12 



KNOWLEDGE. 



January, 1911. 



while suffering from it the patients are dehrious, they 

 are with difticuit\- restrained from \\alking ahout, 



and they cough and spit 

 incessantly. Their sputum 

 is full of plague germs, 

 and is highly infectious to 

 anyone on whom it ma\- 

 be discharged. Rats, also, 

 may have plague pneu- 

 monia : and there is one 

 instance at an\- rate in 

 which a bacteriologist is 

 believed to have contracted 

 plague from the sputum 

 of a plague infected rat. 



Pulex fclis 3 . the cat flea. 



The Great Plague of Lon- 



* t 



V 



don took place in winter, and a large numlier of 

 victims no doubt had plague pneumonia ; hence the 

 virulence and rapidity of the contagion. 



What then is the genesis and progress of an 

 outbreak of plague ? Plague, in the 

 first instance, appears to arise in -^ 



certain foci in Asia and Africa, 

 where it always exists. In a 

 locality such as Bombav, an out- 

 break among human beings is pre- 

 ceded b\' an outbreak among rats. 

 and if a curve be drawn showing 

 the rise, culmination and decline of 

 the plague among rats, it is found 

 that a rise, culmination and fall of 

 plague among human beings takes 

 place in a cur\e almost parallel 

 to that of the plague among rats, 

 but occurring about a fortnight later. 

 The reason is quite plain. In the 

 native dwellings of India generalh*. 

 and in the nati\'e villages of the 

 Punjab, man and the rat live together 

 like friends. There are two kinds of rats in India, as 

 there are two kinds of rats in England, though the 

 numerical proportions of the two kinds are usually 

 reversed. In England the commonest rat is the so- 

 called grey rat, .l/^^^ dcctnuiinns. or Hano\-erian rat, 



which usually inhabits 

 sewers, and is a fierce, 

 strong animal, shy of 

 h u m an beings. I n 

 India the commonest 

 rat is the black rat. 

 Mils idttiis, which is not 

 at all shy, and which, 

 being allowed to do so, 

 ' lives on terms of inti- 

 macy with the Indian 

 natix'c. lUit the black 

 rat is found in England, 

 in the Thames and 

 Mersey warehouses; and 

 the grey rat is found in 

 Indian ports. The reason why the black rat spreads 

 plague in India is that it dwells unmolested in the 



Figure 9. 



Micro-photograph of a 3 PiiKx clicnpia. 

 the Indian rat flea. 



mouse 



Figure 10. 



Ctcnopsylla miiscnli 3 , 

 the mouse flea. 



native \'illages. and in the ramshackle nati\-e quarters 



of towns : and that, consequently, if it has 



any epizootic disease which 



can be transmitted to man. 



there is an initial probabilitx 



of its transmission. In short 



the Indian rat is a reservoir of 



plague, which is constanth' 



being tapped b\- the fleas 



which live on it, and which 



carry it on to the human being 



in the rat's neighbourhood. 



On a rat infected with plague 'f^ 



thirty fleas are by no means Figi're .s. 



an excessive number. In one Sarcopsylla iial/nmcca 



,■1 ■ T) 1 T J- I? , the chicken flea. 



nati\-e house mParel.aii Indian 



village, no fewer than three hundred rats were 



trapped. Multiply the nunil)er of fleas h\- the 



number of rats, and the numlier of possible plague 



inoculators thus arrived at will give an indication 

 of the risk which Indian natives 

 run of contracting plague when 

 there is an outbreak ol it among 

 rats. 



It will be seen, therefore, that 

 even if there were an outbreak of 

 plague among the rats of Suffolk, 

 the chances of the development of 

 a corresponding outbreak are not 

 \'ery large. But they are further 

 narrowed In" the kind of fleas 

 which live on rats in this country 

 and in these latitudes. There are 

 six kinds of fleas which have been 

 found on rats. The human flea 

 ( Piilcx inifans) is found on it 

 sometimes, but not very often. The 

 dog flea I Pulex caiiisj is found 

 more often but still seldom. The 

 flea {Ctcnopsylla luiisciili) (see Eigure 10) 



is also found. 



There are. however, three kinds of fleas \\hich are 



found commonly on rats, and the consideration of 



which is more particularh' rele\'ant to the question 



of the contagiousness of 



plague. The first is a rat 



flea. Typlittpsylld iiuisciiH 



(not unlike the mouse 



flea), which is common 



on rats in some parts of 



Europe; Init which will 



not bite luan. The second 



is the most common Euro- 

 pean rat flea, CeratopsyUiis 



fasciafiis (see Figure 11). 



Hut this flea will not 



feed on man. except 



when starving. The last, 



and most important, flea 



is Piilcx cheopis (see 



Eigure 9), the Indian rat flea, which, unlike the 



liuropean rat flea, readily feeds on a number of 



\ 



X... 



Figure 11. 



Ccratopsylliis fasciatiis 3 , 

 the European rat flea. 



