Host-Parasite Relationships 529 



and yet fail to induce symptoms in a third, although completing a normal 

 cycle and producing gametocytes in each type of host. Thus, under 

 experimental conditions, a parasite may in a single transfer become a 

 dangerous pathogen or show almost no pathogenicity, depending upon 

 the host. Therefore, long association between a parasite and a host is 

 not necessary for the reduction or practical elimination of pathogenicity. 

 In addition, the present pathogenicity of a parasite would seem to be no 

 real guarantee of its amateur standing. As applied to the Endamoebidae, 

 the hypothesis of progressive adjustment to commensalism would imply 

 that man and certain other primates have each acquired Entamoeba 

 histolytica much more recently than their other amoebae, which may 

 approach the status of commensals. It seems just as likely that E. his- 

 tolytica was pathogenic and certain other amoebae were non-pathogenic 

 when they first invaded the ancestral primates, and that the various 

 species have merely retained their original characteristics during the sub- 

 sequent evolution of their hosts. In this connection, the occurrence of a 

 histolytica-like pathogen {Entamoeba invadens) in various reptiles (28, 

 81a, 84) may have some significance. 



Symbiosis 



As an abstract concept, symbiosis (mutualism) is an interesting 

 association. However, examples involving Protozoa as the symbiotes are 

 rare.- The most likely candidates are certain intestinal flagellates of 

 wood-eating termites and wood-roaches (Cryptocerus). The ability to 

 digest cellulose has been reported for some of the termite flagellates (43, 

 91, 92) and flagellates of the wood-roach (22, 91). In addition, the results 

 of defaunation indicate that both types of hosts are dependent, to a 

 considerable extent and perhaps completely, upon certain of their 

 intestinal flagellates (19, 20, 21, 22). The status of the rumen ciliates of 

 herbivores has been disputed (7). There is some morphological evidence 

 that ingested cellulose is digested by certain species (96), and the pro- 

 duction of cellulases also has been reported (44, 45). The results of 

 defaunation have varied from no significant effects (10) to a decreased 

 digestion of roughage (101). Growth-rates of lambs have remained normal 

 in the absence of ciliates (9). If the definition of a symbiote merely re- 

 quires an organism to be beneficial to its host and not necessarily its 

 major means of support, the possession of cellulases might qualify some 

 of these ciliates for participation in symbiosis. If the definition is re- 

 stricted, as it has been occasionally (37), to an organism which is indis- 

 pensable to its host, then there is no justification for listing ciliates of 

 the rumen in this category. 



" The literature on flagellates of termites and ciliates of ruminants has been reviewed 

 by Hungate (45a). 



