The following conclusions appear to us to be justified as a result of the experiments cited above:—
(1) Close contact of plague-infected animals with healthy animals, if fleas are excluded, does not give rise to an epizootic among the latter. As the go-downs were never cleaned out, close contact includes contact with faeces and urine of infected animals, and contact with, and eating of food contaminated with, faeces and urine of infected animals, as well as with pus from open plague ulcers. This conclusion is justified from Experiments I and II and especially from Experiment VI, in which close contact with infected animals was maintained for nearly a month. In Experiment I the animals were inoculated cutaneously, and in many cases had developed ulcers at the site of inoculation.
(2) Close contact of young, even when suckled by plague-infected mothers, did not give the disease to the former.
(3) If fleas are present, then the epizootic, once started, spreads from animal to animal, the rate of progress being in direct proportion to the number of fleas present. Thus, in the experiments during the months of December, January and February, the epizootic was very rapid in those go-downs, namely, Nos. 1 and 2, Experiments IV and V, in which the flea population was abundant and was kept up by a natural supply from the roof: it was much slower in Experiment VII, go-down No. 5, in which the flea supply was kept up artificially; and, finally, it was slowest of all in go-down No. 6, Experiment VI, in which there was no definite natural supply of fleas, and from which the fleas were daily removed for a period of six days, after which removal only a comparatively small number could be caught.
(4) An epizootic of plague may start without direct contact of healthy animal and infected animal. Thus, in the case of Experiment V in go-down No. 1 the healthy guinea-pigs were not put in until the last inoculated guinea-pig had died and been removed.
(5) We have in Part II shown by direct experiment that the rat flea can convey plague from rat to rat. Further experiments (Experiments VI, VII and XI) of a similar nature with the fleas removed from infected go-downs are now recorded.
(6) Infection can take place without any contact with contaminated soil. Thus in go-downs Nos. 1 and 2 (Experiments VIII, IX), guinea-pigs placed in wire cages two inches above the ground developed plague; also the monkey in go-down No. 2 (Experiment XIII) was never in contact with the ground.
(7) Aerial infection is excluded. Thus, guinea-pigs suspended in a cage two feet above the ground did not contract the disease, while in the same go-down those animals allowed to run about and those placed two inches above the floor became infected. Further, the monkey surrounded by tangle-foot was exposed as much to aerial infection as the control animal which contracted the disease.
