Mode of Infection 553 



numbers of the plague bacilli and retained them for a number of 

 days. These bugs did not, however, infect healthy animals when 

 allowed to bite them; but Nuttall was not satisfied that the number 

 of his experiments upon this point was great enough to prove that 

 plague cannot be thus spread. Vergbitski,* however, was more 

 successful and a bed-bug that he caused to bite a patient suffering 

 from plague, subsequently transmitted the disease to a rat. It is 

 quite possible that mosquitoes and biting flies may transmit it. 

 As epidemics of human plague are commonly preceded by epi- 

 demics among the rats which die in great numbers, it early became 

 a question whether the plague among them was not caused by the 

 bites of fleas, and whether it might not also be fleas that infected 

 man. 



M. Herzogt has shown that pediculi may harbor plague bacilli 

 and act as carriers of the disease. 



Ogata found plague bacilli in fleas taken from diseased rats. 

 He crushed some fleas between sterile object-glasses and introduced 

 the jviice into the subcutaneous tissues of a mouse, which died 

 in three days with typical plague, a control-animal remaining well. 

 Some guinea-pigs taken for experimental purposes into a plague 

 district died spontaneously of the disease, presumably because of 

 flea infection. 



Galli-ValerioJ and others think that the fleas of the mouse and 

 rat are incapable of living upon man and do not bite him, and 

 that it is only the Pvlex irritans, or human flea, that can transmit 

 the disease from man to man. Tidswell,§ however, found that 

 of 100 fleas collected frorii rats — there were four species, of which 

 three — the most common kinds — bit men as well as rats. Lisbon|| 

 found that of 246 fleas caught on men in the absence of plague, only 

 one was a rat flea, but out of 30 fleas caught upon men in a lodging- 

 house, during plague, 14 were rat fleas. This seems to show that 

 as the rats die off their fleas seek new hosts, and may thus contribute 

 to the spread of the disease. 



That fleas can cause the transmission of plague from animal 

 to animal has been proved by experiments made in India. These 

 experiments, which are pubhshed as "Reports on Plague Investiga- 

 tions in India," issued by the Advisory Committee appointed by 

 the Secretary of State for India, the .Royal Society, and the Lister 

 Institute, appear in the "Journal of Hygiene" from 1906 onward.** 

 It seems from these experiments that human fleas (Pulex irritans) 

 do not bite rats, but that the rat fleas of all kinds do, though not 



* "Jour, of Hygiene," 1904, vni, 185. 



f'Amer. Jour. Med. Sci.," March, 1895. 



t Ibid., XXVII, No. I, p. I, Jan. 6, 1900. 



§ "British Medical Journal," June 27, 1903. 



II "Times of India," Nov. 26, 1904. 



** "Journal of Hygiene," Sept., 1906, vol. vi, p. 421; July, 1907, vol. vii, p. 324; 

 Dec, 1907, vol. VII, p. 693; May, 1908, vol. viii, p. 162; 1909, vol. nc; 1910, vol. 

 X; 191 1, vol. XI. 



