SCRAPIE Scrapie is a slowly progressive, degenerative disease of the central nervous system of sheep and goats. All breeds of sheep appear susceptible and the peak incidence of clinical disease occurs in sheep of 3 to 4 years of age. It is one of a group of similar disease found in over 20 species of mammals. AETIOLOGY The aetiology of scrapie is still not clearly defined but experimental evidence suggests that the cause is a transmissible, filterable and self-replicating agent. Susceptibility to infection is under genetic control and this allows the opportunity for control using PrP genotyping. There is clear evidence for a single Scrapie Incubation Period (SIP) gene which is dominant for susceptibility. CLINICAL SIGNS Clinical disease develops progressively over months or years and several clinical syndromes have been described. Different strains of scrapie agent causing characteristic clinical and pathological features have been described in goats. The two well recognised strains are termed "drowsy" and "scratching" because of the characteristic clinical signs that each induces. In sheep the "scratching" form is the most common and the affected animal scrapes, rubs and nibbles or bites at its skin due to the presence of a marked itching. The animal, in the early stages of the disease, rubs itself against fixed objects e.g. fence posts, stall divisions, bites and nibbles at its skin and rubs itself frequently with its horns, thus removing large quantities of wool. A "nibbling reflex" can be demonstrated when an affected animal is rubbed gently over the back, rump or flanks, this manipulation resulting in characteristic nibbling movements of the lips and licking movements of the tongue. This "nibbling reflex" is also apparent when the animal rubs itself on objects and can also be seen in animals suffering from intensely itchy conditions such as sheep scab but in the latter case skin lesions will be evident. A second form of the disease in sheep results in changes in excitability when affected animals may become hyperexcitable and tremble markedly when approached or driven. Such animals exhibit fine tremors over the head and neck which produce a nodding action. A small proportion of these sheep become sleepy and stupid. A third and less common form of the disease results in varying degrees of posterior inco-ordination which, in the early stages, is difficult to appreciate being seen best when an affected animal is being driven or made to turn sharply. Later pronounced swaying of the hindquarters becomes apparent with eventual progression to recumbency. The majority of scrapie sheep lose weight and at death are almost at the stage of emaciation but a small number actually put on weight. Affected sheep all die within 1 to 6 months of the onset of the clinical disease. EPIDEMIOLOGY The disease spreads either by vertical transmission to the progeny of affected ewes or by lateral transmission by direct contact between animals or contact with premises which have previously been occupied by infected sheep. Thus the incidence can be expected to increase with intensification. The incubation period of scrapie ranges from 18 months to 3 years before clinical signs become apparent but sheep of less than 12 months of age have developed the naturally acquired disease. TREATMENT No form of treatment is possible. CONTROL Scrapie become notifiable in the U.K. in 1992. At the present time control is limited to the removal and slaughter of clinically affected animals from the flock. To eliminate infection associated with vertical transmission all the progeny of affected cases should be culled. It may be advisable also to remove close relatives of affected male sheep as they may be more susceptible to infection with the scrapie agent but as yet this has not been established.
Posted on: Fri, 02 Aug 2013 20:11:26 +0000
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