496 PLAGUE 



Absolute proof of the possibility of infection by the skin is 

 supplied by several cases in which the disease has been acquired 

 at post-mortem examinations; in the majority of these the 

 lesions of the skin surface were of trifling nature, and there 

 was no local reaction at the site of inoculation. It may now, 

 however, be regarded as established that in the vast majority 

 of cases infection takes place by means of the bites of fleas. 

 It had previously been shown that when fleas were allowed 

 to feed on animals suffering from plague, plague bacilli 

 might be found for some time afterwards in the stomach, and 

 some observers, for example Simond, had succeeded in trans- 

 mitting 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 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 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, 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 



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



