HOST-PARASITE RELATIONS: INTESTINAL PROTOZOA 



that are ''fast" to substances ordinarily destructive. For 

 example, it is customary to speak of emetin fastness in 

 amoebiasis. Loss of resistance may also result from sub- 

 jection to various environmental factors. 



(2) Aggressivity. The term aggressivity is applied to 

 the invasive powers of a parasite. Changes in parasite 

 aggressivity due to residence within a host have been 

 reported although the situation is not yet clear. Thus dif- 

 ferences in invasive powers have been noted among the 

 cysts of the dysentery amoeba, Endamoeba histolytica, 

 which may have been due to changes during residence in 

 the human host. For example, Baetjer and Sellards 

 (1914) state that "chronic cases of long standing, with 

 mild symptoms, often produced an attack in animals 

 which was of comparatively short duration and event- 

 ually ended in recovery"; and Wagener and Thomson 

 ( 1924) were able to infect kittens without difficulty with 

 amoebae from an acute case of amoebiasis but succeeded 

 in only one of fourteen kittens when amoebae were used 

 from a chronic case of amoebiasis. The supposition is that 

 the conditions of chronicity modified the aggressivity of 

 the amoebae until a strain with very little invasive powers 

 was developed. 



9. HOST-PARASITE ADJUSTMENTS DURING AN INFECTION 



( I ) Carriers. During the course of a natural infec- 

 tion as outlined above various adjustments occur between 

 host and parasite. Continued reproduction by the para- 

 site without check would obviously result in the death of 

 the host; this would be a disadvantage to the parasite, 



36 



