LECTURE XXI r. 



CAUSE AND PREVENTION OF DISEASE. 

 PARASITISM. 



Parasitism may be mutual or selfish, temporary or perma- 

 nent, external or internal. 



Mutual parasitism is one in which both organisms are bene- 

 fited. 



Selfish parasitism is where one party is benefited, the other 

 injured. This is the kind we usually deal with in medicine. 



Temporary when only a portion of the parasite's life history 

 is with or upon the host. 



Permanent when its whole life history is with the same host. 



External or internal; i. e., afifecting the body surface or af- 

 fecting the internal organs. 



General classes. — In studying diseases of domestic animals, 

 we deal mainly with two general classes : worms and arthropodes. 



Worms. — Invertebrates with soft contractile bodies, either 

 composed of similar rings or non-articulated ; excretory organs 

 in pairs opening externally. Members of this group affect many 

 of the internal organs and superficial parts as well. 



Arthropodes. — Invertebrates, limbs jointed, bodies composed 

 •of dissimilar rings. In this group also we find both external 

 and internal parasites. 



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 ; through the 

 ■broken skin, or there may be external infection by contact. 



Predisposing causes. — Species of host; age of host; condi- 

 tion of host ; season of year and climate. iMost animal parasites 

 are enormously prolific, but very many eggs and immature forms 

 are destroyed. Others never reach their proper host. Female 

 louse may become ancestor of 10,000 lice in eight weeks; female 

 itch mite may be ancestor of 1,000,000 in three months, or a 

 •certain tape worm will furnish 150.000.000 eggs in one year. 



