474 THE PLAGUE BACILLUS 



[4. Differential diagnosis. The differential diagnosis of the plague bacillus 

 from the organisms most likely to be mistaken for it, viz. : B. pseudo-tubercu- 

 losis rodentium, the bacilli of the hsemorrhagic septicaemia group, and the 

 bacilli of the salmonella group, will depend upon the following observations. 



[1. The inoculation of a white rat will exclude the B. pseudo-tuberculosis 

 rodentium which is non -pathogenic to white rats but gives identical ferment 

 ation reactions. 



[2. The characteristics of the growth in media containing taurocholate of 

 sodium will differentiate the haemorrhagic septicaemia group : the plague 

 bacillus grows well on such media while the growth of the latter group of 

 organisms is inhibited. 



[3. The characteristics of the growth on agar and in broth (absence of 

 stalactites) and the fermentation reactions will distinguish the plague bacillus 

 from the bacilli of the salmonella group. ] 



Post-mortem appearances in rats naturally infected with Plague. 1 



The diagnosis of spontaneous plague in the rat is a matter of much interest [and 

 the following is a brief account of the appearances seen post mortem] : 



[Sub-cutaneous congestion is not infrequently a marked feature : it may be 

 general but in some cases is limited to the neighbourhood of the bubo. Sub -cutaneous 

 haemorrhages occur in about 40 per cent, of rats and are most frequently to be seen 

 in the sub-maxillary region. Buboes are present in the majority of cases but may 

 be absent (15 per cent.) ; when present they occur in the majority of cases in a 

 single situation and most commonly in the neck. The liver may show necrotic 

 changes which have the appearance of an excessive deposit of fat, and a condition 

 of the greatest importance in diagnosis is the occurrence of small necrotic foci 

 scattered over its surface and throughout its substance. The spleen is firm and 

 does not collapse like a soft normal spleen ; granules or nodules may be well-marked 

 in it and may be confluent. The kidneys and supra-renal capsules are often con- 

 gested. Haemorrhages are fairly common in the lungs and visceral pleurae. The 

 presence of pleural effusion is very characteristic and of great value in diagnosis. 



[In naturally infected plague rats the most important features for purposes of 

 diagnosis are : 



1. A typical bubo most commonly in the neck. 



2. Granular liver not seen except in plague rats. 



3. Haemorrhages beneath the skin and in the internal organs are very suggestive. 



4. Pleural effusion. 



In putrid rats, bubo, granular liver and pleural effusion may persist and are of 

 great significance.] 



A microscopical examination of scrapings of buboes and spleen and inoculation 

 tests will clinch the diagnosis. 



t 1 This account is abstracted from the Reports of the Advisory Committee. Journal of 

 Hygiene, vii. p. 324 et seq.] 



