Diagnosis 593 



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

 him, and that it is only the Pulex irritans, or human flea, 

 that can transmit the disease from man to man. Tidswell,* 

 however, found that of 100 fleas collected from rats, there 

 were four species, of which three the most common kinds 

 bit men as well as rats. Lisbon f 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 published as "Reports on Plague 

 Investigations 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 1 906 onward, t 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 willingly, bite men. 

 By placing guinea-pigs in cages upon the floor of infected 

 houses, the fleas of all kinds quickly attack them with result- 

 ing infection, but if the guinea-pigs are kept in flea-proof 

 cages, or if the cages are surrounded by "tangle-foot," or 

 "sticky fly-paper," the fleas, not being able to spring over the 

 barrier, are caught on the sticky surfaces and do not reach 

 the guinea-pigs, which then remain uninfected. What is 

 true of the guinea-pigs, is undoubtedly true of the rats; the 

 disease is transmitted from rat to rat by the fleas (Ceratophyl- 

 lus fasciatus, Ctenopsylla musculi, and Pulex cheopis, which 

 appears to be most common). 



M. Herzog has shown that pediculi may harbor plague 

 bacilli and act as carriers of the disease. 



The plague bacilli can also be transmitted from place to 

 place by fomites, from which they may reach man or rats. 



Diagnosis. It seems possible to make a diagnosis of the 

 disease in doubtful cases by examining the blood, but it is 



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



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



t "Journal of Hygiene," Sept., 1906, vol. vi, p. 421; July, 1907, vol. 

 vn, p. 324; Dec., 1907, vol. vn, p. 693; May, 1908, vol. vin, p. 162; 

 1909, vol. ix; 1910, vol. x; 1911, vol. xi. 



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

 38 



