8o RESEARCH IN PROTOZOOLOGY 



of the chick and set up infections there that remained for periods 

 of from several days to over six months. These results indicate 

 that the chick may be an important transmitting agent in regions 

 where sanitary conditions are not satisfactory and suggest that 

 other domesticated animals may also transmit various species of 

 Trichomonas. There is still much work of this type to be done. 



TRANSMISSION OF INTESTINAL PROTOZOA IN THE CYST STAGE 



This is the most common method of transmission and is the way 



in which eleven of the fifteen human intestinal species succeed in 



invading new hosts. The various percents, in round numbers, of 



the general population that are estimated to be infected by these 



organisms, which give some idea of the effectiveness of this method 



of transmission, are as follows (the number of infections reported 



for those species for which percents are not given is too small to 



furnish reliable figures) : 



Percent 



Endamceba histolytica lo 



Endamocha coli 50 



Endolimax nana 25 



lodamccba williamsi 10 



Dientamceba fragilis 10 



Chilom<istix mesnili 10 



Embadomonas intestinalis — 



Tricercomonas intestinalis — 



Giardia lamblia 15 



Isospora hominis — 



Balantidium coli — 



There is some evidence that racial differences exist with respect 

 to susceptibility to infection with the various species fisted ; and 

 there is no doubt that age has a distinct influence, since children 

 are more often infected than adults. The success of transmission 

 depends to a considerable degree on the number of cysts discharged 

 by the hosts and the ability of these cysts to live outside of the 

 body until they are ingested by new hosts. 



Endamocha histolytica. We know more about the transmission 

 of this species than of any other. The reader is referred to pp. 

 65-83 of the writer's book on Host-Parasite Relations hetzveen 

 Man and His Intestinal Protozoa (Hegner, 1927) for a detailed 

 discussion of this subject. One question of interest is that of the 

 possibility of infections as a result of the ingestion of trophozoites. 

 Experiments with animals indicate that trophozoites may be infec- 



