NON-SPORING BACILLI 257 



Milk is not coagulated. Litmus milk shows slight acid 

 formation. Growth on potato and blood serum shows 

 nothing of differential value. 

 In peptone water, no indol is formed. 



Resistance. Readily killed by heat, like all non-sporing 

 forms ; ten minutes at 65 C., or one hour at 58 C. Drying 

 kills them in six to eight days ; artificial drying in four 

 to five hours. May live in pus or sputum for eight to 

 fourteen days. In a moist dark place, may retain 

 viability for months or even years. Freezing has little 

 effect, bacilli surviving a temperature below o C. for 

 forty days. Direct sunlight kills them in four to five hours ; 

 carbolic 1 per cent in two hours, 5 per cent in ten minutes ; 

 perchloride of mercury 1-1000 in ten minutes. 



Pathogenicity. For man, very pathogenic. For animals, 

 very marked for rats, mice, guinea-pigs, rabbits, and 

 monkeys. In rats and guinea-pigs, the mere rubbing of 

 plague bacilli into the skin will often produce the disease, 

 and this fact is made use of in isolating the bacillus from 

 material contaminated with other bacteria. Animal 

 passage increases virulence ; growth on artificial media 

 diminishes it, when prolonged. After subcutaneous injec- 

 tion, a local inflammatory swelling follows ; then a swelling 

 of the corresponding lymphatic glands ; thereafter a 

 general infection. Mice usually die in one to three days, 

 rats and guinea-pigs in two to five days, and rabbits in 

 four to seven days. Post mortem, the chief changes are : 

 the enlarged glands, congestion of the organs (sometimes 

 with haemorrhages), and enlargement of the spleen. The 

 bacilli are numerous in the lymphatic glands, usually in 

 the spleen, and throughout the blood. The blood of a 

 plague-stricken rat may contain as many as 100 million 

 bacilli per c.c. 



Transmission. Actual contact plays a very minor part 

 in the transmission of the disease, as the virus is not given 

 off by the skin. The chief modes of transmission are two 

 in number, by : (1) Inoculation by biting insects (Limond, 

 1899), tne usual insect being the rat flea (Pulex cheopis) ; 

 (2) Inhalation : this is the mode of spread of the pneu- 

 monic form of plague. 



1. By inoculation by biting we get the bubonic plague, 



17 



