GIARDIA LAMBLIA: CROSS INFECTION 



According- to Thomson (1926) Howitt has succeeded in 

 infecting- dogs with giardias from human feces. 



Most of the experimental work on cross infection has 

 been done with the idea of determining merely whether 

 or not infection is possible. A more interesting and 

 biologically more important question, however, is what 

 factors favor or hinder infection? In this direction lies- 

 the solution of the real problem of cross infection. After 

 the writer (Hegner, 1927a) found that cysts of G. 

 lamhlia excyst in the rat (see p. 157), attempts were 

 made to bring about infection. Ten rats were fed washed 

 cysts of G. lamhlia from man on 1 1 of 12 successive days, 

 and killed at intervals during the succeeding 19 days. 

 Temporary infections were apparently established in 4 

 rats; these infections were highest on the 6th and 7th 

 days, decreased by the 12th and 14th days, and disap- 

 peared by the i6th and 19th days. The trophozoites re- 

 covered from these animals were smaller than those of 

 G. lamhlia: this may have been due to their unusual 

 habitat. The distribution of the giardias differed from 

 that of G. muris; the latter are most abundant in the 

 duodenum, whereas the trophozoites of G. lamhlia were 

 absent from the duodenum and most numerous in the 

 small intestine from 40 cm. to 90 cm. posterior to the 

 stomach. No cysts were passed by the experimentally in- 

 fected rats, which indicates that the rat is not an im- 

 portant transmitting agent of this species. 



The difficulties involved in cross-infection experiments 

 are obvious and the results of such investigations will be 

 unsatisfactory until some method is found of securing 

 experimental animals that are absolutely free from in- 



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