PATHOGENICITY 827 



lot of 20 was fed the same mixture with 10 micrograms of thiamin 

 chloride (vitamin B) per rat daily. The second lot made much greater 

 weight gain during the next ten days than the first. On the tenth to the 

 fifteenth days each rat was inoculated with daily doses of 6,000 sporu- 

 lated oocysts of E. nieschulzi. Twelve rats out of 20 in the first lot 

 succumbed to the infection, and the remaining 8 all lost weight upon 

 recovery. The recipients of vitamin B all lived and, by the time the in- 

 fection had cleared up, all had gained in weight. 



Thus it is evident that the physiological state may be of prime im- 

 portance in determining whether or not an animal survives an infection. 

 In one state it may show few or no outwardly visible symptoms, while 

 in another it may be seriously affected, or even succumb. 



Conclusions 



Such terms as commensalism and true parasitism lose their significance 

 when a comprehensive analysis is made of the circumstances surround- 

 ing an infection with any particular protozoan species. Pathogenicity in 

 the generally accepted sense is a matter of degree, subject in the first 

 place not only to the species, but also to the strain, of the microorganism 

 concerned in the infection. The degree of pathogenicity exhibited by a 

 particular strain in its host may vary from nil to fatal termination, 

 depending upon the inherent defense mechanisms and the other condi- 

 tions affecting the resistance of the host. The effectiveness of this resist- 

 ance, in turn, may vary according to changes in the physiological state 

 of the host. These considerations are of fundamental importance to the 

 investigator who conducts researches on the reaction of any host to the 

 invasion of a protozoan parasite. 



Literature Cited 



Becker, E. R. 1933. Host-specificity and specificity of animal parasites. 

 Amer. J. Trop. Med., 13: 505-23. 



Becker, E. R., and R. C. Derbyshire. 1937. Biological assay of feeding stuffs in 

 a basal ration for rorr/W/V/w-growth-promoting substance. I. Procedure, 

 yellow corn meal, oats, oat hulls, wheat, linseed meal, meat scraps. Iowa 

 St. Coll. J. Sci., 11 (1938): 311-22. II. Barley, rye, wheat bran, wheat 

 flour middlings, soy bean meal. Ibid., 12: 211-15. 



Becker, E. R., and P. C. Waters. 1938. The influence of the ration on mortality 

 from caecal coccidiosis in chicks. Iowa St. Coll. J. Sci., 12: 405-14. 



■ — ■ — - 1939a. Biological assay of feeding stuffs in a basal ration for coccidium- 



