180 PROTOZOAN PARASITISM 



In Porto-Rican children of pre-school age Hill and 

 Hill (1927) found an incidence of 47.2%. The writer 

 (1928) found an incidence of only 2.3% in a medical 

 clientele of 1040 cases, in which, however, children 

 were a small part of the number, and, even at that, 

 45% of the Giardia hosts were under twenty years of 

 age. 



It was long believed that Giardia infection of man 

 was derived from other animals. At the present 

 time there is no reason to suppose such to be the 

 case, nor is there need to go outside humankind to 

 find sufficient explanation of the source of the para- 

 site. While many animals harbor giardias indistin- 

 guishable from that of man by the inexperienced, and 

 some have species very closely similar at expert obser- 

 vation, it is generally considered by the best authori- 

 ties that they are distinct species and that ordinarily 

 the only one with which man is concerned is his own. 



Institutional studies indicate that transmission is 

 commonly by almost direct transfer from one person 

 to another. Close personal contact with the infected 

 leads to spread of the parasite regardless of modern 

 sanitary precautions as to the disposal of sewage, fly 

 control, etc. Undoubtedly, however, the contamina- 

 tion of food and drink by food handlers of unclean 

 habits, the exposure of human excreta, without disin- 

 fection, to flies, and to drinking water, the use of 

 human feces as fertilizer for garden stuffs, are all 

 capable of and probably do play their part in trans- 

 mitting the organism. 



