HOST-PARASITE SPECIFICITY IN THE GENUS GIARDIA 151 



species of Giardia. A mere glance is sufficient to distinguish such 

 species as G. agilis from the tadpole and G. muris from rats and 

 mice. A more careful study is necessary for the recognition of 

 other species. No doubt giardias occur in many species of animals 

 in which they have not yet been found. They should be looked for 

 in the duodenum. Rodents seem to be infected most frequently 

 and careful studies of the giardias in nearly related species of rats, 

 mice and other rodents are indicated. The situation as regards the 

 giardias of birds is in a very chaotic condition and examination of 

 birds of prey, shrikes, water birds and other types that feed 

 largely on small animals would no doubt yield valuable results. 



g. Cross-infection experiments. This subject is treated in gen- 

 eral in another chapter (Chapter IX). Certain investigators are 

 not willing to accept the type of statistical work just described as 

 sufficient for separating species of Giardia. The belief has been 

 expressed that the differences observed may be due to differences 

 in environment in different species of hosts and that one species of 

 Giardia may, therefore, live in several species of hosts and exhibit 

 slightly different but constant characteristics in each host. In order 

 to settle this point cross-infection experiments are necessary. The 

 writer admits that such experiments are desirable and hopes that 

 they will be carried out by competent investigators in the near 

 future. The difficulties involved, however, are considerable and 

 only under exceptional conditions can one obtain parasite-free 

 animals and a sufficient supply of giardias for such work. 



Of particular interest is the situation regarding the possible in- 

 fection of the rat with G. lamblia from man. Boeck (1919) men- 

 tions the discovery in a laboratory rat of a type of Giardia resem- 

 bling G. lamblia. Simon (1922) encountered similar specimens in 

 two laboratory rats which he considered to be of the same type 

 as those reported by Boeck. Lavier (1924) reports three out of 301 

 wild rats collected in the sewers of Paris with giardias morphologi- 

 cally like Giardia lamblia. Since he could not infect laboratory rats 

 with human giardias, Lavier decided that he was dealing with a 

 distinct species which he named G. simoni. Giardias resembling 

 G. lamblia were also described from rats by Nieschulz and Krijgs- 

 man (1925). Potter (1928) found two types of giardias living 

 together in the same laboratory rats. He concludes from a mor- 

 phological study involving specimens of G. muris, G. microti and 

 G. lamblia that he was dealing with G. muris and G. lamblia and 



