SEVENTH ANNUAL YEAR BOOK-PART X. 501 



CAUSE OF COMMON SHEEP SCAB. 



Sheep scab is a strictly contagious disease. Common sheep scab is 

 caused by that species of mites technically known as Psoroptes communis. 

 Parasites of this species cause scab in horses, cattle, sheep, goats and 

 rabbits; but for each of these species of animals there appears to be a 

 distinct variety of this parasite. Although it is more or less difficult to 

 distinguish between these varieties, they differ somewhat in size; also 

 it is found that the Psoroptes communis of the sheep does not cause scab 

 of the horse, ox or rabbit, nor, on the other hand, does the Psoroptes 

 communis of the horse or rabbit cause scab of the sheep. Naturalists, 

 therefore, distinguish the parasite of sheep scab by the name of Psoroptes 

 communis var. ovis. 



The parasite of this disease is one of the larger mites and is quite 

 easily seen with the naked eye. The adult female is about one-fortieth 

 inch long and one-sixtieth inch broad; the male is one-fiftieth inch long 

 and one-eightieth inch broad. These mites are discovered more readily 

 and more clearly on a dark than on a light background, and for that 

 reason the crusts from the affected skin are often placed upon black 

 paper and kept in the sunshine for a few minutes in order to reveal the 

 parasites crawling about. 



The Psoropt inhabits the regions on the surface of the body which are 

 most thickly covered with wool — that is, the back, the sides, the rump, 

 and the shoulders. Its presence is the cause of the true body scab on 

 sheep, and of all parasitic mites it produces the most serious injuries. 



DESCEIPTIOX OF COMMOX SHEEP SCAB. 



The mites of common, or body, scab — ^that is, the Psoroptes — prick 

 the skin of the animal to obtain their food, and probably insert a 

 poisonous saliva in the wound. Their bites are followed by intense itch- 

 ing, with irritation, formation of papules, inflammation, exudation of 

 serum, and the formation of crusts, or scabs under and near the edge of 

 which the parasites live. As the parasites multiply they seek the more 

 healthy parts, spreading from the edges of the scab already formed, thus 

 extending the disease. The sheep are restless; they scratch and bite 

 themselves and rub against posts, fences or stones, or against other mem- 

 bers of the flock. This irritation is particularly noticeable after the 

 animals have been driven, for the itching is more intense when the sheep 

 become heated. The changes in the skin naturally result in the falling 

 of the wool. At first slender "tags" are noticed; the fleece assumes the 

 condition known as "flowering;" it looks tufty or matted, and the sheep 

 pulls out portions with its mouth or leaves tags on the objects against 

 which it rubs. Scabs fall and are replaced by thicker and more ad- 

 herent crusts. The skin finally becomes more or less bare, parchment 

 like, greatly thickened, furrowed and bleeding in the cracks. With shorn 

 sheep, especially, a thick, dry, parchment-like crust covers the greatly 

 tumified skin. Ewes may abort or bear weak lambs. 



