CHAPTER XIX 



PARASITES AND PARASITIC DISEASES 



GENERAL CONSIDERATIONS 



Parasites are living organisms which obtain their sustenance 

 in whole or in part at the expense of other living organisms. 

 More or less damage is always done the host by their presence. 



In keeping with their location we are able to recognize external 

 or ectoparasites and internal or entoparasites. All forms may be 

 placed in one or another of the following divisions — permanent 

 parasites, which spend their entire hfe with one host; temporary 

 parasites, which spend but a part of their existence with one host; 

 facultative parasites, which have the power to change from one 

 host to another of a different species; obligatory parasites, which 

 can live only on or in one species of animal. 



The effects of parasitism depend on the number and position 

 of the invaders. Injury is caused by the parasites from irritation; 

 from drain by feeding upon the tissues and juices; from the ab- 

 sorption of poisons produced by the parasites; from their wander- 

 ings in the body tissues, and from obstruction of the natural 

 passages. 



All animal parasites of the horse belong to some one of the 

 three following sections of the animal kingdom — protozoa, worms, 

 or arthropodes. 



Protozoa are the very lowest form of animal life. They are 

 microscopic in size and possess the simplest form of digestive and 

 respiratory apparatuses. 



Worms have soft contractile bodies. They comprise a great 

 number of parasites which find their habitat in the digestive, res- 

 piratory, circulatory, and other body organs. 



Arthropoda are provided with jointed legs. The subdivisions 

 of this group are (1) the arachnida, represented by the mange 

 mites; (2) the insecta, common examples of which are the parasitic 

 lice and flies. 



228 



