110 PARASITES OF THE DOMESTIC ANIMALS 



among sheep, psoroptic scabies may be regarded, from an economic 

 standpoint, as the most important of all scabies affecting live stock. 

 That the disease has been known for many centuries is evident through 

 references to it in early writings, including the Bible. The relationship 

 of the mite to the disease, however, was not determined with certainty 

 until the nineteenth century, during the first half of which the complete 

 life cycle of the parasite was demonstrated. It was shown that mites, 

 like larger animals, are the offspring of ancestors and are not of sponta- 

 neous origin or accidental occurrence. It was further proven by animal 

 experimentation that the mites were not present as a result of the scab, 

 as had been supposed by some, but that the scab resulted from the 

 presence of the mites and could be produced in no other way. 



The psoropt of sheep scab (Figs. 68 and 69) lives upon the surface of 

 the body, where it is most thickly covered with wool, as the back, sides, 

 and shoulders. From their seat of invasion the colonies will spread and 

 these areas may coalesce, involving large patches, though the regions of 

 short wool, as the belly and front of the chest, are rarely attacked. 



Symptoms and Lesions. — The first indications of the disease are 

 rubbing and gnawing at the wool and general unrest caused by the itch- 

 ing. As the changes in the skin progress loosened tufts become raised 

 over the surface of the fleece. These tags are soon rubbed or pulled 

 away by the sheep, and the fleece over the affected parts becomes ragged 

 and matted, the skin finally becoming more or less bare and showing 

 at this stage a thickened, furrowed, and bleeding condition. 



If the skin is examined before the shedding of the wool there will be 

 seen small yellowish nodules and papular elevations of the epidermis. 

 The latter with their serous exudate dry up, forming an accimaulation 

 of fatty yellowish scales upon the skin and among the deeper parts of 

 the hairs. The papules are closer together as the punctures of the psoropt 

 become more nmnerous. As they become confluent the skin thickens, 

 and the drying exudate and papular debris form a massive yellowish 

 crust. This, as it increases layer by layer, envelopes and mats the 

 wool, lifting the fibers from their follicles and raising large bunches 

 above the surface of the fleece. These detached patches will soon fall 

 away, the denuded skin showing a variation of lesions common to 

 scabies. It will be thickened, cracked, and scabby, and there may be 

 here and there excoriations, with perhaps sloughing and ulcerated areas. 

 The acari forsake the more central and older lesions for the periphery 

 of the denuded patch where they may be found in large numbers at 

 the roots of the incrusted wool which in its turn will fall away. Due to 

 direct exposure to the atmosphere, old denuded or sheared areas dry 

 out and become uniformly covered with a dry parchment-like crust 

 beneath which the skin is thickened and fissured. 



Course and Prognosis. — The course and termination of the- disease 



