GIAEDIA IN ANIMALS 705 



of the naturally infected animal. Two of the cats died on the fourth day, 

 when a careful examination of the intestines was made both in the fresh 

 condition and in stained sections. There was no sign of any infection 

 with Giardia. The naturally infected cat and one other survived for 

 three weeks. They completely recovered from the intestinal disturbance 

 caused by the inoculation. The naturally infected animal alone continued 

 to pass cysts. The ajiimals were killed, and, as was expected, Giardia was 

 found only in the one which was already infected before the experiment. 



GIARDIA IN ANIMALS. 



As already remarked, a number of different species of Giardia have 

 been described, but, with the exception of G. agilis of the tadpole, the 

 various forms are very uniform in appearance. Simon (1922) and Hegner 

 {1922a) have introduced a biometric method for the separation of species 

 similar to that which has been employed in the case of trypanosomes. 

 They maintain that if a sufficiently large number of individuals is 

 measured, species can be separated by constructing curves showing the 

 percentage of flagellates of any one size. Simon and Hegner claim that in 

 this wa}^ it is possible to separate the human form from that of rats and 

 mice, and Hegner those occurring in the dog and rabbit from each other 

 and from those of human beings and rats (Fig. 296). Hitherto the occur- 

 rence of Giardia in different hosts has been the chief factor which has 

 influenced observers in the establishment of species. Grassi (1879o) 

 gave the name Dimorphus muris to the form in the mouse, while later 

 (1881a) he noted the occurrence of Giardia in human beings and also in 

 the cat, dog, rabbit, sheep, rat and mouse — Mus muscidus, Rattus rattus, 

 R. decumanus, M. sylvestris, Arvicola (Microtus) arvalis — and introduced 

 the new name Megastoma entericum. Grassi and SchewiakofE (1888) 

 added to the list of hosts A. (Microtus) amphibius. These observers 

 regarded the animal forms as belonging to the same species as that 

 occurring in man. In the writer's experience, Giardia is commonly 

 present in cats, dogs, rats, mice, and rabbits in England. Kofoid and 

 Christiansen (1915) have described as G. microti a form which occurs in 

 Microtus calijornicus of California. They believe it to be distinct from 

 G. muris, which they found, not only in mice, but also in Peromyscus 

 maniculatus gambeli. Davaine (1875) described as Hexamita duodenalis 

 a flagellate from the duodenum of the rabbit. It is undoubtedly a species 

 of Giardia, so that the correct name for the form in the rabbit is G. duo- 

 denalis, though Bensen (1908) proposed to name it G. cuniculi (Fig. 296, b). 

 Fonseca (1916) observed it in both the rabbit and Ccendu viUosus of South 

 America. A form he saw in the monkey {Cebus caraya) he regarded as 

 identical with the human G. intestinalis. Splendore (1920) gave the name 



I. 46 



