GIARDIA LAMBLIA CYSTS 



mens of Giardia canis from dogs were injected into the 

 stomach of guinea-pigs ; they became distributed through- 

 out the small intestine within one or two hours, remam- 

 ing active during this period. Of particular interest is 

 the fact that they appeared to congregate in the duode- 

 num which is their optimum habitat. These experiments 

 suggest that trophozoites may be infective. They may be 

 ingested by the host with contaminated food or drink 

 and carried through the digestive tract in the intestinal 

 contents. It seems probable that conditions in the duode- 

 num stimulate them to attach themselves to the epithelial 

 cells of the intestinal wall by means of their sucking 

 discs. Those that do not succeed in doing this are carried 

 down and out of the intestine. Entrance to the body and 

 distribution within the body are thus primarily due to 

 the host, but the establishment of the flagellates in the 

 primary site of infection probably depends on the reac- 

 tions of the parasite. 



Infection by cysts. Very little is known regarding the 

 length of life of the trophozoites of giardias while out- 

 side the body, but it is apparently very short and hence 

 active specimens are probably very seldom ingested in 

 a living condition. The cysts, therefore, are more impor- 

 tant in bringing about infections. Cysts of Giardia lamb- 

 lia are of common occurrence. The incidence of infection 

 among human beings, and consequently the number of 

 hosts passing cysts, differs in various parts of the world, 

 according to the results of various surveys. Data from 

 35 surveys published during the years 19 16-19 19 gave an 

 average incidence of about 12 per cent in 20,000 per- 

 sons (Hegner and Payne, 1921). Boeck and Stiles 



155 



