46 



NATURAL RAT PLAGUE. 



Two groups of cases are distinguished, first, those in which a large 

 number of B. pestis are found in the liver and in the spleen. In the 

 spleen this is accompanied by hemorrhages and congestion of the 

 pulp sinuses and in the liver with congestion of the capillaries. 

 These are early cases. 



In the second group, or the later cases, there are extensive reaction 

 changes in the tissues. In the spleen this leads at tunes to distinct 

 abscess formation, but more frequently to a walling off of the foci 

 of necrosis. In the liver more or less focal necrosis is found; some- 

 times the areas of " necrosis" may be so extensive that little healthy 

 liver tissue remains. Bacilli are usually to be demonstrated in 

 these areas of necrosis. Giant cells of the Langhans type may be 

 found in the neighborhood of these foci. 



The granular appearance of the liver is attributed to " hemorrhages 

 and the focal necroses, together with the fatty changes in the liver 

 cells. It must be understood, however, that a peculiar honeycomb- 

 like vacuolar degeneration of the liver cell protoplasm was far more 

 frequent than any actual, coarse, fatty infiltration. The granular 

 appearance of the spleen is due partly to endothelial catarrh and 

 partly to subcapsular changes." 



In experimental rat plague Ledingham found the lesions to resemble 

 those of the first group of cases referred to above. There is usually 

 marked bacteraemia; focal necroses of the liver are scanty. 



In a chronic case, minute abscesses were found scattered through 

 the spleen. In the center of the abscesses were found clumps of 

 degenerated bacilli. The areas were walled off by epithelioid and 

 spindle cells and numerous giant cells of the tubercular type. 



IMMUNITY OF RATS. 



Contrary to the general impression the wild rat is not an animal 

 especially susceptible to plague infection. The Indian Plague Com- 

 mission (19) found that when rats are inoculated by the cutaneous 

 method from the spleen of infected rats 59 per cent are immune to 

 infection. A series of experiments conducted in the federal laboratory 

 in San Francisco showed that when inoculated with highly virulent 

 cultures of B. pestis there is an immunity which is, however, more 

 frequent among the large rats. When inoculated cutaneously with 

 tissue containing large numbers of B. pestis from plague infected 

 human beings, rats, or squirrels, about 15 per cent of small rats and 

 about 50 per cent of large ones were found to be immune. There is 

 no good reason for believing that this immunity of San Francisco 

 rats was due to a previous attack of the disease. Indeed, it was known 

 beyond a doubt that some of the immune rats had never had an 

 opportunity of becoming infected with plague in nature and thereby 



