﻿ETIOLOGY OF DENGUE FEVER. 129 



blood of the infected individual, and that the parasite is probably 

 ultramicroscopic in size, as proven by the positive results of our experi- 

 ments with the filtered blood, we turned our attention to the problem 

 of mosquito transmission. Unfortunately for the fullest success of 

 our work in this direction, we were forced because of lack of other 

 volunteers, to use a number of men who had already passed unharmed 

 through the epidemic at Fort McKinley and the majority of whom were 

 immune, as is proven by the negative result of intravenous inoculation of 

 dengue blood. Thus, of the nine men in whom we endeavored to produce 

 dengue by exposing them to the bites of infected mosquitoes, three were 

 proven in this way to be absolutely immune, one may have had a slight 

 attack of dengue previous to exposure, while three probably possessed 

 a relative immunity, for while they developed dengue from the inocula- 

 tion of a comparatively large amount of dengue blood, the symptoms were 

 mild in character, and in one case the incubation period was greatly 

 prolonged. In one instance already described (see Case 4) no immunity 

 existed to the disease, but the mosquitoes refused to bite the man under 

 any conditions we could devise. 



The mosquito used. — In looking over the geographical distribution of 

 dengue and various species of mosquitoes, we found but one species 

 (Culex fatigans Wied.,) of this insect that apparently occurred wherever 

 dengue did. We do not wish to be understood as stating conclusively that 

 this mosquito is the only one which may be present in all dengue- 

 infected regions, but only that, so far as we have been able to determine 

 from the literature available, this species is constantly found and is 

 mentioned by almost every recent investigator as being very numerous 

 during epidemics of this disease. In Theobald's monograph the map 

 illustrating the known distribution of Culex fatigans Wied. might al- 

 most be used to illustrate the distribution of dengue fever, and if to 

 this map be added the regions in which this mosquito has been demon- 

 strated since it was published, the association of dengue and Culex 

 fatigans Wied., is still more striking. 



For this reason, and because this mosquito was employed by Graham 

 in his experiments, we decided to work with this species at first, and in 

 the event of our results being negative, to extend our work to ernbrace 

 other species. 



We have used mosquitoes reared in captivity, and also those caught in 

 natural surroundings. However, in our successful case produced by 

 the mosquito, we used mosquitoes reared by us from the egg, and thus 

 we are sure that ho infection occurred in these insects before they bit 

 the dengue patients. 



Our mosquito experiments were conducted as follows : The patient 

 suffering from dengue was placed in a bed beneath a mosquito net in 

 a mosquito-proof tent. At night from twenty to thirty mosquitoes were 

 liberated beneath the mosquito bar and collected in the morning; almost 



