664 INTESTINAL PROTOZOA AND HOST-PARASITE RELATIONS 



known only in the oocyst stage. When fully developed the oocyst contains two spores each 

 of which has within it four sporozoites. 



Balantidium coli (Fig. ii), the only well-authenticated ciliate that occurs in man, is 

 comparatively large and cannot possibly be confused with any other organism. 



These intestinal protozoa are limited to certain definite parts of the body. The mouth is 

 inhabited by Endamoeba gingivalis and Trichomonas buccalis; the duodenum by Giardia 

 lamblia; the small intestine by Isospora hominis; the large intestine by five species of amebae, 

 four species of flagellates, and one ciliate as follows: Endamoeba histolytica, Endamoeba coli, 

 Endolimax nana, lodamoeba williamsi, Trichomonas hominis, Chilomastix mesnili, Embado- 

 monas intestinalis, Triccrcomonas intestinalis, and Balantidium coli; and the vagina or uri- 

 nary tract by Trichomonas viginalis. 



The regions indicated may be considered the primary sites of infection of the 

 various species listed. Several of these species, however, have been recorded from 

 other parts of the body, which constitute the secondary sites of infection. 



HOST-PARASITE RELATIONS OF INTESTINAL PROTOZOA 

 HOST-PARASITE RELATIONS 



This term is meant to include the relations between host and parasite not only 

 during the period of the infection of the host by the parasite but also during the perior] 

 when transmission takes place. It involves studies of the life-cycle of the parasite, 

 especially during the period when it is outside the body of the host; such phenomena 

 as encystment, excystation, the distribution and localization of the parasite within 

 the host, the resistance of the parasite to the defenses of the host, changes in the 

 parasite due to residence in the host, the method of attack of the parasite, and the way 

 in which the parasite escapes from the host so that new hosts may be infected. The 

 reactions of the host to the parasitic invasion are also included. These concern the 

 part played by the host in transmission and the localization of the parasite wathin 

 the body, the natural resistance of the host to the parasite, and the changes in the host 

 brought about by the attack of the parasite. Besides these subjects the term "host- 

 parasite relations" involves the fascinating problems of host-parasite adjustments and 

 host-parasite specificity. Studies of host-parasite relations are of both scientific ^nd 

 practical importance since any increase in our knowledge of relations between host 

 and parasite adds to the effectiveness of our attempts at prevention and control. 

 Heretofore studies have, for the most part, been made by zoologists who are primarily 

 interested in the parasite or by medical men who are primarily interested in the host. 

 The present attempt is to bring together these two points of view and add that of the 

 student of hygiene and public health. It is believed progress in prevention and control 

 can be made most rapidly by such a combination. 



EPIDEMIOLOGY 



The methods of transmission of intestinal protozoa are fairly well known although 

 exact data are difficult to obtain. Thus we believe, but are not certain, that Enda- 

 moeba gingivalis and Trichomonas buccalis, that live in the mouth, are transferred from 

 one host to another by kissing; that Trichomonas vaginalis is transmitted by con- 

 tamination or cohabitation; and that the other twelve species of intestinal protozoa 

 escape from the body in the feces and are ingested by new hosts in food contaminated 



