The Plague Fleas 603 



the rats, is now transmitted to men. Human fleas may also transmit the infec- 

 tion from man to man, but the bulk of the transmission probably takes place 

 through rat fleas. 



When the plague spreads from the rat to ground squirrels or to marmots, rare 

 fleas may engage in the transmission of the disease from animal to animal and 

 from man to man, but ordinarily it is the common rat fleas that are responsible 

 for it. 



Both rats and fleas vary in prevalence and in relative frequency in different 

 parts of the world. Thus there are three common rats: Mus decumanus, the 

 brown or sewer rat, Mus rattus, the black or house rat and Mus norvegius, the 

 Norway rat. In Northern Europe, the Mediterranean coast, Egypt and North 

 America, the Norway rat has colonized more or less successfully. Where it 

 preponderates Ceratophyllus fasciatus is a common flea. Where Mus decumanus 

 and Mus rattus alone are found, or are preponderant, Xenopsylla cheopis is the 

 common flea. In the Orient, Xenopsylla cheopis is the chief flea that is to be 

 taken into account in plague transmission. The dog flea Ctenpcephalus canis 

 is common everywhere as is Pulex irritans, the human flea. It is likely that any 

 or all of these engage in plague transmission when once an epidemic has started, 

 but the most active vector of the disease, the world over, and the most important 

 agent in starting human epidemics of plague is Xenopsylla cheopis. 



Much interesting and valuable information concerning the biology, bionomics 

 and relation of rats and fleas to plague, will be found in the "Reports of the 

 India Plague Commission" many of which are to be found in the "Journal of 

 Hygiene," vols, i-xiv; 



The following illustrations and tabulations will enable the student to identify 

 the common genera of fleas. For more intimate systematic study he must be 

 referred to "A Text-book of Medical Entomology," by Patton and Cragg.* 



*" Christian Literature Society of India, London, Madras and Calcutta," 1913. 



