ETIOLOGY OF YELLOW FEVER 453 



must now be considered merely as an organism which may 

 occur in the organs and tissues in yellow fever as a secondary 

 infection, but without any etiological significance. 



The facts of importance which have been established 

 regarding the etiology of the disease are due to the labours of 

 the United States Army Commission, which began its work in 

 1900. The members of the Commission first directed their 

 inquiries towards determining whether the bacillus icteroides 

 was present in the blood during life, and a series of cases were 

 investigated bacteriologically, with entirely negative results in 

 each instance. They then resolved to test the hypothesis of 

 Dr. Carlos Finlay of Havana, promulgated several years pre- 

 viously, that the disease was carried by mosquitoes. Selecting 

 mosquitoes which they reared from eggs, they allowed them to 

 bite yellow fever patients and then to bite healthy men. Of 

 several experiments of this nature two were successful in the 

 first instance, the first individual to be infected in this way 

 being Dr. James Carroll, a member of the Commission, who 

 passed through a severe attack of typical yellow fever. Experi- 

 ments were then performed on a larger scale, with completely 

 confirmatory results, as to the conveyance of the disease by 

 mosquitoes. Of twelve non-immunes living under circumstances 

 which excluded natural means of infection, ten contracted 

 yellow fever after having been bitten by mosquitoes which had 

 previously bitten yellow fever patients ; happily all of these 

 recovered. Two of the men who were thus infected had been 

 previously exposed to contact with fomites from yellow fever 

 patients without .results. These results were confirmed by 

 Guiteras, whose investigations were carried out along similar 

 lines; of seventeen individuals bitten by infected mosquitoes, 

 eight took yellow fever, and three of these died. 



The species of mosquito used by the American Commission 

 was the Stegomyia fasciata, and up to the present time no other 

 species has been found capable of carrying the infection. It has 

 also been determined that a certain period must elapse after the 

 insect has bitten a yellow fever patient before it becomes infec- 

 tive to another subject. In summer weather this period is about 

 twelve days ; at a lower temperature somewhat longer. This 

 probably means that, as in the case of malaria, the parasite must 

 pass through certain stages of development before it reaches the 

 salivary gland and is thus in a position to be transferred to a fresh 

 subject. Infected mosquitoes, however, retain the power of 

 infection for a considerable time afterwards, probably as long as 

 sixty days. It has also been shown that mosquitoes may become 



