374 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



quito and in no other way. Since that time many other ex- 

 periments have been made, and it is now definitely known how 

 this disease spreads through an infected region. The virus 

 that causes the fever occurs in the blood plasma of the human 

 host, but the yellow-fever patient is a source of infection for the 

 mosquito only during the first three or four days after the fever 

 manifests itself. The virus must undergo an incubation period 

 of twelve to fourteen days in the mosquito before she is capable 

 of transmitting the disease. After this incubation period the 

 mosquito is infective for the rest of her life. The parasite 



FIG. 164. Yellow-fever mosquito, Stegomyia fasciata. (About eight times 



natural size.) 



that causes the disease has never been seen, probably because 

 it is too small, but it is believed that it is one of the large group 

 of Sporozoan parasites. 



One of the members of the Yellow Fever Commission died of 

 yellow fever during the course of the experiments, and another 

 member contracted the disease but recovered. The applica- 

 tion of the knowledge gained by these studies enabled the 

 officers soon to check the epidemic of yellow fever then existing 

 in Cuba. A few years later when the disease appeared in 

 virulent form in New Orleans it was stamped out in a re- 

 markably short time by waging a ceaseless war against the 

 mosquitoes. The Panama Canal Zone has long been regarded 

 as one of the worst regions in the world for yellow fever and 

 malaria. Since the United States began work on the Canal 

 persistent efforts have been made to control the mosquitoes 

 there. These efforts have been so successful that there has 

 been no yellow fever in the Canal Zone for several years, and 

 cases of malaria are comparatively rare. 



