Mode of Infection 551 



tion. The spleen and liver are enlarged, the former often pre- 

 senting an appearance similar to that observed in miliary tuber- 

 culosis. Sometimes there is universal enlargement of the lymphatic 

 glands. Bacilli are found in the blood and in all the internal organs. 

 Skin eruptions may occur during life, and upon the inner abdominal 

 walls petechias and occasional hemorrhages may be found. The 

 intestine is hyperemic, the adrenals congested. Serosanguinolent 

 efiEusions may occur into the serous cavities. 



In the latter, they sometimes have encapsulated caseous nodules in 

 the submaxillary glands, caseous bronchial glands, and fibroid 

 pneumonia, months after infection. In all such cases virulent 

 plague bacilli are present. 



In and about San Francisco the extermination of rats for the 

 eradication of the plague was unexpectedly complicated by the 

 discovery that other rodents with which the rats came into contact 

 also harbored the plague bacilli. McCoy and Smith* found this 

 to be true of the prairie dog, the desert wood rat, the rock squirrel, 

 and the brush rat. To insure security against the recurrence of 

 the disease among men necessitates continued observation of these 

 animals and the extermination of diseased colonies, as well as their 

 complete extermination in the neighborhood of human habitations. 



Devellf has found frogs susceptible to the disease. 



Mode of Infection. — The plague bacillus may enter the body by 

 inhalation, from an atmosphere through which it is disseminated, 

 under which circumstances it usually causes the pneumonic type 

 of the disease which is not unlike other forms of pneumonia. The 

 lung is consolidated, enormous numbers of plague bacilli occur in 

 the sputum, the fever is high, and death occurs in a few days. 



Plague pneumonia does not necessarily imply infection through 

 inhalation of the bacilli, however, for it occasionally occurs as a 

 comphcation in the bubonic form of the disease. 



Klein found that animals fed upon cultures of the bacillus or 

 upon the flesh of animals dead of the disease became ill and died 

 with typical symptoms. Simond has confirmed his results and 

 it is not improbable that the disease is sometimes acquired by 

 rats through feeding upon their companions that have died of it. 

 The micro-organisms seem able to penetrate any of the mucous mem- 

 branes, so that infection usually follows their appHcation to the un- 

 injured conjunctiva, nasal, buccal, vaginal or gastro-intestinal 

 surfaces. 



Cutaneous and Subcutaneous Inoculation. — All susceptible ani- 

 mals quickly become infected if a needle infected with 'a culture 

 of the bacilli or with material from a bubo or other infective lesion 

 be used to puncture or scratch the skin. Wyssokowitsch and 

 Zabolotnyf found monkeys highly susceptible to plague, especially 



* "Journal of Infectious Diseases," 1910, vil, p. 374. 

 t "Centralbl. f. Bakt. u. Parasitenk.," Oct. 12, 1897. 



