CROSS-INFECTION EXPERIMENTS 89 



indicate that this amoeba is interchangeable. The only logical con- 

 clusion in the light of these data is that these amoebae of man, of 

 the monkey, of the rat and of the domestic pig all belong to the 

 same species and that they have merely gained access to different 

 hosts. It may of course be found that cultural or physiological dif- 

 ferences, similar to those employed in differentiating bacteria, will 

 give more delicate means of species differentiation among the 

 protozoa than we now know. In such event these amoebae of ani- 

 mals which are morphologically similar to the amoeba of man may 

 exhibit serological differences which will warrant their being clas- 

 sified as different species. 



In all probability all amoebae that exhibit similar morphology 

 have a common origin and the problem left with us is to determine 

 whether the environments of the digestive tracts of different 

 animals present a sufficiently wide variation in biochemical range 

 to have altered individual parasites sufficiently to restrict them to 

 a specific host. 



METHODS IN GENERAL 



Experimental cross-infection work in relation to the problems 

 mentioned above must all be accomplished with the lower animals 

 since man cannot be used safely for experimental infections on 

 account of the dangers associated with his acquiring protozoan 

 infections and bacterial infections of uncertain pathogenicity that 

 might be transmitted to him during the experiment. 



Animal transfer experiments, however, are imperative as aids 

 in determination of species identity and to problems in host-para- 

 site relationships. These must be performed with the greatest 

 caution and the necessity of procuring absolutely parasite-free 

 animals for experimental work cannot be too greatly emphasized. 



A. Selection of Negative Animals 



Animals negative for parasitic infection may be selected by the 

 following methods : 



I. Examination of feces for the detection of intestinal protozoa. 

 Feces for examination may be collected in a variety of ways. 



(a) Collection of normally passed feces. As a general rule this 

 method may be held to be more suitable for animals that normally 

 have only one or two evacuations daily than for animals that pass 

 small pellets of feces frequently, e.g., rats and mice. Experience 



