30 



of the infected rodents were found dead, the remainder being trapped. 

 This is probably due to the fact that the dicease is one of several 

 days' duration, from two to six most frequently, and during this 

 period there are more chances of catching the sick rodent in a trap 

 than there are of finding the body after death, unless the immediate 

 surroundings are known to harbor infected animals and an especially 

 careful search is made for cadavers in the places, often difficult of 

 access, where rats have their burrows and nests. 



As plague is a disease that gives rise to such characteristic gross 

 pathological lesions in man and in laboratory animals, it is but 

 reasonable to expect that equally distinctive lesions would be found 

 in the rat, and this we find to be the case. 



Skschivan (1), Kister and Schumacher (2), and other writers have 

 observed and recorded the gross lesions of plague in rats. It re- 

 mained, however, for the Indian Plague Commission (3), which had 

 the opportunity of examining an enormous number of plague rats 

 in Bombay and elsewhere in India, to crystallize our knowledge of 

 this subject and to point out its field of usefulness. 



As to the comparative value of microscopical and macroscopical 

 methods of diagnosis, the Indian Plague Commission (3) states 

 that: "The results of tests carried out for the purpose of comparison 

 make it manifest that the naked eye is markedly superior to the 

 microscopical method as an aid in diagnosis, and as the result of 

 our experience we are prepared to make a diagnosis of plague on the 

 strength of the macroscopical appearances alone, even though the 

 other results of cutaneous inoculation and culture are negative and 

 the animal shows signs of putrefaction.' 7 



Our experience with rat plague, though limited, leads us to the 

 same conclusion as that arrived at by the Indian Commission in 

 regard to the value of the gross lesions of plague in making the 

 diagnosis. To one who is acquainted with them, these lesions are 

 as characteristic as those of any infectious disease in man. It is 

 quite true that occasionally atypical cases are encountered where the 

 majority of the gross lesions are wanting, and in such cases it becomes 

 necessary to resort to the inoculation of animals or to cultural inves- 

 tigations in order to make a diagnosis. Such cases are, however, if 

 anything, rarer than are atypical post-mortem findings in pneu- 

 monia or in typhoid fever in man. 



MODE OF EXAMINATION. 



A brief description of the actual manner of examining rats for 

 plague infection will be given here. 



The rats are immersed in any convenient solution for the purpose 

 of killing fleas and other ecto-parasites that might be capable of car- 

 rying infection from a plague-infected rat. 



