E. histolytica: host resistance and susceptibility 



infections are not as severe as are those of foreigners and 

 that, therefore, a real difference exists in susceptibiHty 

 and resistance of the different races to E. histolytica. 



Age. The relation between age in man and host-sus- 

 ceptibility to E. histolytica is not well known. Young ani- 

 mals seem to be more frequently infected with parasites 

 than adults. Kessel and Svensson (1924), however, re- 

 port conditions among the Chinese that do not coincide 

 with this view. Of 100 individuals between the ages of 

 1 and 15 years, 25.3 per cent were infected, between 16 

 and 50, 29.2 per cent, and over 51, 34.4 per cent were 

 infected. Kessel (1923) in an earlier report noted that 

 older rats are more resistant to infection with human 

 amoebae than younger rats. It is well known that kittens 

 are more readily infected with E. histolytica than are 

 older cats. 



The character of the infection may also differ with 

 age. Kittens usually come down with an acute infection 

 whereas Wagener and Thomson (1924) obtained chronic 

 infections in 3 adult and 3 half -grown cats with amoebae 

 from an acute human infection; evidently the amoebae 

 encounter greater resistance in the older animals. This 

 resistance affects the size and number of amoebae; those 

 from acute cases being large and numerous, whereas 

 those from chronic cases are smaller and few in number. 

 Experiments to determine the relative susceptibility of 

 old and young cats were also carried out by Eguchi 

 (1925). Of 15 kittens that were fed cysts of E. histoly- 

 tica, 6 or 40 per cent developed dysentery and one liver 

 abscess ; but of 23 cats over 500 gm. in weight only 2 or 

 8.6 per cent became infected. Eguchi believes this result 



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