14 



BULLETIN 248, U. S. DEPARTMENT OF AGRICULTURE. 



Fig. 3. — The European rat flea : Larva. 

 (Original.) 



Greatly enlarged. 



together with the favorable conditions as regards rats and fleas in 

 seaports, accounts for the fact that the disease usually first breaks out 

 in such places. The pestilence, when once introduced, is carried 

 rapidly from rat to rat by the fleas. This spread is increased by the 

 fact that most of the fleas leave the rats as they die and pass to others. 

 It is these fleas, set free by the death of their plague-stricken hosts, 

 which form the chief menace to man. 

 The method by which fleas convey plague germs has received con- 

 siderable attention, 

 and various theo- 

 ries have been ad- 

 vanced from time 

 to time in an effort 

 to explain the mech- 

 anism of transmis- 

 sion. It appears 

 that the two most important methods are by contamination 

 of the skin of man or other host by excrement voided by the 

 infected fleas while feeding and the subsequent rubbing or scratch- 

 ing in of the germ-laden material and by the injection of the 

 disease organism into the wound made by the flea at the time of 

 feeding. Kesearches made by Mr. A. W. Bacot, of the Lister Insti- 

 tute, have proven this last method to be an important one. He 

 showed that the entrance to the stomach of some of the fleas becomes 

 plugged by a growth of the plague germs. This ultimately prevents 

 the passage of food backward into 

 the stomach, but does not prevent 

 the flea from sucking up small 

 quantities of blood, some of which 

 is forced back into the wound 

 after becoming laden with the 

 disease organism. 



Close trade communication be- 

 tween the nations of the world 

 gives increased channels for the 

 dissemination of various pests and 

 diseases as well as opportunities for the furtherance of knowledge and 

 the exchange of trade commodities. The colonization of new lands 

 in the Tropics, the opening of a great artery of trade intimately 

 connecting many of our ports with the commerce of the world, the 

 immigration brought about by the present European strife — these, 

 when considered together with the fact that plague is now present 

 in many quarters of the globe, should impress all with the importance 

 of exercising great care to exclude the disease from our shores. 

 Knowing that this pestilence spreads among people only as a result 



'--%, 



Fig. 4. — The European rat flea : Cocoon. 

 Greatly enlarged. Note the particles 

 of sawdust and dirt adhering to the 



surface. (Original.) 



