92 CAUSES AND PREVENTION OF DISEASE. 



How nounshed. — Parasites usually live on the exudate or 

 excretion which results from their presence. Some have mouth 

 and digestive organs ; e. g., round worms of the intestines. Oth- 

 ers receive their nourishment by surface absorption or osmosis; 

 e. g., tape worms. 



Effect on health of host depends on the organ or organs 

 invaded ; the rapidity of multiplication ; the amount of nutrition 

 used by parasite, and amount of irritation caused. 



Intestinal parasites cause trouble by obstruction ; by irritat- 

 ing and abstracting blood from the mucous membrane; by me- 

 chanical irritation, and by using nutrition. Hence we have symp- 

 toms of indigestion, colic and unthrift. 



Liver parasites cause jaundice and general anaemia. 



Limg and bronchial parasites cause bronchitis or pneumonia. 



Parasites in the blood vessels cause disease of the vessel walls, 

 and may indirectly cause obstructions and colic. 



Parasites in the brain are more rare and usually fatal. 



Parasites in mnscular tissue may cause little disturbance to 

 the animal host ; but be very serious to human health ; e. g., pig 

 measles, trichina, actinomycosis (lumpy-jaw). 



General prevention.— Parasitism would be impossible if ev- 

 erything that comes in contact with the animal body were free 

 from parasites. Saddles, harness, blankets, posts and fences are 

 the usual agents which spread Texas itch or mange among 

 horses, and round intestinal worms generally gain entrance in 

 the egg or immature form, with the water drank. Ponds, wells 

 which receive surface drainage, sluggish streams and marshes, 

 should therefore be regarded with suspicion. Sound animals 

 should not be allowed in sheds, yards or barns where animals 

 diseased by external parasites, like lice or scab mites, have been, 

 until such structures have been disinfected. Dogs are danger- 

 ous factors in sheep pastures, because of the tapeworms which 

 they furnish to sheep. Intestinal worms which appear in the 

 manure of horses should be destroyed with boiling water, — not 

 merely crushed and thrown away. 



- General treatment. — Medical treatment must be such as will 

 destroy or remove the parasites. Medicines may aid us in re- 

 moving parasites, by suffocating them ; e. g., as with oils ; by 

 intoxication of the parasites; or by caustic or irritating chem- 

 ical effect on the bodies of the parasites. Many parasites dis- 



