PARASITIC DISEASES 



LECTUEE XXV 

 PARASITISM 



Parasitism may be temporary or permanent, external or in- 

 ternal. 



Parasites of domestic animals cause greater losses than is gen- 

 erally realized. Every stockman should be informed concerning 

 the most common and serious parasites, especially those of sheep. 

 Parasitism may be temporary when only a portion of the para- 

 site's life history is with one host; permanent, when its whole 

 life history is with the same host; external or internal, when it 

 affects the body surface or the internal organs. 



Sources and causes of parasitic diseases. — Parasites may be 

 received into the digestive apparatus with food or drink, they 

 may gain entrance through the respiratory tract, or broken 

 skin; or there may be external infection by contact, as with lice 

 or scab and mange mites. 



Nourishment. — Some parasites have mouth and digestive or- 

 gans, e.g., roundworms of the intestines; others receive their 

 nourishment by surface absorption or osmosis, e.g., tapeworms. 



Effect on health of host. — This depends on the organ or or- 

 gans invaded, the rapidity of multiplication, the amount of 

 nutrition used by parasite, and the amount of irritation caused. 



Intestinal parasites cause trouble by obstruction, by irritating 

 and abstracting blood from the mucous membrane, by mechani- 

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

 toms of indigestion, colic, and unthrift. 



Liver parasites cause jaundice and general anemia. 



Lung and bronchial parasites cause bronchitis or pneumonia. 



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

 and may indirectly cause obstructions and colic. 



Parasites in the hraim are more rare, and usually fatal. 



Parasites in muscidar tissue may cause little disturbance to 



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



measles and trichina. 



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