CHAPTER II 



HOST-PARASITE RELATIONS BETWEEN ANIMALS 

 AND THEIR PROTOZOAN PARASITES 



By 



Robert Hegner 



The Johns Hopkins University School of Hygiene and 



Public Health 



As pointed out in Chapter I the study of protozoology has been 

 and is being carried on by men with various viewpoints. An at- 

 tempt to bring together the various phases of the subject of interest 

 to zoologists, medical men, veterinarians and public health workers 

 has resulted in a program which we call host-parasite relations. 

 This program presupposes a knowledge of the morphology, 

 taxonomy and life-cycles of the protozoa concerned. It is devoted 

 more especially to the relations between protozoa and their hosts 

 and involves the medical as well as the biological aspects of the 

 subject. The writer has in previous publications presented a gen- 

 eral outline of this program (Hegner, 1926) and a more detailed 

 presentation of the host-parasite relations between man and his 

 intestinal protozoa (Hegner, 1927c), hence it will be unnecessary 

 to consider in extenso an account of this subject. 



The logical point to begin a study of host-parasite relations is 

 with the entrance of the parasite into a clean host. This involves 

 the study of the organism outside of the body or in an interme- 

 diate host and its method of entrance into the vertebrate host. 

 Then follow in order the subjects of the distribution and localiza- 

 tion of the parasites within the host, data regarding primary and 

 secondary sites of infection, types of resistance exhibited by both 

 host and parasite, the parasite's method of attack, changes in the 

 host as a result of this attack as indicated by symptoms, pathologi- 

 cal lesions and immunological reactions, changes in the parasite as 

 a result of residence in the host, adjustments between host and 



4 



