﻿XIII. VIRULENCE OP THE PLAGUE BACILLUS. 



INCREASE OP VIRULENCE OP THE ORGANISM. 



The behavior of the pest bacillus in relation to its virulence is very 

 interesting and in many respects quite unusual, when it is compared 

 with that of the majority of the microorganisms giving rise to our other 

 common infectious diseases. The question obviously is also a very im- 

 portant one from an epidemiological standpoint and therefore will be 

 discussed somewhat at length. 



In 1895, Yersin, Calmette and Borrel 08 maintained that in a series of passages 

 of the pest bacillus through a given species of animal, an increase of virulence 

 ("a fixed virulence") was obtained for this species of animal only, and that by 

 continued passages through a series of animals of one species, its virulence for 

 other species became reduced. Dujardin-Beaumetz, 0J during the present year 

 also speaks of a hyper-virulence of the pest bacillus obtained by passage through 

 guinea pigs and rats. Hankin 10 ° found that the virulence of plague bacilli 

 became reduced by passage through rats and that he was not able to kill more 

 tiian three or four rats in a series, even though the culture in the beginning was 

 fully virulent. On the other hand, he found that the virulence of the organism 

 could be increased by passage from mouse to mouse. 



Walton, 101 also observed that a virulent strain of pest became attenuated by 

 successive passages from rat to rat. He used a suspension of a portion of the 

 spleen of the dead animal for infecting the second one, and so on. He also was 

 not able to infect more than three rats in a series by this method. Otto, 102 from 

 numerous carefully performed experiments, concluded first that in the passage 

 of pest bacilli from guinea pig to guinea pig without growth, on artificial media, 

 there resulted no dimunition in virulence of the organism either for the guinea 

 pig or for the. rabbit, rat or mouse, and second, that it did not appear to be pos- 

 sible by passing the organism successively through guinea pigs to obtain a 

 substantial increase in the virulence of a culture of sufficient pathogenesis at 

 the time of the beginning of the experiment, to cause the death of a guinea pig. 



The Plague Commission in India under the direction of Martin and Lamb 103 

 have also very recently reported upon the effect upon its virulence of the passage 

 of Bacillus pestis through series of rats, when the organism was given by sub- 

 cutaneous inoculation without intermediate culture. Each rat in the series was 

 infected with a suspension of the spleen of the animal which had died previously. 



^ Ann. de I'Inst. Pasteur (1895), 9, 589. 



m Bull. de I'Inst. Pasteur (1906), 4, 473. 

 100 Rept. of the Indian Plague Commission (1899), 2, 22. 

 m Rept. of Indian Plague Commission (1900), 3, 337. 



10 °-Zeit. f. Hyg. u. Infeetionskrankh. Leipz. (1902), 41, 380; (1904), 48, 430. 

 103 Jour, de Hyg. (1906), 6, 496, 502. 



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