478 A MANUAL OF BACTERIOLOGY 



multiply in some of the fleas to such an extent as to occlude 

 the entrance to the stomach. Such fleas will still bite, but 

 on ceasing to suck, some of the blood with numerous 

 bacilli in it regurgitates into the wound and thus infects. 1 

 The seasonal prevalence of plague coincides with the 

 prevalence of rat-fleas. Humidity favours the longevity 

 of the rat-flea, and the decline of plague epidemics in 

 Northern India with the onset of the hot weather is 

 probably correlated with this factor. The manner in 

 which the periods in the year when human plague does 

 not occur are bridged over is unknown. In such periods 

 rats suffering from plague have been found, but these 

 are regarded as having a retrogressive form of the disease 

 rather than a chronic infection. The destruction of 

 rats, either by trapping, poisoning, or asphyxiating, or 

 by the use of the Danysz or other rat virus (see p. 443), 

 is, therefore, one of the means to be adopted in fighting 

 the disease. The extermination of rats seems quite impos- 

 sible, but by rat destruction there is a likelihood of 

 destroying infected animals and the subsequent develop- 

 ment of a healthy race. On the other hand, objection 

 has been taken to rat-destruction, it being surmised that 

 if the epizootic be allowed to proceed, the susceptible 

 rats will be exterminated and a race of rats relatively 

 insusceptible to plague will ultimately be established. 

 It is still more important to prevent access of rats by 

 rendering buildings rat-proof. 



On Plague, see Simpson, Treatise on Plague (Cambridge Univer- 

 sity Press) ; Klein, Bacteriology of Oriental Plague ; " Eeports on 

 Plague Investigations in India," Journ. of Hygiene (extra numbers), 

 vols. vi et seq. 



Clinical Examination 



If it cannot be examined immediately, plague material may be 

 placed in a solution containing glycerin 20 c.c., distilled water 



1 Bacot and Martin, Journ. of Hygiene, xiii, Plague Supp. iii, 1914, p. 423. 



