45 



gfends will be found to be surrounded by an infiltration, and the 

 gtend itself will be purulent or less frequently caseous. Such lesions 

 are occasionally met with in rats in which there is no suspicion of 

 plague inflection; but they are seen so frequently among rats that 

 hare survived artificial inoculation with B. pestis, there is no doubt 

 but that in these cases they are the result of the inoculation. In 

 several such cases pest-like organisms have been demonstrated 

 in smears, and acute plague has been produced in guinea pigs 

 by inoculation with the pus found in these lesions. Not infrequently 

 in these cases the spleen will be found enlarged and looking very 

 much like the organ in acute plague, but cultures from this organ 

 in such cases have in my experience remained sterile. 



In other cases the only lesions will be found in the spleen. The 

 organ is enlarged and contains a number of caseous nodules. These 

 nodules vary in number from four or five to thirty or forty and in 

 size from the head of a pin to a lesion 0.3 centimeter in diameter. In 

 a number of such cases the nature of the lesion has been demonstrated 

 by animal inoculation. For example, in a series of experiments 

 carried out to determine the susceptibility of San Francisco rats to 

 plague infection a large Mus rattus died on the eleventh day after 

 inoculation. The post-mortem examination showed nothing except 

 an enlarged spleen which contained about a dozen caseous nodules, 

 the largest of which was not over 2 millimeters in diameter. The 

 nodules were very firm and the capsule smooth, so that they were held 

 with difficulty with dressing forceps. Cultures from the liver and 

 the spleen remained sterile, but a piece of the spleen was placed 

 beneath the skin of a guinea pig. This animal died of acute plague, 

 and a pure culture of B. pestis was isolated from its liver. In some 

 of these cases the liver will show large, distinct, whitish caseous foci. 

 In another case a small Mils norvegicus was killed on the twelfth day 

 after a cutaneous inoculation from an artificially infected squirrel. 

 No lesion was found except in the spleen which was not materially 

 enlarged, but which presented two small whitish caseous granules 

 on the surface, neither being over 1 millimeter in diameter. A piece 

 of the spleen containing one of these granules was put under the skin 

 of the belly of a guinea pig. The guinea pig died on the fourth day 

 with the usual lesions of acute plague. Occasionally in these cases 

 of chronic plague punctate hemorrhages or even areas of consolida- 

 tion are found in the lungs. 



THE HISTOLOGY OF RAT PLAGUE. 



The most recent and satisfactory work on this subject is that of 

 Ledingham (14), who has studied the lesions of both natural and 

 induced plague in rats. The following is a very brief abstract of his 

 work. The reader is referred to the original for a full study of the 

 subject. 



