E. histolytica: aggressivity 



of one strain? Can a strain retain its aggressivity against 

 the acquired resistance of the host or against drugs ? 



Human experiments. The experiments of Walker and 

 Sellards (191 3) seem to prove that the amcebse in contact 

 carriers are sufficiently aggressive to give rise to acute 

 amcebiasis and that the condition of the host is the impor- 

 tant factor, not the aggressivity of the parasite. They 

 found that 2 of the 20 men employed in their experi- 

 ments never became infected ; that several of the remain- 

 ing 18 had to be fed cysts more than once before positive 

 results were obtained ; and that cysts from a convalescent 

 carrier produced a contact carrier v^hen fed to a second 

 man, and that cysts from him produced another contact 

 carrier when fed to a third man, but when cysts from 

 this individual were fed to a fourth man an acute case 

 developed 20 days later. It is possible that rapid passage 

 through several hosts increased the aggressivity of the 

 strain to such an extent as to bring about an acute attack 

 in the fourth man but this does not seem probable. 



Animal experiments. Several investigators, on the 

 other hand, have found that when kittens are used as 

 experimental animals the percentage of infections ob- 

 tained and the severity of the attack depend upon the 

 character of the amoebae used. The evidence indicates that 

 the amoebae differ in virulence and that material from 

 chronic cases is not as infective as that from acute cases. 

 Thus Baetjer and Sellards (1914) state that "chronic 

 cases of long standing, with mild symptoms, often pro- 

 duced an attack in animals which was of comparatively 

 short duration and eventually ended in recovery." Wage- 

 ner and Thomson (1924) had no difficulty in producing 



105 



