HOST-PARASITE RELATIONS: INTESTINAL PROTOZOA 



agents that regulate the activity of these cells. There is 

 formed a reflex arc consisting of a receptor which re- 

 ceives the stimulus, an afferent nerve which carries it to a 

 nerve center, and an efferent nerve which conveys it to 

 the effector. A modification of any part of this arc results 

 in an abnormal response, i. e., a symptom. The effector 

 may be a muscle, gland, or the sensorium. The blood 

 stream no doubt also plays an important role in the pro- 

 duction of symptoms by transporting toxic substances 

 from one part of the body to another. The genesis of 

 symptoms in protozoan diseases is open to experimental 

 study, but very little is known about the subject. 



(2) Pathogenesis. Many of the protozoa of man are 

 not pathogenic so far as is known ; this is true of Enda- 

 niceba coli and other commensals. A few, such as Enda- 

 mocha histolytica and the coccidia are apparently unable 

 to exist without direct attacks on the host tissues. These 

 attacks bring forth more or less definite responses on the 

 part of the host and usually a rather definite series of 

 changes occur during the course of the infection. This 

 series never proceeds beyond the earlier stages in contact 

 carriers (see p. 107) and stops short of the end when 

 the patient is treated or undergoes spontaneous recovery. 

 Only when the death of the host ensues may the final 

 stages be observed. Each parasite maintains its own 

 method of attack and the host responds usually in a per- 

 fectly definite way to the inroads of each species of 

 protozoon. The changes in the host have definite efifects 

 upon the progress of the parasitic attack and it is thus 

 possible to obtain a dynamic view of host-parasite reac- 

 tions during the course of the infection. The complete 



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