Back to Transactions of RHASS Volume 1940 Transactions

Transactions of RHASS Volume 1940 - Page 048

Image details

Year 1940
Transcription
OCR Text 84 SCRAPIE.
month on the same number of ewes affected with the disease
He states that the lambs all remained healthy.
The possibility that the disease can be transmitted from
ewe to lamb through the medium of the milk must remain
for the present undetermined.
(6) Infection, by Sayrcosporidia.—I\I‘Gowan advanced the
hypothesis that scrapie was due to a heavy infestation of
the muscles by Sarcosporidia (Sarcocystis tcnellay).
This hypothesis is open to serious objection.
M‘Gowan believed that this Sarcosporidiosis occurred in its
greatest intensity in gimmers, and that the heavily infested
dam could transmit the infestation to her lamb both by con-
genital infection and through the medium of her milk; he
believed that scrapie was propagated by breeding from the.
progeny of gimmers (‘ ginnner—gimmer breeding '), and he
sought to explain the enzootic character of the disease on
the grounds that this method of breeding was practised within
the scrapie enzootic area.
But. as is well known. 8. tcnella is ubiquitous and infests
sheep throughout the entire world. The organism is, in
fact. to be regarded as a commensal rather than a parasite
in that its infestation is not known to produce any evidenc-
of disease. It is also recognised that particularly heavy
infestations by sarcosporidia are associated (as secondary
infections) with chronic debilitating diseases other than
scrapie. Further. it is a common practice in flocks in which
scrapie is prevalent to breed from the ewes of all ages, while
‘ gimmer—ginimer breeding’ is practised in flocks which
remain free from the disease.
(7) Pasture Infection—For a number of years there had
been brought to the writer’s notice several instances which
suggested the possibility that Scrapie could be transmitted
from diseaSed to healthy sheep through the medium of the
pasture. It was found, however, to be a matter of extrema:
difiiculty to obtain evidence in the field that the disease was
thus transmitted, since there so frequently existed the possi-
bility that transmission had occurred by other means, such
as congenital infection or immediate contact. However,
there were observed instances of which the following may
be cited as an example that strongly indicated the probability
of pasture infection.
For three consecutive years Farm G, on which scrapie was
prevalent. obtained Cheviot ewe lambs from Farm E, which
was known to be free from the disease. There was no evidenmx
that any of these animals had been in immediate contac»
either with a scrapie ram or with the general sheep stock on
Farm Gr, yet, when the lambs of each y'ear’s purchase became
two years old, numerous cases of scrapie appeared in them.
but their fellows on Farm F remained unaffected. It seemed,
SORAPIE. 85
therefore, that it might reasonably be deduced that an exaltmg
pathogenic factor was in operation on Farm Gr but not on
1“;vrm F. i _ _
In view of this and a number of Similar observations, one
was led to the tentative conclusions (or) that. the dlsease was
due to a factor capable of mediate transmrssron, (b) that this
ffa'l'OI‘ was present on some farms and was absent on others,
and (c) that it probably represented a hvmg mfective agent.
similar views had been reached by other workers, notably
M'Fadyean and Stockman. .
In 1932 there was observed an occurrence which appeared
1 . alford evidence confirmatory of this view. On a certain
“rm there had been maintained for sixty-seven years a
\.Lll12tb10 flock of pedigreed Border Leicesters. This flock had
l:»-en entirely free from scrapie until 1928, in whlch year the
(useasc made its first appearance. On the farm there was
-.- ten-acre paddock in which no case .of Scrapie had been
Luown to occur, but in 1930 a ewe clinically affected With
the disease was confined in the paddock until she eventually
(Med, a period of approximately five weeks. Several months
Her two rams (A and B) were kept in the paddock for about
me month, and thereafter no sheep had access to it until
1 .irteen months later, when three others rams (C, D, and E)
more confined there for about four weeks. .
Each of these five rams developed scrapie approxrmately
Haven months after their having been first introduced to the
paddock. Although a close inquiry revealed the facts that
i) the case of rams A and B there was reason to believe that
wrapie existed in the flocks from which they had been pur—
thased, and that in the case of ram C the ammalhad been
wed for service on a pasture during a period in which a case
or scrapie had occurred there in a ewe, there was defimte
widence that the remaining rams D and E, which were
Home bred, had never previously been in direct or indirect
untaet with the disease. In view of theSe circumstances,
end eSpecially since all five rams developed clinical Symptoms
[.i' the disease after, in each case, the expiry of a period of
eleven months, it appeared that possibly in all five cases,
: ml very probably in at least two of these (rams D and E),
the disease could be attributed to infection from the pasture.
’i‘he disease, in the case of each ram, manifested itself during
Inc breeding season, and this may have been the cause of
[He apparently short incubation period, since, as has been
..;-evi0usly stated, the sexual crises, with good reason, may
be presumed to precipitate the clinical manifestation of
sci-a 1e.
11? consideration of these and other similar kinds of data,
I"? was therefore resolved to attempt to determine by‘ means
of the experimental method whether scrapie could, in fact,




























Title Transactions of RHASS Volume 1940 - Page 048