818 PATHOGENIC MICROORGANISMS 



common flea found on rats is the Xenopsylla cheopis. The disease 

 can also be transmitted by Ceratophyllu-s fasciatus and by Puiex 

 irritant. Fleas habitually infesting dogs and cats may also infest 

 rats which means that flea extermination must be general. It also 

 indicates that the climatic and geographical distribution of fleas, 

 as well as that of rats, must be taken into account in dealing with 

 the disease. 



In rats the first development is a generalized blood infection 

 during which enormous numbers of bacilli may be present in the 

 blood. These are then taken into the intestine of the flea where 

 they can live for a long time, and may be deposted upon the skin 

 of the victim during feeding, since the flea is apt to regurgitate 

 blood and to deposit feces at this time. It may also be that some 

 of the bacilli are directly introduced with the bite, but it is probably 

 more common that the organisms thus distributed will be rubbed in 

 either by the clothing or iri scratching the fleabite. 



It is thus established with considerable certainty that while 

 contact infection and other means of direct and indirect transmis- 

 sion may, of course, occur, the usual manner of spread of plague 

 is from rat to rat, rat to man, or man to man, by the agency of fleas. 

 It is the Epimys rattus which lives in closest relationship to man, 

 and is perhaps the most dangerous for this reason. The ordinary 

 rat flea leaves the body of the rat within about three days of its 

 death and is capable of remaining alive about three or four weeks. 

 The plague bacilli may multiply tremendously in the intestine of 

 the flea during the period between feedings. In the California 

 outbreak infection from ground squirrels to man was definitely 

 shown in a number of cases, and in Manchuria the tarbagan men- 

 tioned above has also been suspected of being the direct source. 



McCoy 22 in 1921, summarizing the results of recent plague studies, 

 states that in the United States natural infection has taken place 

 among ground squirrels of California, the black rats of Hawaii, 

 and a species of wood rat and field rodent in Louisiana. Human 

 cases have been unquestionably traced to ground squirrels, and 

 almost always, he says, have the peculiarity of showing the primary 

 bubos in the axillae, because the fleas in the course of the infection, 

 attack the upper extremities, whereas when the disease is contracted 

 from rats, the fleas are more apt to bite on the legs. Squirrel 



22 McCoy, Amer. Jour. Hyg., March, 1921. 



