The Plague Fleas 561 



that of the preferred host, ovulation follows in about twenty-four hours. The 

 eggs are relatively large, and small numbers are produced. 



In the case of Sarcopsylla penetrans, a flea that has no known interest in con- 

 nection with plague transmission, the female after copulation imbeds itself in 

 the skin of the host and suffers an enormous saccular distension of the abdomen 

 where many ova are produced. Ordinary fleas never imbed themselves but sim- 

 ply bite and suck blood, leaping off of the host when satisfied. 



Epidemics of plague among men are commonly preceded by epizootics of 

 plague among rats. The mortality of the rats being high and their number 

 diminishing, many fleas are unprovided for and seek human hosts upon whom 

 to satisfy their appetites. In this way, the plague which was at first transmitted 

 by the fleas to the rats, is now transmitted to men. Human fleas may also trans- 



Fig. 232. Various fleas, magnified about 30 diameters. The specimens are 

 treated with hot 20 per cent, caustic potash for a few minutes, dehydrated in 

 alcohol, cleared in xylol and mounted in balsam, a, Ctenocephalus canis, d" ; b 

 Ctenocephalus canis, 9 ; c, Ctenocephalus felis, d" ; d, Ctenocephalus f elis, 9 

 (Bacot, in Journal of Hygiene, "Plague Supplement in, 1914"). 



mit the infection 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 Ctenocephalus canis 

 36 



