TRANSMISSION OF PLAGUE 401 



animals, or their excretions, infected with plague, and 

 not from a saprophytic form of the organism. 



Certain animals, especially the rat (Mus rattus and Mus 

 decumanus), are important agents in spreading the disease. 

 The association of sickness and of death among rats 

 with an epidemic of plague has been established by a 

 number of observations, and in some instances the epizootic 

 among rats has been definitely shown to precede the 

 epidemic in man. The epidemics at Sydney are perhaps 

 the most striking instances of rat-borne plague ; discussing 

 the first one Tidswell says : " The one clear fact in our 

 epidemic was that human beings were not becoming infected 

 from one another." In the first epidemic the mode of 

 introduction of the disease was never traced to any human 

 source. During an epidemic rats may be found in 

 all stages of illness and plague bacilli can be found in 

 large numbers in their carcases. In the various epidemics 

 at Sydney, cases of plague first occurred among the rats 

 and mice, followed after an interval of days or weeks by 

 human cases. Other animals may also occasionally be 

 the means of disseminating the disease. The experiments 

 of the Advisory Committee on Plague Investigation in 

 India have conclusively shown the important part played 

 by rats in the dissemination of the disease, though the 

 origin of the primary infection in rats is doubtful. They 

 may possibly become infected from the dust of earthen 

 floors of the native houses soiled with excreta or discharges 

 of plague patients, or from their clothing, poultices or 

 dressings, but the readiest method is probably by feeding 

 on the dead. Once the epizootic has started, further 

 infection is simple ; rats fight, and so may directly inocu- 

 late one another ; the sick rats may soil grain or other 

 food-stuffs, and the dead rats are eaten by their fellows. 

 Moreover, parasitic insects, especially fleas, undoubtedly 

 may transmit the disease from one animal to another. 



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