634 COMMUNICABLE DISEASES OF LABORATORY ANIMALS 



different dealers into the infected group (laboratory stock) is followed by an epidemic 

 with a typical and remarkably constant mortality curve, and then a return to an 

 endemic distribution of the infection. The survivors of the epidemic may or may not 

 be carriers or immune to subsequent infections. When the factors involved in these 

 occurrences were analyzed it was found by Topley and by Webster that the distribu- 

 tion, the virulence of the microbes, and the host susceptibility were in all probability 

 responsible for the outbreaks. Topley, for example, demonstrated that the pre- 

 epidemic period was preceded by a marked rise in the fecal excretion of paratyphoid 

 bacilli, and it is reasonable to assume that a certain dosage increases the chance of 

 the mice to ingest the specific bacteria. Regarding the influence of the virulence, the 

 views of Topley are diametrically opposed to those of Webster. According to Lock- 

 hart,' significant variations in the invasiveness of B. aertrycke may occur under various 

 conditions, while Webster believes that the inherent virulence of each strain remains 

 constant, uninfluenced by repeated animal passage and varied natural environmental 

 circumstances, and is a relatively fixed quality. However, the fact remains that some 

 strains are highly virulent, others less so. The factors which induce these conditions 

 are unknown. There seems to be ample evidence that the fundamental factors which 

 influence the course of a mouse epidemic is the host susceptibility. Webster^ and 

 Pritchett^ have shown that different breeds and strains of mice vary in susceptibil- 

 ity, and that individual variations from extreme susceptibility to complete resistance 

 are influenced by hereditary, acquired immunity (perhaps in consequence of a latent 

 infection [Topley, Wilson, and Lewis]), ^ diet (on the McCollum diet the mice are 

 distinctly more resistant than on bread-and-milk diet although the intestinal flora 

 plays no part [Webster and Pritchett]),^ seasonal and other environmental factors. 

 In this connection it is interesting to record that the inoculation of killed cultures 

 yields some definite protection against infection per os (Webster, Lange and Yoshio- 

 ka,^ Topley and Wilson^). However, it is doubtful if vaccination would check a 

 mouse typhoid epidemic in a breeding station (Lynch). Preventive measures are 

 much more effective, but cannot be practiced with any degree of assurance until 

 suitable methods for the detection of carriers and latent infections have been de- 

 veloped. 



Although mice are highly susceptible to Pasteurella organisms, spontaneous epi- 

 demics are rare. Reference to such infections are, however, found in the papers by 

 Greenwood and Topley (1925); Greenwood, Newbold, Topley, and Wilson;* and 

 Baudet.5 The autopsy findings are not characteristic; pulmonary lesions and en- 



'Lockhart, L. P.: J. Hyg., 2s, 50. 1926. 



» Webster, L. T.: op. cit., 36, 71, gj. 1922; 37, 21, 33, 231, 269, and 781. 1923; 38, 33, 45. 1923; 

 39, 129, 879, 265. 1924; 40, 397. 1924; 42, I. 1925. 



3Pritchett, I. W.: loc. cil. 



1 Topley, W. W. C, Wilson, J., and Lewis, E. R.: /. Hyg., 23, 421. 1925. 



5 Webster, L. T., and Pritchett, I. W.: /. Exper. Med., 40, 397. 1924; Webster, L. T.: ibid., 

 37. 21. 1923- 



^ Lange, B., and Yoshioka, M.: Ztschr.f. Hyg. u. Injektionskrankh., loi, 451. 1924. 



7 Topley, W. W. C, and Wilson, J. : loc. oil. 



'Greenwood, M., Newbold, E. M., Topley, W. W. C, and Wilson, J.: /. Hyg., 25, 336. 1926. 



"Baudet, E. A. R. F.: Tijdschr. v. dicrgcneesk., 51, 15. 1925. 



