ETIOLOGY OF YELLOW FEVER 681 



called bacillus icteroides, and which he considered to be the 

 cause of yellow fever ; it was probably identical with the bacillus 

 x of Sternberg, but subsequent observations made by others 

 gave conflicting results. The bacillus icteroides, as described 

 by Sanarelli, belongs to the paratyphoid group, possessing lateral 

 flagella growing on gelatin without liquefaction, and fermenting 

 glucose but not lactose. Reed and Carroll found that it was 

 practically identical with the bacillus of swine cholera. It 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 was investigated bacterio- 

 logically, with entirely negative results in each instance. They 

 then resolved to test the hypothesis of Dr. Carlos Finlay of 

 Havana, promulgated several years previously, that the disease 

 was carried by mosquitoes. Selecting mosquitoes which they 

 had 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. . Experiments were then per- 

 formed 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- 



