Role of Fleas in the Transmission of Plague 173 



But, however clear the evidence that fleas are the most important 

 agent in the transfer of plague, it is a mistake fraught with danger 

 to assxmie that they are the only factor in the spread of the disease. 

 The causative organism is a bacillus and is not dependent upon any 

 insect for the completion of its development. 



Therefore, any blood-sucking insect which feeds upon a plague 

 infected man or animal and then passes to a healthy individual, 

 conceivably might transfer the bacilli. Verjbitski (1908) has shown 

 experimentally that bed-bugs may thus convey the disease. Hertzog 

 found the bacilli in a head-louse, Pediculus humanus, taken from a 

 child which had died from the plague, and McCoy found them in a 

 louse taken from a plague-infected squirrel. On account of their 

 stationary habits, the latter insects could be of little significance in 

 spreading the disease. 



Contaminated food may also be a source of danger. While this 

 source, formerly supposed to be the principal one, is now regarded as 

 unimportant, there is abimdant experimental evidence to show that 

 it cannot be disregarded. It is believed that infection in this way 

 can occur only when there is some lesion in the alimentary canal. ^ 



StiU more important is the proof that in pneumonic plague the 

 patient is directly infective and that the disease is spread from man 

 to man without any intermediary. Especially conclusive is the 

 evidence obtained by Drs. Strong and Teague during the Manchurian 

 epidemic of 1910-11. They fotmd that during coughing, in pneu- 

 monic plague cases, even when sputum visible to the naked eye is 

 not expelled, plague bacilli in large numbers may become widely 

 disseminated into the surrounding air. By exposing sterile plates 

 before patients who coughed a single time, very numerous colonies 

 of the baccilus were obtained. 



But the great advance which has been made rests on the dis- 

 covery that bubonic plague is in the vast majority of cases transmitted 

 by the flea. The pneumonic type forms a very small percentage 

 of the human cases and even with it, the evidence indicates that the 

 original infection is derived from a rodent through the intermediary 

 of the insect. 



So modem prophylactic measures are directed primarily against 

 the rat and fleas. Ships coming from infected ports are no longer 

 disinfected for the purpose of killing the plague germs, but are fumi- 

 gated to destroy the rats and the fleas which they might harbor. 

 When anchored at infected ports, ships must observe strenuous 



