Animal Resistance 



477 



This is strikingly illustrated by the experimental infection in the 

 rabbit which, as a rule, is comparatively mild but in exceptional 

 instances may assume a more malignant form or persist in an 

 active state with severe constitutional symptoms for months or 

 even years and in the end cause the death of the animal. 



Little has been known as to the nature or source of the forces 

 employed by the animal in combating this disease but there is 

 a great deal of evidence to indicate that resistance to this in- 

 fection is a function of animal economy and is subject to the 

 same influences as are concerned in the regulation of growth 

 and development, and the maintenance of general metabolic ac- 

 tivities. Moreover, there have been specific indications that re- 

 sistance was influenced to some extent by the endocrine system. 



This possibility was first suggested to us by the occurrence 

 of marked seasonal changes in the severity of the disease but 

 was most forcibly impressed upon us by the abnormal resistance 

 displayed at times by pregnant and lactating females. Finally, 

 when we came to investigate the crossed immunity relationships 

 between syphilis and a malignant tumor which had developed in 

 a syphilitic rabbit, 1 ' 2 we were again impressed by the importance 

 of the constitutional element in resistance. In the case of the 

 tumor, it was perfectly obvious that we had to deal with the en- 

 docrine mechanism and this led us to carry out on rabbits infected 

 with Treponema pallidum a parallel series of investigations in- 

 tended in part as a control for the work on the tumor but pri- 

 marily as a means of determining whether any definite changes 

 which appeared to bear a relation to the resistance displayed 

 by the animal to syphilitic infection could be detected in the 

 animal organism. 



EXPERIMENTAL 



Since November, 1921, a total of 139 rabbits infected with 

 virulent strains of Treponema pallidum has been killed and 

 examined at various stages of the infection, the time ranging 

 from 1 week after inoculation (first incubation period) to sev- 

 eral months after the development of complete latency. All 

 organs were weighed and studied grossly and microscopically 

 and the conditions found were analyzed with reference to the 



i Brown, W. H., and Pearee, L., J. Expcr. Med., 1923, xxxvii, 601. 

 - Pearce, L., and Brown, W. H., ibid., 631. 



