PARASITISM. 



93 



appear by means of their own activity and habits ; e. g., ticks, 

 bots in horses, grubs in backs of cattle, and grubs in the nasai 

 passages and head cavities of sheep. It is well to remember 

 also that many parasites can resist stronger medicines than the 

 organs or tissues they invade; e. g., bots in the horse's stomach. 



EXTERNAL PARASITES. 



Most serious are the mite diseases : itch, mange, and sheep 

 scab, lice, ticks and ringworms. 



Mite diseases {acariasis). — The various forms of itch or 

 mange in horses, cattle, and hogs ; scab in sheep ; scabies in cats 

 and dogs are all similar in cause, contagiousness, prevention, dam- 

 age done and in treatment. 



FIG. 33. MANGE MITE. 



The cause of one form of horse 

 mange. Psoroiitca communis equi. 



FIG. 34. CATTLE LOUSE. 

 FEMALE. 

 Haematopinus curysicrnus. 



Causes. — Three types of mites, or acari, affect man and the 

 lower animals, all very small: (a) sarcoptes, (b) psoroptes, (c) 

 symbiotes. 



Sarcoptes burrow channels and live in or beneath the cuticle. 

 The eggs are laid and young hatched in these channels. These 

 mites may live 14 days in ordinary stables on harness, blankets, 

 or woodwork. 



Psoroptes live on skin surface. They can move around on 

 the surface and spread rapidly. 



Symbiotes affect regions near the feet and move about very 

 little. 



Diagnosis is made on the condition of the skin, which be- 

 comes dry and wrinkled or leathery with hairs partly gone ; in- 



