36 



It is recognized that the data frcim the San Francisco records is so 

 much smaller than that from the Indian report that perhaps no just 

 comparison is to be made. However, the figures are quite similar, 

 except for the small percentage of buboes and of liver lesions in the 

 work of Wherry, Walker, and Howell. The work of these observers 

 was done in the early part of the epizootic in San Francisco while 

 the other figures from that city are drawn from records later in the 

 campaign. 



No single sign is pathognomonic, though only once have we been 

 deceived by what was regarded as a typical plague bubo. This was 

 in a rat that presented no other suspicious lesions and the inocula- 

 tion test resulted negatively. 



It is a combination of two or more of the signs that is of moment. 

 The subcutaneous injection with a typical liver or these signs asso- 

 ciated with a typical spleen afl^ord good grounds for a diagnosis. 

 A rat showing a typical liver associated with a pleural effusion will 

 usually prove to be plague infected, and if a large, dark, firm spleen 

 is also found a diagnosis may be considered as practically established.. 



As has been pointed out by several writers gross lesions of plague 

 may be distinguished even in rats that are badly decomposed. 



CHRONIC PLAGUE. 



No case of natural chronic plague has been encountered in San 

 Francisco. Only one case was found among the many hundreds of 

 plague rats examined by the Indian Plague Commission (3, p. 457) 

 in Bombay. However, this commission encountered a considerable 

 number of cases among Mus rattus in the Punjab villages of Kasel 

 and Dhand. The lesions were purulent, or caseous foci. They 

 classify these cases as follows: Chronic plague of the visceral type, 

 which is further subdivided into splenic nodules and abscesses, and 

 mesenteric abscesses; chronic plague of the peripheral type in which 

 abscesses are situated in the regions of the peripheral lymph 

 glands. 



Plague bacilli were either absent or very scanty upon microscopical 

 examination. They were, however, quite frequently recovered by 

 cultural methods, and in the great majority of the cases the organ- 

 isms were fully virulent. No evidence was forthcoming to show that 

 this chronic rat plague had anything to do with the recurrence of 

 acute plague among the rats. 



We have diligently sought for chronic plague among the rats in 

 San Francisco, but, as we said above, without success, although a 

 considerable number of lesions that correspond perfectlv to the 

 description of chronic plague have been submitted to the guinea-pig 

 inoculation test, but invariably with a negative result. An account 

 of the lesions of chronic plague as observed among inoculated rats 

 is given in another part of this paper. 



