812 PATHOGENIC MICROORGANISMS 



In rats, spontaneous infection with plague is common and plays 

 an important role in the spread of the disease. The pneumonic 

 type of the disease is common in these animals and has been pro- 

 duced in them by inhalation experiments. During every well- 

 observed plague epidemic, marked mortality among the domestic 

 rats has been noticed. 



Although it was formerly supposed that rat infection took place 

 because of the gnawing of dead cadavers by other rats, the work 

 of the British Indian Plague Commission has shown that rats, like 

 man, are spontaneously infected by means of fleas which pass from 

 the infected to the uninfected animal. 



In his work in California McCoy showed that the weasel and 

 chipmunk are susceptible to plague infection, and therefore, poten- 

 tial means of spread if once infected. 



Toxin, Formation. The systemic symptoms of plague are largely 

 due to the absorption of poisonous products of the bacteria. Al- 

 brecht and Ghon, 9 Wernicke, 10 and others were unable to obtain 

 any toxic action with broth-culture filtrates and concluded that the 

 poisons of B. pestis were chiefly endotoxins, firmly attached to the 

 bacterial body. Kossel and Overbeck, 11 however, on the basis, of a 

 careful investigation, came to the conclusion that, in addition to 

 the endotoxin, there is formed in older broth cultures a definite and 

 important true, soluble toxin. 



This, however, is unlikely in the light of a general survey of 

 experimental work and conditions as they exist in the disease itself. 

 It is most likely that the toxic symptoms here are those generally 

 spoken of as endotoxin and also, we believe, perhaps some of the 

 proteose substances suggested by us in connection with other 

 bacteria. 



Immunization. A single attack of plague usually protects human 

 beings from reinfection. A second attack in the same individual is 

 extremely rare. Immunization in animals produces specific agglu- 

 tinating and bacteriolytic substances which are of great importance 

 in the bacteriological diagnosis of the bacillus. The agglutinating 

 action of the serum of patients is clinically important in the diagnosis 

 of the disease, even in dilutions of one in ten, since undiluted normal 

 human serum has no agglutinating effect upon plague bacilli. 



8 Albrecht und Ghon, loc. cit. 



10 Wernicke, Cent. f. Bakt., Ref ., xxiv, 1898. 



11 Kossel und Overbeck, Arb. a. d. Gesundh., xviii, 1901. 



