EPIDEMIOLOGY OF PLAGUE 477 



numbers in their carcases. In the various epidemics at 

 Sydney, cases of plague first occurred among the rats and 

 mice, followed after an interval of days or weeks by human 

 cases. Other animals may occasionally be the means 

 of disseminating the disease, e.g. the ground squirrel 

 in California and the marmot in Manchuria. The experi- 

 ments of the Advisory Committee on Plague Investigation 

 in India have conclusively shown the important part 

 played by rats in the dissemination of the disease, though 

 the origin of the primary infection in rats is obscure. They 

 may possibly become infected from the dust of earthen 

 floors of the native houses soiled with excreta or discharges 

 of plague patients, or from their clothing, poultices or 

 dressings, but the readiest method is probably by feeding 

 on the dead. Once the epizootic has started, further 

 infection is simple ; rats fight, and so may directly 

 inoculate one another ; sick rats may soil grain or other 

 foodstuffs, and dead rats are eaten by their fellows. 

 Moreover, parasitic insects, especially fleas, undoubtedly 

 may transmit the disease from one animal to another. 

 Thus it is found that if guinea-pigs be placed in a plague - 

 infected compound, many of the animals contract plague ; 

 but if the animals be placed in cages of wire-gauze, the 

 mesh of which is small enough to prevent access of fleas, 

 the animals do not contract plague. The transmission 

 of the bubonic form of the disease from rats to man 

 is similarly due to transmission by fleas. The great 

 majority of rat fleas are Xenopsylla cheopis, Cwaiophyllus 

 fasciatus, Cer. anisus, Ctenopsylla musculi, and Ctenoph- 

 thalmus agyrtes, of which the first is most prevalent in the 

 tropics and subtropical regions, the second in cooler 

 regions and in this country. 1 Walker 2 has found that 

 bed-bugs may occasionally transmit plague. The bacilli 



1 See Chick and Martin, Journ. of Hygiene, vol. xi, 1911, p. 122. 



2 Ind. Med. Gaz., May, 1910. 



