37 



Pound (7) believes that recovery from plague in rats is shown by 

 the presence of pigmented lymphatic glands. Kister and Schumacher 

 (2) mention pigment deposits in the inguinal region, but remarked 

 that they are not characteristic of plague, a view which I believe is 

 correct, as we have frequently seen them in San Francisco among the 

 older rats, in which there was no reason to suspect previous plague 

 infection, and they have been almost uniformly absent in the case of 

 rats that have been experimentally infected with plague but have 

 recovered. 



RAT PLAGUE WITHOUT GROSS LESIONS. 



Plague infection may be present in a rat without bringing about 

 any recognizable gross lesions. For example: Dunbar and Kister 

 (8) mention a rat, which came from a ship on which plague rats had 

 been found, that had no lesions, and cultures were negative ; but a 

 guinea pig cutaneously inoculated died of plague. 



Among a considerable number of inoculated rats we have very 

 rarely, perhaps once or twice in a hundred cases, found nothing at the 

 post-mortem examination that would suggest plague infection, yet 

 cultures or inoculation of guinea pigs would demonstrate the presence 

 of B. pestis. Such cases are very infrequent, but it should be kept 

 in mind that they do occur. When a large number of rats are to be 

 examined it would be impracticable to inoculate a guinea pig from 

 each rat; and even if one did this the occasionally resistent guinea 

 pig would introduce a larger error than exists by placing dependence 

 upon the gross lesions for a diagnosis. 



MICROSCOPICAL EXAMINATION. 



The exact weight to be given to the morphology of the organisms 

 found in smears from the organs of a rat suspected of being plague 

 infected is a matter of individual judgment. Smears from a bubo and 

 from the spleen may show no organisms at all, or none even remotely 

 resembling B. pestis, and yet by culture and inoculation methods we 

 may be able to demonstrate that the animal is plague infected. 

 Attention has been called to this point by several observers, and every 

 worker in this field has the experience sooner or later. 



In other cases the smears will show such numbers of perfectly 

 typical bipolar bacilli and " involution" (coccoid) forms as to leave 

 scarcely any doubt as to the nature of the organism. But even here 

 cases that are not plague are encountered that will deceive even the 

 most experienced. We have been accustomed to put great depend- 

 ence on the " coccoid" forms of the organism, but late in the San 

 Francisco experience, smears from a splenic nodule that was not 

 regarded as due to plague showed perfectly typical "involution" 

 ("coccoid") forms. Animal inoculations and cultures showed that 

 the tissues contained no plague bacilli. 



