SYMPTOMATOLOGY 



7. CHANGES IN THE HOST CAUSED BY THE PARASITE 



The reactions of hosts to infection may be considered 

 under the headings symptomatology, pathogenesis and 

 immunology. If the parasite lives on the tissues of the 

 body or injures the body in any way it is pathogenic. 

 If the injuries are severe, changes occur in the functions 

 of certain organs sufficient to bring about symptoms. In 

 certain cases the body reacts to the infection by building 

 up a resistance to the parasite which we call immunity. 



(i) Symptomatology. The symptoms that are char- 

 acteristic of the various diseases due to parasitic proto- 

 zoa are in most cases well known. Comparatively little is 

 known, however, regarding the genesis of these symp- 

 toms. The Century Dictionary defines a symptom as 

 "one of the departures from normal function or form 

 which a disease presents, especially one of the more evi- 

 dent of such departures." In other words, symptoms 

 arise when the function of an organ is modified. Usually 

 one species of parasitic protozoon brings about a large 

 number of symptoms since more than one organ may be 

 disturbed. Some of these are localized at the point where 

 injury is being done, whereas others appear at a dis- 

 tance. 



The most recent and probably the most successful at- 

 tempt to determine the mechanism of symptom produc- 

 tion is that of Sir James Mackenzie (1923). His argu- 

 ment is as follows: Symptoms are produced by organs 

 whose functions are stimulated to unusual activity, or 

 depressed or suspended. Functional activity is dependent 

 on the cells of the organ and on the nerves and other 



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