396 A MANUAL OF BACTERIOLOGY 



the practical disinfection of native houses a 1 : 250 solution 

 of sulphuric acid may be employed. A temperature of 

 65 C. kills the organism in about fifteen minutes. Desic- 

 cation 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, ground 

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

 pig and rabbit are also susceptible to inoculation. The 

 horse, cattle, sheep and goat are relatively insusceptible, 

 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 con- 

 firmed by other workers). Birds are not easily susceptible, 

 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 simple prick in the thigh with an infected 

 needle being sufficient to induce the disease. 



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, 



1 Report on the Plague in Hong Kong. 



