PATHS AND MODE OF INFECTION 483 



in the majority of cases to take place by means of the bites 

 of fleas. For some time it had been known that plague bacilli 

 might be found for some time afterwards in the stomach of fleas 

 allowed to feed on animals suffering from plague, and some 

 observers, for example Simond, had succeeded in transmitting 

 the disease to other animals by means of the infected insects. 

 Most observers, however, had obtained negative results, and it 

 was only by the work of the Advisory Committee referred to 

 above, 1 that the importance of this means of infection was estab- 

 lished. By carefully planned experiments, the Committee 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 produced by fleas 

 removed from plague rats and 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, comparatively 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 susceptible 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 



1 See Journal of Hygiene, vi. 421 ; vii. 323. 



