Transactions of RHASS Volume 1940 - Page 048
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| Year | 1940 |
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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 |
