CHAPTER XV 



HOST-PARASITE SPECIFICITY IN THE GENUS 

 GIARDIA 



By 



Robert Hegner 

 The Johns Hopkins University School of Hygiene and Public 



Health 



INTRODUCTION 



The genus Giardia is of particular value for the study of host- 

 parasite specificity since each species of host seems to be infected 

 by a distinct species of parasite. The Giardia of man was first 

 described by Leeuv^enhoek in 1681 (Dobell, 1920). Giardias v^^ere 

 reported from mice, rats and cats by Grassi (1879-1881). Metzner 

 (1901) gave a good description of giardias from the rabbit. All of 

 these giardias were supposed to belong to a single species and 

 cross-infection between different species of hosts in which they 

 were found was supposed to be the rule. Bensen, however, in 1908 

 distinguished three species which we now know as Giardia lamblia 

 from man, Giardia muris from rats and house mice, and Giardia 

 duodenalis from the rabbit. Practically all protozoologists who ac- 

 cepted these as separate species still believed until quite recently 

 that cross-infection occurred and it is even supposed by many 

 physicians at the present time that human infections are due to 

 contamination of food or drink with cysts from rats or mice. 



Twenty or more species of Giardia have been described, 

 especially during the past fifteen years, on the basis of morphology 

 or occurrence in a host species in which they had not previously 

 been reported. The following table presents a list of these species 

 with the name of the describer and date and that of the host. The 

 validity of all these species is not certain. For example, G. simoni 

 is probably G. lainhlia that has succeeded in setting up a temporary 

 infection in rats. Giardia xenopi may belong to the species Giardia 



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