390 THE ETIOLOGY OF TROPICAL DISEASES 



somewhat limited one, namely, a definite area in Asia known as the 

 " Plague Belt." From Mesopotamia, as a sort of focus, the disease 

 spreads northwards to the Caspian Sea, westwards to the Eed Sea, 

 southwards as far as Central India, and eastwards as far as the 

 China Sea. This constitutes the "belt," but the disease may take 

 an epidemic form, and is readily, though very slowly, conveyed by 

 infection or contagion. It appears to be infectious by means of 

 infective dust, and contagious by prolonged and intimate contact 

 with the plague-stricken. 



Rats and Plague. — Eats have been shown to be the agents 

 for conveying the disease from port to port, and even infecting 

 man. It is probable that rats are not the only agency acting 

 in this way.* Nevertheless, it is true that rats contract the 

 disease more readily than any other animals, and that when 

 suffering from it they may spread the infection. How it is 

 thus spread is not known. Cantlie and Yersin have pointed 

 out that previously to an epidemic of plague rats die in enormous 

 numbers, and Manson has declared that rats supply " the best and 

 most probably the initial opportunity" for the bacillus of plague. 

 " Were I asked," he continues, " how I would protect a state from 

 plague, I would certainly answer, exterminate the rats as a first and 

 most important measure." But, to be effective, this measure must be 

 employed in anticipation of the advent of the disease. " When the rats 

 are tumbling about drunk with plague it is too late." We may quote 

 Sir P. Manson's simile of the position of the rat in epidemic plague. 

 " I would compare a plague-threatened, but as yet not invaded, city," 

 he says, " to a grate in which the fire is laid all ready for lighting. 

 There is the refractory though combustible coal on top, there is the 

 greasy paper and dry, resinous, inflammable wood underneath, and 

 there is the lighted match ready to be employed. Drop the match 

 on the top of the coal ; it flickers for a second and goes out — the 

 coals do not catch fire. But apply it to the paper and sticks under- 

 neath, and in a moment there is a blaze : the sticks are consumed, 

 the coals catch, and in a little while the fire burns merrily. The 

 coals will now burn by themselves, or, if they threaten to go out, 

 another stick or two will quickly revive the fire. In my simile the 

 coals stand for the human inhabitants, the sticks for the rodent 

 inhabitants, and the lighted match for the plague germ that has 

 dodged the quarantine intended to protect that threatened city. No 

 sticks, no fire ; no rats, no plague epidemic." f 



Dr Doriga, the Principal Medical Officer of Health for Venice, 

 has set forth a brief r2sum4 of the chief facts relating to the 



* See Indian Plague Commission Report, 1902, and Report on Plague, at Sidney, 

 1903 (Ashburton Thompson). 



t Bi-it. Med. Jour., 1899, vol. ii., p. 924. 



