The
researchers found that toxoplasma can be inherited
in sheep – passed from ewes to their
lambs in the womb. If she is right, the parasite
could be bred out of the national flock by
allowing only toxoplasma-free ewes to reproduce,
saving British agriculture around £80m
every year in sheep deaths and abortions.
Most humans catch the bug from infected lamb,
so a healthy flock also means a healthy public.
Dr
Smith said: “It’s always been
thought that sheep pick up the disease-causing
parasite by eating grass contaminated with
the faeces of infected cats. Our study of
infected flocks shows that the parasite is
passed ‘vertically’ down the generations.”
The alternative explanation is that sheep
inherit susceptibility to the disease, rather
than inherit the parasite itself, but Smith
believes that’s unlikely: “We’ve
found that almost all infections are from
one strain of the parasite; genetic evidence
that transmission from ewe to lamb may be
much more important than sheep becoming infected
as adults.”
The
research overturns the conventional wisdom
on toxoplasma control. Smith explains: “For
years farmers were told to breed from ewes
who had previously aborted a lamb because
of toxoplasma, since it was thought that previous
aborters would have developed immunity. Far
from controlling the disease, farmers were
actually spreading it, by breeding from infected
ewes who pass the parasite on to their young.
It does seem crazy now, but some earlier studies
had showed that immunity can reduce rates
of abortion – what vets hadn’t
realised is that while abortions decreased,
transmission to live lambs continued.”
In
a wild population, the parasite might be naturally
bred out by natural selection, but until now,
the disease has continued to thrive in the
UK’s managed flocks.
The
major strain of toxoplasma in Europe is thought
to come from a cross between two parental
strains that occurred 10,000 years ago –
around the time people first started farming.
It is likely that the parasite then spread
as domestic animals expanded across Europe,
and that toxoplasma has dogged us since agriculture
began.
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