CHAPTER XVII 



CERTAIN ASPECTS OF PATHOGENICITY 

 OF PROTOZOA 



Elery R. Becker 



It is customary to recognize three functional categories of parasitic 

 Protozoa: (1) commensals, which neither harm nor abet the host; (2) 

 symbionts(=symbiotes), which aid the host; and (3) true parasites 

 or pathogenes, which disarrange the host organism to a greater or less 

 degree. This practice may be defended on academic grounds, since it 

 serves to clarify concepts and to attract students' interest to animal 

 microorganisms and the roles they play in the lives of other animals and 

 plants, but it is in reality highly artificial. The ensuing discussion will 

 be developed principally about this point, with the deliberate intention 

 of provoking wide consideration of the subject, particularly as regards 

 the "pathogenic" aspects of parasite activity, as was done with the 

 subject of host-specificity of parasites a number of years ago (Becker, 

 1933). Such terms as commensalism, symbiosis (symbiotism), and 

 pathogenicity can represent no more than an expression of the state of 

 adjustment between two separately functioning entities, the host and the 

 parasite, coexisting in one of the most intimate relationships, and as 

 such are subject to analysis. 



Problems of Virulence and Pathogenicity 



The functional categories have no counterparts in the zoological 

 scheme: that is to say, there are no classes, orders, or families which 

 have as their distinguishing character that they are pathogenic or other- 

 wise. The statement applies also to genera, for, as a matter of fact, we 

 recognize "pathogenic" and "non-pathogenic" members of Trypanosoma, 

 Trichomonas, Entamoeba, and other genera. The situation is seen, at 

 the outstart, to limit itself almost entirely to a consideration of "patho- 

 genic species," but it is actually still more complicated than that. There 

 is indisputable evidence that many species of pathogenic Protozoa are 



