458 YELLOW FEVER. 



The results of Sanarelli were certainly striking, but they have not been 

 confirmed by recent observations. Of the investigators immediately following 

 Sanarelli, some stated that they found the B. icteroides in a certain proportion 

 of cases, whilst others obtained negative results. 



Reed and Carroll regard this organism as identical with the bacillus of 

 hog cholera; and culturally, it parallels the growth characteristics of B. 

 enteritidis (Gaertner) . 



2. The Mosquito Theory. — The most extensive and carefully 

 planned inquiry into the etiology of yellow fever has been that 

 of the United. States Army Commission (i 900-1), and the result 

 of their labours has been the bringing forward of an entirely 

 new order of facts. In the first place, they failed to find the B. 

 icteroides in the blood of patients suffering from the disease ; in 

 twenty-four cases blood was withdrawn from a vein and cultures 

 made on various media with negative result. Yet they found 

 that a small quantity of such blood (.5-2 c.c), when injected into 

 a healthy subject, was sufficient to produce the disease. Further- 

 more, in three instances they found that the blood serum of a 

 yellow-fever patient, diluted and passed through a Berkefeld 

 filter, still retained its pathogenic properties, but it was found 

 that if the pure or diluted serum, either filtered or unfiltered, 

 were heated for ten minutes at 55° C. and injected in quantities 

 of 1.5 c.c, it was perfectly innocuous. 



From this result, and from the negative result of microscopic 

 examination, they surmised that the virus was not one of the 

 ordinary bacteria and was probably ultra-microscopic in nature.^ 

 The next important conclusion arrived at was that the disease 

 is not communicable by direct contact with those suffering from 

 the disease, with their fomites, etc. In a specially constructed 

 house seven men were exposed to the most intimate contact 

 with the fomites of yellow-fever patients for a period of twenty 

 days each, the soiled garments worn by the patients being in 

 some instances actually slept in by these men; the result was 

 that not one of those thus exposed contracted the disease. By 

 far the most important result of this investigation, however, is 

 the eslablishment of the part played by mosquitoes in the trans- 

 mission of the disease. Of twelve non-immune persons who 



1 In several diseases the existence of such causal factors is suspected. The other 

 examples are foot-and-mouth disease, South African horse-sickness, and the contagious 

 pleuro-pneumonia of cattle. 



