PROBLEMS IN HOST-PARASITE SPECIFICITY 



Musca domestica, as Herpetomonas musccB-domes- 

 ticcB. These results have been confirmed and extended by 

 Drbohlav (1925b). 



3. SOME PROBLEMS IN HOST-PARASITE SPECIFICITY 

 AMONG INTESTINAL PROTOZOA 



Biological studies of the relations between protozoan 

 parasites and their hosts, especially man, have within the 

 past thirty years brought about a marked change in our 

 ideas regarding host-parasite specificity. Until quite re- 

 cently the belief was prevalent that cross-infection is the 

 rule in nature; for example, that man is infected with 

 protozoa of lower animals and that lower animals are 

 regularly parasitized by human protozoa. Thus, where 

 several decades ago one species was supposed to inhabit 

 a number of species of hosts we know to-day that in 

 many cases each species of host is parasitized by its own 

 species of parasites, which appear to be rigidly adjusted 

 to it and unable to live in any other species of host. Some 

 of the problems involved in the study of host-parasite 

 specificity are stated in the following paragraphs and 

 suggestions are presented to account for the facts ob- 

 served. The conclusion reached is that we know very lit- 

 tle about this interesting and important subject, but that 

 further experimental study is possible and desirable. 



(l) To WHAT EXTENT DOES THE BEHAVIOR OF THE 

 HOST AND THAT OF THE PARASITE DETERMINE HOST- 

 PARASITE SPECIFICITY? This problem involves particu- 

 larly the question of transmission (Hegner, I926d). It 

 is obvious that host and parasite must be brought to- 

 gether under favorable conditions when the host is sus- 



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