HOST-PARASITE RELATIONS! INTESTINAL PROTOZOA 



fection. It is unnecessary here to point out the possibiH- 

 ties of error in the work described. The conclusion seems 

 warranted that Giardia lamhlia has not been proved 

 capable of living in any other animal ; and that it is rig- 

 idly limited to one host, man. How many of the species 

 of Giardia listed above are "good" species remains to be 

 determined by more careful study and experimental 

 infections. 



Relation of age and susceptibility. It is generally rec- 

 ognized that the young of both man and the lower ani- 

 mals are more susceptible to infection and more fre- 

 quently infected with protozoa than are adults. Thus 

 it has become the custom to select young animals for 

 laboratory experiments. In the case of Giardia lamhlia 

 considerable information exists on this subject. Dobell 

 (1921b) has collected the following data: 



Investigator 



Matthews and Smith 



Campbell 



Miss Nutt 



Miss Nutt 



McLean 



Similar results have been obtained by Maxcy (1921) 

 in the United States and by Simon (1924) in Germany. 

 Maxcy found 17 per cent of infection in children one to 



5 years of age, but almost 40 per cent in children from 



6 to 12 years old. Simon (1924) found 23.4 per cent of 

 infection in a group of 137 persons ; 77.4 per cent of those 

 who were under 15 years of age gave an incidence of 27.4 

 per cent. His youngest case was that of a child 9 months 



168 



