Transactions of RHASS Volume 1940 - Page 047
Image details
| Year | 1940 |
|---|---|
| Transcription |
|
| OCR Text |
33 SCRAPIE. verSely from ewe to rain. the writer still awaits proof that such transmission occurs and that despite careful and pro— longed personal investigation of the conditions of occurrence of scrapie in the field. (3) Congenital Infections—Congenital infection implies that an animal while it is still unborn is infected with the living causal agent of a particular disease. It does not- necessarily imply that the disease manifests itself at birth; indeed, a considerable period may elapse before its symptoms become evident. It therefore seemed probable that the numerous cases in which Scrapie occurred in the progeny of affected parents could be referred to congenital infection. This. nevertheless. was not very easy of acceptance. since, while the congenital transmission of an infective agent can readily be understood in its conveyance from ewe to lamb. it is difiicult prima facie to conceive how such infection can be conveyed from the rain through the ewe to the lamb. since. as has been indicated. one can find no solid evidence that the disease can be transmitted by direct venereal infection from ram to ewe. From what has been stated there is reason to believe that the disease may exist in a rain for a prolonged period in a latent form and that during this period of latency the carriei animal may be capable of infecting his progeny in that his Semen contains an infective agent. Since this implies that the ewes mated to such scrapie rams must also become infected. the fact that the disease seldom, if ever, manifests itself in them may be explained on the assumption that such ewes also harbour scrapie in a latent form; it is even possible. although we have no evidence to support this further assump» tion, that they may remain infective carriers. In this way. too, the fact that an apparently healthy ram may transmit the disease to his progeny may also be explained by a siniilai working hypothesis which was independently postulated by Stockman. The factors which arouse the latent infection and convert it into an active disease condition with frank clinical inani festations are not known. but may be represented by tliOSe discussed under predisposing; causes. Our observations have already afforded evidence that the central nervous system of sheep in the pre-clinical stage 0: the disease is highly infective, and indeed it isvconceivablc that sheep harbouring the latent infection possess a higher degree of infectifity than those in which the disease is clinically manifest. The following experiment carried out at Moreduii Institute under the direction of Mr \Y. A. Pool may be cited in respect of this latter statement. Ten Cheviot ewes obtained from a ‘clean’ flock were mated in November and December 1928 to a rain then actually SCRAPIE. 83 :ii'l‘ected with the disease. The rain died shortly after serving the ewes, but they and their lambs were kept under observ‘ - min for four years, at the end of which period no clinical evidence of scrapie had developed either in the ewes or in their progeny. The negative nature of this result appears 1.» provide some evidence that scrapie is not transmitted -. enereally from ram to ewes or congenitally toltheir progeny, wt in this cztSe the ram employed was exhibiting clinical \yiiiptoms of scrapie, and it is conceivable that a ram in the fire-clinical stage of the disease might transmit infection much more readily than one in which the signs of the disease were manifest. It is, of course, recognised that the number of experimental sheep in this particular case was small, and that in consequence no definite inference can be drawn. . (4) Immediate Contagion.—So far we are yet Without ev1dence iliat scrapie can be transmitted by immediate contact. M‘Fadyean carried out experiments in which eight healthy zest sheep were placed in loose-boxes in immediate contact with cases of scrapie for periods varying from thirty—four to winetv-two days. The test animals were kept under observa- ‘ion for further periods of from eighteen months to three tears eleven months. The result was entirely negative. . V Bertrand, Carré, and Lucam mixed eight sheep (including 1:)111‘ lambs) with an infected flock numbering approxnnately ‘wo hundred, in which the incidence of the disease was about ten per cent. The contact animals intermingled With the llock for a period of five months. Thereafter they were Lept under observation for a further period of over two cars. At the conclusion of the experiment none of the i-untacts had developed scrapie. These authors also quote Hainiiiann (1869), who kept healthy sheep (number not mted) iii-immediate contact with scrapie for periods ranging 1min nine to twenty-two months with negative result. While the results of these experiments are negative, it could he argued that they are not conclusive, since the number of the contact animals was relatively small and since, especially .n the experiment of Bertrand ct a1 and in that of Dammann, the period over which the observations were made should properly have been extended. ' (5) Infection through the Milk. M‘Gowaii had Six lambs, the .»i'ogeny of healthy ewes, suckled upon six scrapie ewes until lllt‘Se died or became too weak to nourish the lambs (a period of about one month), after which the lambs were reared on cow's milk. M‘Gowan stated that “ in due course ” all these xix lambs died from scrapie, while no caSe. of the disease occurred in lambs born from and nursed on healthy mothers, which were closely intermingled with these experimental Hiinbs during the whole course of the experiment. {oche-Lubin had thirty healthy lambs suckled for one |
| Title | Transactions of RHASS Volume 1940 - Page 047 |
