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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 organicms (either animal or vegetable) 

 which live on (ecto- parasites) or within (endo- parasites) other organisms. 

 They are divided into parasites 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. For convenience' sake, I venture to enlarge this definition, so 

 as to include insects which annoy horses by their presence. 



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

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

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

 foetus. This mode of infection 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 find hereditary predisposition more or less marked. 



PREDISPOSITION AND PREVENTION.— 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 also be a further cause. As a rule, a state of good health 

 in the horse is antagonistic to the development of parasites, wnich conse- 

 quently 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 speak- 

 ing, 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 exception 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 

 th6 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 irritation or disease. Some 

 parasites are capable of producing millions of eggs in a year. 



