42 



temperature was 73 to 78 F., against 14 per cent, successes 

 when the temperature was 82 to 85 F. Transmission 

 experiments were therefore carried out simultaneously at 

 70 and at 85 F., in suitable chambers, both upon rats and 

 guinea-pigs. The results were consistent with previous 

 experience. The possibility that the virulence of the bacillus 

 might undergo some variation if grown at one or other 

 temperature was examined by the Commission and excluded. 

 The claims of flea transmission to be the predominating 

 mechanism of spread from rat to man may be briefly 

 summarised as follows : 



1. The experimental evidence that plague is easily trans- 

 mitted from animal to animal by rat fleas. 



2. That in presence of fleas, the epizootic, if started, 

 varies as regards severity and rate of progress with the 

 number of fleas present and the season of the year, 

 whereas all attempts to induce epizootics in the absence of 

 fleas have failed. 



3. That under natural conditions (experiments in plague 

 houses, &c.) an animal can be protected from infection by 

 any simple procedure which will exclude the visits of fleas. 



4. The only discovered infection in plague houses resides 

 in plague-infected fleas. 



5. The rat fleas A', oheopis and Ceratophyllus fasciatus 

 readily bite man. 



6. The conclusions drawn from animal experiments, when 

 applied to the problem of the spread of plague amongst 

 human beings, afford a reasonable interpretation of every 

 cardinal epidemiological fact. 



The Part taken by the Human Flea. 



It is possible to transmit plague experimentally by means 

 of P. irritans. Nevertheless, the direct transmission of the 

 disease from man to man cannot, at the present time, be of 

 frequent occurrence, or we should have evidence of direct 

 infection instead of dependence upon the epizootic. 



The reason why the human flea is ineffective is because in 

 human cases the average degree of septicasmia before death 

 is so much less than in rats that the chance of a flea 

 imbibing even a single bacillus is small. A variation of the 

 plague bacillus in the direction of greater infectivity, with 

 perhaps diminished toxicity leading to a higher degree of 

 septicaemia in man, would permit of direct transmission by 

 human fleas. Bubonic plague would then be independent 

 of the rat, and spread directly from man to man. For 



