Transactions of RHASS Volume 1940 - Page 043
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| Year | 1940 |
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| OCR Text |
74 SCRAPIE. While this type of evidence seems to suggest the existence of local controlling factors, it is to be understood that if such exist their action is only limited in degree, since, as will later appear, there is now evidence that the actual cause of scrapie is a specific infective agent. (2) I nbrccding.—It is widely believed that inbreeding is an important predisposing factor in the development of scrapie. and it is true that the disease not infrequently occurs in sheep the parents of which are closely related. Continental authori- ties have observed that the periods when scrapie was most prevalent coincided with those during which intensive inbreed— ing for the purpose of breed-improvement was practiSed. These observations. as has already been indicated, may be explained on the assumption that because of the greater attention then given to sheep husbandry the occurrence of the disease became more clearly recognised. It is also true that scrapie occurs commonly in Half-Breds obtained as first crosses from a Border Leicester ram and Cheviot ewe> and in Cross Suifolks, the progeny of a Suffolk ram with Half—Bred ewes. and while inbreeding has been intensiveh practiSed in Border Leicester‘s, Cheviots and Suffolks. the cross progeny might be expected to be relatively free from scrapie if inbreeding per sc were an important predisposing factor. (3) Influence of Sexual Crises. The disease most commonly attacks gimmers—that is, ewes at about two years old— towards the end of their first pregnancy and during tht period of subsequent lactation. Further evidence that a sexual crisis may precipitate a1: attack is found in several observations to the effect that when gimmers, because the ' are in poor condition, are not mated until they reach the age of approximately two years six months, the disease in these most commonly develops at three years old—that is, at about the time they give birth to their first lamb. Serapie also frequently appears in older ewes soon after they have been mated. In rams, the disease 00111- monly develops soon after they have completed their period at stud, and also occurs shortly before the stud season corn mences. On the Continent it is believed that breeding from immature sheep may cause them to contract the disease. These facts suggest that the physiological crises associated with mating, pregnancy, and parturition in some manner precipitate the manifestation of the disease. (4) Feeding—While it is true that scrapie occurs in sheep on ‘poor keep7 as commonly perhaps as it occurs in thOSl‘ receiving a nutritious diet, there are several published observa tions which indicate that heavy feeding tends to prediSpose to attack. It is also believed that fat ‘soft’ sheep are par» ticularly susceptible. The records relating to the alleged SCRAPIE. 7 5 effects of nutritional factors are, however, of a highly conflicting t ' I 1aracter. INCIDENCE. When the disease is established in a flock, the incidence rate presents wide variation and commonly falls between 4 per cent and 20 per cent. In some flocks the incidence is so low that only occasional cases occur throughout a period of several _\ea1‘s. Very occasionally, without obvious reason, the number at cases in a scrapie flock may rise rapidly and may even ittain a figure of over 50 per Cent. SYMPTOMS. The onset of the disease is insidious, and only an experienced aliepherd may be able to recognise the earliest signs. The sheep is nervous, apprehensive, and more excitable ihan usual. If closely watched fine tremors extending over .he head and neck are observed ; these cause slight but very ‘.l1)l(l nodding movements. If the animal be rounded up it :wcomes tense with excitement and rapid muscular tremors uu'ticularly affecting the thighs and flanks are evident. The read and neck are carried high and somewhat stifliy; the I01le expressmn is staring and fixed, and the ears not in- oequently assume an unnatural position. Grinding movements wl‘ the teeth are commonly evinced. Thirst is a frequent symptom, and the affected sheep may .irink large quantities of water. There is no diarrhoea, but '2 the animal be excited quantities of faeces and urine may ire involuntarily passed at short intervals. The fleece becomes lighter in colour and loses its lustre. The bleating is somewhat 1eeble, husky, and tremulous. The most characteristic clinical feature is the development e1 pruritus, or itch, which, frequently commencing in the iegion of the rump and the loins, extends over the Whole 1mdy. When the skin, particularly that of the back, is rubbed die ‘ scratch reflex,’ evidenced in nibbling movements of the lips and Vigorous wagging of the tail, is exhibited; in many instances the itch becomes extreme and the animal is unable to rest for any considerable length of time. Although the appetite remains unaffected almost through out ‘Llefflltll‘B course, feeding and rumination may be imperfectly performed because of the severe skin irritation. The sheep repeatedly rubs itself against fixed objects, but, apart from the abras1ons on the skin and the loss of wool which con- s<~quently result, no lesion of the skin is observable. As a iesult probably of the constant torment of irritation emaciation |
| Title | Transactions of RHASS Volume 1940 - Page 043 |
