198 



COMMON DISEASES. 



are most certainly recognized when they are seen to move, and it 

 is sometimes difficult to distinguish otherwise between the mites 

 and particles of light colored dust and epithelial scales from the 

 skin surface. It should be borne in mind when examining a case 

 of head scab that the mites which cause this disease burrow be- 

 neath the surface of the skin, and it is necessary to scrape deeper 

 than for ordinary bodv scab. 



Varieties. — Each of these three types of mites may affect dif- 

 ferent domestic animals, including horses and cattle, but there 

 seems to be a different variety for each domestic animal. The va- 

 riety which causes scab in sheep is very similar to. but not iden- 

 tical, with the variety which causes mange in horses, and the one 

 which affects horses also differs from the variety of the same spe- 

 cies which causes mange in cattle. The variety which belongs to 

 the horse, for instance, does not thrive and multiply upon any 

 other animal. 



Male. 



I'l'Qisle. 



FIG. 73. SHEEP SCAB MITES. 

 Psoroptes. male (^Cur- Saifopte.s (Linujcr). Sarcoptes {Pettit). 



tice). Body scab. 



Head scab. 



Head scab. 



Possible mistakes. — It is well to bear in mind that certain oth> 

 er conditions may be mistaken for sheep scab, particularly disor- 

 ders of the skin, produced by other external parasites ; for instance, 

 lice and sheep ticks. We have had in Minnesota an interesting 

 outbreak of skin disease among sheep which on superficial exami- 

 nation resembled rather closely sheep scab, but was due to the 

 awns of a certain wild grass (Stipca sparta) wild oats. It devel- 

 oped that these sheep had been pastured where this grass was 

 abtnidant, ami at a tinn- wlun the awns were readily separated. 

 These little spear-like bodies gradually worked their way through 



