402 A MANUAL OF BACTERIOLOGY 



Thus it is found that if guinea-pigs be placed in a plague- 

 infected compound, many of the animals contract plague ; 

 but if the animals be placed in cages of wire-gauze, the 

 mesh of which is small enough to prevent access of fleas, 

 the animals do not contract plague. The transmission 

 of the disease from rats to man is similarly due to trans- 

 mission by fleas (except in the pneumonic forms in which 

 infection is direct from the sick to the healthy). The great 

 majority of rat fleas are Xenopsylla cheopis, Ceratophyllus 

 fasciatus, Cer. anisus, Ctenopsylla musculi, and Ctenoph- 

 ihalmus agyrtes, of which the first is most prevalent in the 

 tropics and subtropical regions, the second in cooler regions 

 and in this country. 1 Walker 2 has found that bed-bugs 

 may occasionally transmit plague. The bacilli 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. 3 

 The seasonal prevalence of plague coincides with the 

 prevalence of rat-fleas. 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 

 rat virus (see p. 373), is, therefore, one of the means to be 

 adopted in fighting the disease. The extermination of 

 rats seems quite impossible, but by rat destruction there 

 is a likelihood of destroying infected animals and the 

 subsequent development of a healthy race. On the other 

 hand, objection has been taken to rat-destruction, it being 



1 See Chick and Martin, Journ. of Hygiene, vol. xi, 1911, p. 122. 



2 Ind. Med. Gaz., May 1910. 



3 7Bacot and Martin, Journ. of Hygiene, XIII, Plague Supp. Ill 

 914, p. 423. 



