HOST-PARASITE RELATIONS: INTESTINAL PROTOZOA 



a week or longer after amoebse appeared in their stools, 

 but a negative result when serum from normal cats or 

 from cats infected less than a week were used. 



Intradermal reactions. Scalas (1923) obtained re- 

 markably successful results in his experiments with intra- 

 dermal reactions. He used an antigen prepared from the 

 fresh feces of a case of acute dysentery which he injected 

 intracutaneously into patients suffering from acute, sub- 

 acute and chronic stages of amoebic dysentery and into 

 non-dysenteric subjects. The 9 infected patients gave a 

 positive result, i. e., a swelling which itched and was hot ; 

 the 23 non-infected persons gave only negative results, 

 — erythema without itching or heat. 



9. CHANGES IN THE PARASITE DUE TO RESIDENCE IN 

 THE HOST 



(i) Aggressivity. The problems involved. As 

 already mentioned, many persons are ''contact carriers" 

 who never have shown symptoms but "carry" the amoe- 

 bae in their intestine and pass cysts in their feces. The 

 discovery that E. histolytica can live, grow and repro- 

 duce in culture without access to tissue elements indi- 

 cates that this organism can live in the lumen of the 

 intestine and suggests that the intestinal wall of contact 

 carriers may never be invaded by the amoebae. This brings 

 up the important problem of parasite aggressivity. Are 

 strains that inhabit contact carriers lacking in aggres- 

 sivity? Are they capable of bringing about acute infec- 

 tions? Can the aggressivity of a strain be increased by 

 rapid passage through a number of hosts? Are there 

 adolescent and senile strains or periods in the life-cycle 



104 



