922 



DISEASES OF SHEEP. 



the skin, deriving its nourishment from sucking the fluids of the 

 system. It is a minute mite, which attaches itself to the skin, and 

 penetrates the surface, lodging itself in the tissues, and causing in- 



Fig. 1221.— The Scab. 



tense irritation or itching, and the secretion of a matter which dries 

 on the surface and forms a scab. Scab maybe of spontaneous origin, 

 as well as the product of contagion. One female acarus can pro- 

 duce a million and a half of progeny in ninety days, and this ex- 

 plains the rapidity with which scab spreads in sheep. Of parasitic 



Fig. 1222.— Serious Case of Scab. 



acari, there are three principal species : one that burrows in the 

 scarf-skin, one which lives on the surface among the scabs, and one 

 which lives in the fatty glands of the skin in sheep and dogs. In 

 scab, the animal shows its aggravation by moving its body, and by 

 rubbing against fences or other objects (Fig. 1221). The wool is 

 often torn off from a good portion of the body. 



