300 RESEARCH IN PROTOZOOLOGY 



b. Parasitologically — is there any reduction in the number 



of parasites present or in the duration of the infection 

 in the latter attacks ? 



c. Immunologically — is it possible to demonstrate the pres- 



ence of circulating antibodies ? 



2. How long does this resistance last? 



3. Does the resistance increase with the age of the host which 

 has not been infected? 



4. What is the mechanism of the immunity? (See section on 

 Host-Parasite Specificity. ) 



5. Is it possible to protect animals from severe, naturally ac- 

 quired infections by quantitative inoculation to produce a mild 

 infection under carefully controlled conditions? 



6. Just what are the factors which result in the production of 

 an acute attack or a chronic infection or a change from the one 

 to the other? 



Pathogenicity. While it is known that among the lower animals 

 coccidia may appear as pathogenic, or even lethal, parasites it is 

 clear that in the great majority of cases the host survives the 

 infection with little or no apparent injury. Data regarding patho- 

 genicity of coccidia in man are few, but some of the contributions 

 to the subject are of high quaHty. The situation in man seems to 

 be a reflection of the situation in animals, namely, that coccidia 

 may upon occasion be severely pathogenic. One of the first ques- 

 tions for investigation is a determination of the factors which 

 predispose toward pathogenicity, and should include a considera- 

 tion of the following variables on the part of individuals : age, sex, 

 previous experience with the disease, faulty nutrition, concomitant 

 infections, derivation of infection from acute or chronic strain 

 of disease, etc. 



The mechanism of pathogenicity should be investigated. Does 

 the parasite kill or injure the host with a toxin? Does it affect 

 carbohydrate metabolism? Does it interfere by means of destruc- 

 tion of intestinal epithelium with the proper absorption of food? 

 Is there sufficient destruction of blood elements to provoke the 

 symptoms observed? Are the chemical and physical equilibria of 

 the blood disturbed in the course of the infection? Some work on 

 determinations of the blood changes have been reported by Leger 

 (1926) in man, Yakimoff, ct al. (1926, 1926^) in cattle and white 

 rats, and Sanders ( 1928) in kittens. 



