PATHS AND MODE OF INFECTION 495 



referred to above, 1 that the importance of this means of infection 

 was established. By carefully planned experiments, the Com- 

 mittee showed that the disease could be transmitted from a' 

 plague rat to a healthy rat, kept in adjacent cages, when fleas 

 were present; whereas this did not occur when means were 

 taken to prevent the access of fleas, though the facilities for 

 aerial infection were the same. The disease can also be pro- 

 duced by fleas removed from plague rats apd transferred directly 

 to healthy animals, success having been obtained in fully 50 

 per cent, of experiments of this kind. When plague-infected 

 guinea-pigs are placed amongst healthy guinea-pigs, compara- 

 tively few of the latter acquire the disease when fleas are 

 absent or scanty ; whereas all of them may die of plague when 

 fleas are numerous. This result demonstrates the comparatively 

 small part played by direct contact, even when of a close 

 character. Important results were also obtained with regard to 

 the mode of infection in houses where there had been cases of 

 plague. It was found possible to produce the disease in sus- 

 ceptible animals by means of fleas taken from rats in 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 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, 



1 See Journal of Hygiene, vols, vi.-x. 



