INSECTS IN RELATION TO DISEASE 327 



The practical effect of exterminating the mosquito is shown 

 by the decrease of malaria and yellow fever (see Fig. 155). 

 During the various attempts of the French engineers to con- 

 struct the Panama Canal, disease made the completion of the 

 work very nearly impossible. When the United States took 

 over the enterprise, the first step was the establishment of sani- 

 tary conditions, and the largest part of the problem was the ex- 

 termination of mosquitoes through draining and filling in, and 

 the inspection of inhabited regions to prevent the maintenance 

 of breeding-places for the insects. Similar methods have since 

 been used in many parts of the world. The health department 

 of the government of Peru reports that there has been no yellow 

 fever in that country since August, 192 1. 



247. Other disease-bearing insects. Fleas are found to be in- 

 volved in a very serious combination injurious to man. The bubonic 

 plague, which has in past times been the most dreaded of diseases, espe- 

 cially in Asia, was found (in 1894) to be caused by a specific bacillus ; 

 but the mode of infection was not known until quite recently. The 

 Chinese had observed, centuries ago, that there was some connection be- 

 tween the dying of rats in large numbers and the appearance of a plague 

 epidemic. Modern scientists set to work to find out whether the rat 

 plague was in any way related to the human disease, and they found that 

 the same bacillus was the cause of both. Then it was found that the 

 plague spreads from rat to rat not by contact of the animals but through 

 fleas that suck the blood of the sick rats and later bite others, thus trans- 

 ferring the infection. Further study showed that the plague is primarily 

 a disease of rats, and gets into human beings when the fleas abandon 

 dead rats and infect men and women. The plague has spread from the 

 Orient, and cases have appeared at several ports in the United States at 

 various times. The methods of dealing with this danger are directed not 

 toward killing the bacteria but toward killing the rats and fleas. A ship 

 coming from an affected port is thoroughly fumigated to kill the fleas 

 and rats ; special devices are attached to ropes and chains to keep the 

 rats from leaving the vessel or from coming aboard ; and a search is 

 made for hiding-places in which rats may be concealed. In California it 

 was found that the ground squirrels have become infected with the 

 plague bacillus, and systematic patrols had to be established to catch 

 rats and ground squirrels, which are regularly examined for possible 



