1923] Kessel: Experimental Infection of Rats and Mice 423 



Young amoeba-free rodents have acquired an infection of amoeba 

 in 80 per cent of the cases fed, while old amoeba-free rodents have 

 acquired an infection in only 20 per cent of the cases. This, together 

 with the facts that 27 per cent of rats of known amoebic infection 

 have later been known to rid themselves of the infection (Kessel, 

 19236) and that the rate of amoebic infection is much lower in old 

 rodents than in middle-aged rodents (Kessel, 1923a), indicates that 

 as the animals progress in age they are able to establish an active 

 resistance to amoebic infection. 



III. Application op Cross-Infection Experiments from the 

 Point of View of Medical Zoology 



The fact that the amoebae common to the intestinal tract of man 

 have been successfully established in rats and mice is important, first, 

 from the standjjoint of preventive medicine. Infection of rats and 

 mice with the amoebae of the human digestive tract has been suc- 

 cessful in 55 per cent of all the attempts made, while the cross- 

 infection of these rodents with amoebae common to themselves has 

 been successful in 53 per cent of all the attempts made. In the 

 experimental work of this investigation we thus .see that even slightly 

 greater success has attended attempts to infect rats with amoebae 

 common to man than with the amoebae common to rodents (see 

 Ke.ssel, 19236). 



The fact of transfer of human infections to rats and mice throws 

 heavy suspicion upon these rodents as facultative carriers of the 

 causative organisms of amoebiasis in man and supports the findings 

 of Lynch (1915) and Brug (1919). Whether the amoebae thus 

 experimentally infected into rodents still retain their pathogenicity 

 for man, it is still impossible to state, but the fact that they may 

 be passed from one rodent to another indicates that the viability 

 of the amoebae is not destroyed by passage through one rodent, and 

 that an infection once established among these rodents may spread 

 to other rodents. 



The second important application to experimental medicine of 

 facts determined in this investigation is that the young rats may 

 be used successfulh' in further investigation relating to pathological 

 conditions resulting from amoebiasis. Experimental infection of 

 kittens with E. dysenteriae has resulted in an acute form of amoebiasis 

 (Dobell, 1917) and the amoebae recovered are thought to be abnormal 



