194 STRONG AND TEAGUE. 



have been made that there is often a marked difference in viru- 

 lence in the different cultures isolated, during this epidemic of 

 pneumonic plague the organism seems to have retained a very 

 high degree of virulence throughout. The cultures isolated from 

 a number of cases near the close of the epidemic, upon inoculation 

 into animals, proved to be fully as virulent and to kill animals 

 as quickly and in the same doses as did those cultures isolated 

 near the beginning. That the organism retained such a stable 

 virulence throughout the epidemic is, perhaps, not surprising 

 when one considers that infection occurred directly from man to 

 man or, frequently one might say, from lung to lung and without 

 the passage of the organism through rodents as ordinarily occurs 

 in bubonic-plague infection. Morever, from the results of pre- 

 vious experiments relating to infection of animals with pneu- 

 monic plague by inhalation, we would expect that the organism 

 would have retained its maximum virulence throughout this 

 epidemic. 



For these reasons and, also, from the fact that the acute course 

 and mortality of the disease were not changed toward the close 

 of the epidemic and especially, from the experimental proof 

 furnished by the inoculation of animals with cultures isolated 

 near the beginning and near the close of the epidemic, we must 

 conclude that the sudden decline and cessation of the epidemic 

 was not due to any marked change in the virulence of the strain. 

 Such a decline and cessation must have depended upon other 

 causes. The plague bacillus, whether isolated from pneumonic 

 or from bubonic epidemics, usually exhibits marked stability in 

 virulence. While it is not a very resistant organism in nature 

 and easily becomes destroyed under certain conditions, it usually 

 does not become markedly attenuated in passage through the 

 animal body, and even on artificial culture-media, after many 

 months, its virulence is usually fully retained. Instances of 

 spontaneous loss of virulence in culture-media have been reported, 

 but this is not usually the case with fresh, virulent cultures. 

 This quality of stability of virulence of the plague bacillus, so 

 different, for example, from that of the cholera vibrio, is of 

 particular interest from an epidemiological standpoint. 



AGGLUTINATION TESTS. 



Theoretically the agglutination test has two applications in 

 plague: (1) The diagnosis of the disease by the demonstration 

 of antibodies in the patient's serum and (2) the identification of 

 the organism cultivated from a suspected case by means of the 

 serum of an animal immunized against the plague bacillus. 



