84 RESEARCH IN PROTOZOOLOGY 



indicate that rats may become infected and pass cysts of E. histoly- 

 tica. Under favorable conditions rats might become infected in 

 nature but how important a role they play in the dissemination of 

 E. histolytica cysts is unknown. 



Comparatively little experimental work has been carried on with 

 cysts of the other intestinal protozoa. It seems probable, however, 

 that they are subjected to the same factors as are those oi E. his- 

 tolytica and are transmitted in much the same way. Many of the 

 investigators who carried out experiments with the cysts of E. his- 

 tolytica used also the cysts of other intestinal protozoa and their 

 findings may be found in their publications. The cysts of the 

 human coccidium, Isospora hominis, appear to be more resistant 

 to conditions outside of the body than are the cysts of the intestinal 

 amoebse and flagellates. Haughwout (1921), for example, exposed 

 oocysts in the sporoblast stage to the sun for three hours every 

 day for a week and found that at the end of this period some of 

 them developed sporozoites when water was added ; and Wen- 

 yon (1926) reports the completion of development within the 

 oocysts of Eimcria sticdce from the rabbit after being subjected 

 to a fixing solution of sublimate, stained with hematoxylin, de- 

 hydrated, cleared, and mounted in balsam. 



Of particular interest is the situation regarding Balantidium coli 

 (see Chapter XXIV). Cysts of this species seem to be rare in 

 man, not so rare in monkeys, but more common in pigs. A careful 

 study of cyst production in these three types of hosts, as well as 

 in guinea-pigs, and the resistance of these cysts to factors in the 

 environment outside of the body would well repay investigation. 



On the whole it seems to the writer that our knowledge of the 

 transmission of intestinal protozoa, which is so important from 

 a public health standpoint, is based on fragmentary and in many 

 cases uncontrolled experiments. Furthermore, most of these ex- 

 periments have been conducted in the laboratory and in many 

 cases do not represent satisfactorily conditions as they are in 

 nature. We have here an important field for investigation in which 

 careful work would furnish results of considerable importance. 



