434 PLAGUE 



plague houses. When animals were placed in plague houses 

 and efficiently protected from fleas they remained healthy ; 

 whereas they acquired the disease when the cages were free 

 to the access of fleas in the neighbourhood. 



The following are some of the experiments which were conducted. A 

 series of six huts were built which only differed in the structure of their 

 roofs. In two the roofs were made of ordinary native tiles in which rats 

 freely lodge ; in two others flat tiles were used in which rats live, but in 

 which they have not such facilities for movement as in the first set, and in 

 the third pair the roof was formed of corrugated iron. Under the roof in 

 each case was placed a wire diaphragm which prevented rats or their 

 droppings having access to the hut, but which would not prevent fleas 

 falling down on to the floor of the hut. The huts were left a sufficient 

 time to become infected with rats, and then on the floor in each case 

 healthy guinea-pigs mixed with guinea-pigs artificially infected with 

 plague were allowed to run about together. In the first two sets of huts 

 to which fleas had access the healthy guinea-pigs contracted plague, 

 while in the third set they remained unaffected, though they were freely 

 liable to contamination by contact with the bodies and excreta of the 

 diseased animals. In the third set of huts no infection took place as 

 long as fleas were excluded, but when accidentally these insects obtained 

 admission, then infection of the uninoculated animals commenced. 

 Other experiments were also performed. In one case healthy guinea-pigs 

 were suspended in a cage two inches above a floor on which infected and 

 flea-infested animals were running about. Infection occurred in the cage, 

 but if the latter were suspended at a distance above the floor higher than 

 a flea could jump, then no infection took place. Again, in a hut in which 

 guinea-pigs had died of plague, and which contained infected fleas, two 

 cages were placed, each containing a monkey. One cage was surrounded 

 by a zone of sticky material broader than the jump of a flea. The 

 monkey in this cage remained unaffected, but the other monkey con- 

 tracted plague. 



Other experiments showed that when plague bacilli were 

 placed on the floors of houses, they died off in a comparatively 

 short period of time. After forty-eight hours it was not found 

 possible to reproduce plague by inoculation with material from 

 floors which had been grossly contaminated with cultures of the 

 bacillus. Afterwards, however, animals placed in such a house 

 might become infected by means of fleas. In all these experi- 

 ments the common rat-flea of India pulex cheopis (Rothschild) 

 was used, but it has been shown that this flea, when a rat is 

 not available, will bite a man. These results are manifestly of 

 great practical importance. They show that direct infection by 

 dust and other material through small lesions of the skin plays, 

 probably, a comparatively small part in the spread of the disease, 

 fleas apparently being in the majority of cases the carriers of in- 

 fection. They also point to important preventive measures, 

 which will no doubt be put to a practical test before long. 



