152 RESEARCH IN PROTOZOOLOGY 



that his rats were actually infected with giardias from man. It is 

 not certain that Potter was dealing with what may be called a 

 permanent infection. The writer (Hegner, 1927) encountered 

 lamblia-Yike giardias in two laboratory rats that were being used 

 for studies of excystation. Later, attempts were made to infect 

 rats by feeding them cysts of G. lamblia. Temporary infections 

 were apparently established in four rats ; these infections were 

 highest on the sixth and seventh days, decreased by the twelfth and 

 fourteenth days and disappeared by the sixteenth and nineteenth 

 days. The trophozoites recovered from these animals were smaller 

 than those of G. lamblia from man; this may have been due to 

 their unusual habitat. No cysts were passed by the experimentally 

 infected rats which suggests that the rat is not an important trans- 

 mitting agent of the human Giardia. Extensive and important ex- 

 perimental work of a similar nature could be carried out with 

 giardias from man and a number of lower animals. 



h. Transmission. It seems probable that the cyst is the infective 

 stage of Giardia. The trophozoites might be able to reach the 

 duodenum in a viable condition ( Hegner, 1926) but the chances 

 that they would live outside of the body long enough under ordi- 

 nary conditions to be ingested by another host are very slight. 

 Experiments have been carried out to determine resistance of cysts 

 of Giardia to various factors such as temperature (Boeck, 1921) 

 encountered outside of the body of the host. The work of Stiles 

 and Keister (1913), Wenyon and O'Connor (1917), and Root 

 (1921) indicates that flies may be important transmitting agents; 

 and several investigators have implicated the cockroach (Porter, 

 1919; Pessva, 1927). Further experiments regarding the viability 

 of both trophozoites and cysts of giardias outside of the host and 

 of their transmission by insects are desirable. 



i. Age resistance. A number of investigators, especially Dobell 

 (1921), Maxcy (1921), and Simon (1924), have pubHshed data 

 which indicate that the resistance to infection with Giardia lamblia 

 is much higher in children than in adults. Whether the young of 

 other animals are more frequently infected than the adults is not 

 known. A study should be made of this problem. It is customary 

 to speak of age resistance when facts of this type are revealed but 

 the factors involved in age resistance are practically unknown. 

 Perhaps, as Dobell suggests, the higher incidence of giardias among 

 children may be due to the greater ease of finding the organisms 



