PATHOGENIC IMPORTANCE 117 



posed have been furnished recently by the findings in returned 

 British soldiers, in whom uncomplicated infections with flagel- 

 lates have been found in many dysenteric cases, and also by the 

 investigations of Lynch, Barlow, Escomel and others in various 

 parts of the world. Still further evidence is furnished by the 

 fact that parasites very closely allied to species found in man 

 have recently been shown to be unquestionably of pathogenic 

 importance, at least under certain conditions, in lower animals. 



Obviously, however, in view of the large number of infected 

 persons, the intestinal protozoans must often have little or no 

 pathogenic effect. There is, nevertheless, much individual dif- 

 ference in susceptibility, and different strains of the same para- 

 site seem to vary in the effects they produce. Moreover it is 

 highly probable that a great many slight and perhaps almost 

 unnoticed symptoms, resulting in a certain amount of interference 

 with the digestive tract and in a general lowering of the health, 

 may find their ultimate cause in intestinal parasites, either pro- 

 tozoans or worms or both. The health of people living in warm 

 and tropical countries, even aside from the effects of malaria and 

 other warm-climate diseases, is proverbially less perfect than that 

 of people in the usually more sanitary northern countries. It is 

 quite probable that intestinal Protozoa may play a part in this 

 lowering of the tone of health. 



In the paragraphs below a brief account of the more important 

 intestinal flagellates and ciliates is given, with what is known of 

 their pathogenic effects, in the following order: (1) the bi-flagel- 

 late forms, Bodo, Cercomonas and Prowazekia; (2) the multi- 

 flagellate forms, Trichomonas, Macrostoma and Giardia; and (3) 

 the ciliate, Balantidium. 



Bi-flagellate Protozoa 



Most primitive of the intestinal flagellates are the bi-flag- 

 ellated forms, several genera of which have been found in the 

 human intestine. These are, namely, Bodo, Cercomonas and 

 Prowazekia (Fig. 29). The relation of these animals to the 

 still more primitive mono-flagellated trypanosomes and their 

 allies is shown by the parasites of the genus Trypanoplasma 

 found in the intestines of fishes and in a number of invertebrate 

 animals. The animals of this genus resemble trypanosomes in 



