1923] 



Kessel: Infection with Intestinal Amoebae 



407 



C. INCIDENCE OP INFECTION AT DIFFERENT AGES 



In choosing: animals for experimental infections it is advisable to 

 select those most susceptible to infection. The present investigation 

 has shown that the incidence of amoebic infection among culture 

 rats in this laboratory is 34 per cent. The rate of infection is much 

 higher in middle-aged animals than it is in very young animals or 

 in very old animals as is shown in table 3. 



TABLE 3 



Incidence of iNrECTiON op Amoebae in Cultuke Rats and Mice at 



Different Ages 



From this table it is seen that animals between the ages of two 

 and ten months show by far the highest rate of infection. The infer- 

 ences are that young animals, before weaning, either because of their 

 milk diet (Kessel, 19236) or because of the minimum degree of 

 exposure to infection, have not established an infection; that the older 

 animals have, in some way and to some extent, cleared themselves of 

 infection ; that the animals midway between these two ages not living 

 on a diet of milk exclusively, are subject to constant exposure, and, 

 since they have not had time to establish an immunity, show the 

 highest degree of infection. 



In this investigation, in most cases, old amoeba-free animals failed 

 to become infected when fed cysts by mouth, while young amoeba- 

 free animals fed on the same material established an infection (see 

 Kessel, 1923a and &). 



It may therefore be concluded that the most advantageous time 

 to choose rats for feeding experiments is shortly after weaning or 

 at about the age of two months. Among rats of this age will be found 

 the greatest number free from amoebic infection and also those rats 

 with the greatest susceptibility to infection. 



