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CHAPTER VIII. 



Parasitic Skin Diseases. 



general remarks on parasites flies leeches maggots lice 



ticks -poultry mites' — mange insects parasitic ringworm 



— bursatee. 



General Remarks on Parasites. 



DEFINITION. — Parasites are organisms (either animal or vegetable) 

 which live on (ccto^parasitcs) or within (en do-para sites J other organism?. 

 They are divided into xjarasites proper, such as tape-worms, which live on 

 the tissues of their hosts, and commensals, or guests, which feed on the food of 

 their hosts. Fu: convenience sake, I ventui-e to enlarge this definition, &o as 

 to include insects which annoy horses by their presence. 



ORIGIN AND HEREDITY.— Parasites of every kind, external, internal, 

 inicrobic and non-microbic, obtain their origin from without. We have seen 

 on page 21, that microbes may be conveyed by the blood of the dam to the 

 fcetus* This mode of inspection would be rarely if ever possible in the case 

 of non-microbic parasites. As like tends to produce like, and as certain states 

 of health are favourable to the development of parasites in an animal, we may 

 Jind hereditary predisposition more or less marked. 



PREDISPOSITION AND PREVENTION.— As explained by Neumann, 

 young animals are more liable to the attacks of certain parasites on account 

 of their tissues being more easily pierced than those of older ones. The 

 greater vascularity of their tissues may al>o be a further cause. As a rule, 

 a state of good health in the horse is antagonistic to the development of 

 parasites, which consequently invade debilitated and old horses, more readily 

 than the strong and mature. "We have here the question of " seed and soil." 

 Locality also plays a large part in this connection. For instance, bots, 

 practically speaking, infest only those horses which feed in the open ; 

 and leeches, only those that reside on or travel through ground in which 

 these parasites live. The experience I have gained in many parts of the 

 world convinces me that internal non-microbic parasites, with the ex- 

 ception of bots, are conveyed to the horse, as a rule, in the water he drinks 

 and on the grass or other " green meat " he eats. The horse, being a 

 clean eater, is comparatively free from internal non-microbic parasites. 

 The majority of dogs, on the contrary, are infested with them. From these 

 remarks we may see that our interests as horse-owners are concerned in 

 obtaining for our animals, food and drink which are free from hurtful forms 

 of life ; in attending to the health of our horses ; in protecting them from 

 parasitic invasion ; and in destroying all parasites got rid of by them, so that 

 neither these parasites nor their progeny may become future sources of irri- 



