332 SPECIFIC MICRO-ORGANISMS 



by heat produce fatal poisoning in guinea-pigs and rabbits. 

 The poisons obtained so far are much less powerful than the sol- 

 uble toxin of B. diphthericB or the endotoxins of the typhoid and 

 cholera germs. 



Rodents, especially rats and guinea-pigs, are very susceptible 

 to inoculation, even a needle prick carrying the minutest quantity 

 of a virulent culture being sufficient to kill in a few days. At 

 autopsy the adjacent lymph nodes are found greatly swollen 

 and surrounded by hemorrhagic edema. The spleen is greatly 

 swollen: Everywhere are enormous numbers of the bacilli. 

 Infection by feeding gives positive results in about half the ex- 

 periments. Inhalation of the bacilli produces typical pneumonic 

 plague in rats. Monkeys are susceptible and present lesions 

 similar to human plague. 



Bubonic plague can be recognized in descriptions of epidemics 

 in very ancient records. Rufus of Ephesus who lived at the time 

 of Trajan (A. D. 98) mentions specifically a very fatal acute 

 bubonic plague ("pestilentes bubones"). Great epidemics oc- 

 curred in Europe in the 6th century (527-565 A. D.), in the four- 

 teenth century (1347-1350 A. D.). Each of these was foUowted 

 by smaller outbreaks persisting in the latter epidemic up to about 

 1850. It is estimated that 25 million persons died of the plague 

 in the " Great Mortality" of the 15th century. Another pandemic 

 of plague began in 1893. Its progress has been slow and un- 

 doubtedly hampered by the prophylactic measures made possible 

 by the discovery of Yersin and Kitasato. It exists as a persistent 

 infection among rodents or human beings, or both, in central 

 Asia, central China, northern India, Arabia, southern Egypt, 

 and, more recently, seems to have established itself in California. 

 Outbreaks of plague in man in new localities have usually been 

 preceded or associated with mortality among rodents, especially 

 rats. When an epidemic begins in a seaport town, the sewer rats 

 (Mus decumanus) are first attacked. Two to three weeks later 

 the house rats {Mus rattus) begin to die, and about four weeks 

 later the epidemic of human plague begins. The transmission 



