

TRANSMISSION OF DISEASES BY INSECTS 261 



>y the bite affords entrance to plague bacilli when the bodies of the 

 insects are crushed or when the infected feces are introduced by the 

 ubbing or scratching of the wound. 



The species of rat-flea most common in the orient is the cosmo- 

 politan " plague flea/' Xenopsylla cheopis. 



In the United States the most common rat flea is Ceratophyllus 

 fasciatus. The common cat and dog flea, Ctenocephalus canis, affects 

 rats as does the human flea, Pulex irritans; and all these species are 

 known to bite man. 



Plague in San Francisco. Plague, long dreaded in American sea- 

 ports, finally entered San Francisco in 1900, killed 114 persons in the, 

 next four years, became dormant and broke forth again, with violence, 

 in 1907. The city, just beginning to recover from the great fire of the 

 year before, was in a frightful sanitary condition and most of the popu- 

 lation, engaged in the work of reconstruction, paid little attention to 

 the deaths from plague and at first gave little aid toward the suppression 

 of the disease. As may be imagined, the campaign against the dis- 

 ease undertaken by the U. S. Public Health and Marine-Hospital 

 Service was carried on in the face of great odds. It was, however, con- 

 ducted most efficiently and successfully under the command of Dr. 

 Rupert Blue (later Surgeon- General), who wisely attacked the disease 

 by attacking the rat population. 



The labor involved in starving out the rats, trapping or poisoning 

 them, and making buildings rat-proof by the use of concrete or sheet 

 iron, was immense; but the undertaking was nevertheless carried to a 

 successful conclusion. More than one. million rats were killed and the 

 disease was checked. 



In California plague affects ground squirrels, which doubtless con- 

 tract the disease from .the rats that use the runways of the squirrels in 

 the fields. 



TRYPANOSOMIASES 



Some of the diseases known as trypanosomiases are among the dead- 

 liest that affect man and other vertebrates, and pathogenic trypano- 

 somes the organisms causing these diseases have received an 

 immense amount of study during recent years. 



Trypanosomes. The organisms under consideration are flagellate 

 protozoans. A typical trypanosome, for example, T. lewisi (Fig. 274) 

 of the rat, is essentially an elongated cell, tapering at each end, serpen- 



