﻿294 STRONG. 



It was found possible to carry. on the series in this way as far as the twenty-sixth 

 passage, when apparently the experiments were discontinued. The rats were all 

 wild, Bombay rats and were not selected with reference to species. No evidence 

 was obtained by these experiments that the virulence of the bacillus used for the 

 passages underwent any alteration nor was any given by those in which cutaneous 

 inoculation of a series of rats was attempted. Notwithstanding the failure of 

 Otto and of the Indian Plague Commission perceptibly to increase the virulence 

 of pest bacilli by repeated inoculation from guinea pig to guinea pig or from 

 rat to rat, Dieudonne 10i states that pest cultures which have become weakly 

 virulent may again be rendered more infectious by repeated passages through 

 such animals as guinea pigs and rats. He also recommends the passage from 

 the pneumonic lung of the infected animal to the healthy lung of the second 

 animal, such as the rat or ape, and so on through a series as a means of increas- 

 ing the virulence of this organism. 



Balzaroff 105 also found that the virulence of a pest strain could be increased 

 if it was passed successively for several generations from the pneumonic lung 

 of an animal, by nasal infection, to the healthy lung of another one. 



An organism which had not the power of killing animals by hypodermic inocu- 

 lation was said to be restored to virulence by inoculating a guinea pig with it 

 in the nostril and upon the death of this animal, inoculating by the same portal 

 of entry a second one with a portion of the spleen of the first. After the third 

 or fourth passage the organism was said again to have become virulent. 



Marsh I0 ° claimed that if the organism was cultivated at certain temperatures 

 in mixtures of carbonic acid gas and ordinary air, the bacillus increased in 

 virulence and retained its vitality for a long time. 



Albrecht and Gohn, 107 and Kolle and Martini 10S also found that by means of 

 repeated animal passages, without growth on artificial media, a strain of the pest 

 bacillus was rendered more virulent for the species of animal employed in the 

 experiment, and also for other species which were markedly susceptible to pest. 

 On the contrary, the transmission of the pest virus from rabbit to rabbit ap- 

 parently resulted in a reduction of the virulence of the organism. 



Kolle and Hetsch im also state that some strains of pest bacilli can be 

 rendered more virulent by animal passage. 



My experiments, which had the object of increasing the virulence of 

 the pest bacillus, have been carried on in guinea pigs and monkeys and 

 both virulent and avirulent strains of the organism have been used. 

 However, no perceptible increase in virulence has been obtained for either 

 rats, guinea pigs or monkeys. The strain "Pest Virulent," as has already 



10i Handb. d. path. Mile. (Kolle and Wassermann) (1903), 2, 51C. 

 105 Ann. d. Vinst. Pasteur (1899), 13, 385.. 



IU0 Eep. of Indian Plague Commission (1898-99), 3, 73; also (1898-99), 5, 

 App. Ill, 480. 



107 Loc. cit. 



108 Deutsche med. Wchnschr. (1902), 28, 1. 



109 Die Exper. Bakt. u. die Infektionskrankh., Berl. and Wien (190G), 214. 



