PATHS AND MODE OF INFECTION 497 



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 com- 

 menced. 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 contracted 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 ex- 

 periments the common rat-flea of India Pulex cheopis (Roths- 

 child) was used, but it has been shown that this flea, when a 

 rat is not available, will bite a man. Recent observations show 

 that not only is plague transferable by means of fleas, but that 

 this is practically the only method obtaining in natural condi- 

 tions, with the exception that rats may become infected by eating 

 the carcases of other animals containing large numbers of 

 plague bacilli. It is improbable from the experiments made 

 that plague is transmitted by direct contact even when of a 

 close nature; in fact, it has been shown that plague-infected 

 guinea-pigs may suckle their young without the latter acquiring 

 the disease. The general results show that in the human 

 subject 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 

 nearly all cases the carriers of infection. 



The more recent work of the Committee has supplied in- 

 formation of the highest value with regard to the epidemiology 



3 2 



