472 A MANUAL OF BACTERIOLOGY 



ture of 65 C. kills the organism in about fifteen minutes. 

 Desiccation over sulphuric acid at 30 C. is also rapidly 

 fatal. 



Vitality and virulence of cultures. Cultures retain their 

 vitality for at least a month. As regards virulence, the 

 organism varies much according to the source from which 

 it is obtained. Under cultivation it gradually loses its 

 virulence unless subcultured in the following manner : 

 The cultures are made every week on surface agar, are 

 placed in the blood-heat incubator for twenty-four hours, 

 and are then removed and kept at room temperature. If 

 inoculated into animals the virulence may be heightened 

 for a particular species by successive passages, but in 

 so doing is diminished for other species. 



Pathogenic action. In addition to man, the following 

 animals are liable to contract plague under natural con- 

 ditions the monkey, cat, rat, mouse, squirrel, hare, 

 ground squirrel, ferret, bandicoot, and marmot. The- 

 guinea-pig and rabbit are also susceptible to inoculation. 

 The horse, cattle, sheep and goat are relatively insus- 

 ceptible, though Simpson stated that calves and poultry 

 may be infected by feeding, and suffer from a chronic form 

 of the disease (this observation of Simpson's has not been 

 confirmed by other workers). Birds are not easily sus- 

 ceptible, and vultures feeding on the corpses of the 

 plague-stricken do not seem to contract the disease. The 

 mouse, rat, and guinea-pig are the animals chiefly used 

 for experimental purposes in the laboratory ; the first 

 two are highly susceptible. 



A guinea-pig inoculated with plague material or with 

 a pure cultivation usually dies in from two to seven days, 

 the symptoms being sluggishness and loss of appetite, 

 sometimes a discharge from the eyes, and towards the 

 end, staring coat and perhaps convulsive and paralytic 

 attacks. The post-mortem appearances are extensive 



