CHAPTER X 



METHODS AND PROBLEMS OF CROSS-INFECTION 



EXPERIMENTS WITH INTESTINAL PROTOZOA 



OF MAN AND OF CERTAIN LABORATORY 



AND DOMESTIC ANIMALS 



By 



John F. Kessel 

 School of Medicine, University of Southern CaHfornia 



IXTRODUCTIOX 



With the discovery that most of the common laboratory and 

 domestic animals harbor intestinal protozoa similar to those found 

 in man there has been much speculation as to whether animals 

 could serve as reservoir hosts in the transmission of the intestinal 

 protozoa to man and also as to whether the species of intestinal 

 protozoa of man are identical with or different from those found 

 in the lower animals. Similar questions obviously arise with refer- 

 ence to the interrelationships of the intestinal protozoa of the 

 various lower mammals. 



Many of the opinions stated in the past have been formed with- 

 out resorting to adequate experimental measures and much dis- 

 cussion that has taken place reminds one of the debates reputed 

 to have occurred during the Aliddle Ages concerning the number 

 of teeth possessed by the horse, all without the participants having 

 resorted to the simple measure of counting the horse's teeth. It is 

 obvious, of course, that the difficulties encountered in the problems 

 under present consideration are not to be overcome so easily but 

 will require the collection of much data by tedious experiments. 



As a consequence of the speculation on this subject in the past 

 two contrasting views have become evident. The first assumes that 

 the intestinal protozoa of man and many of the lower animals with 

 which he is associated are identical, that the host-parasite relation- 

 ships are not rigid and that the lower mammals may serve as 



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