﻿128 ASHBURN AND CRAIG. 



small in cultures that it passes through a Berkefeld filter. While this 

 may prove to be true as regards the dengue organism, we feel justified in 

 stating, that, so far as present evidence goes, the organism causing dengue 

 is ultramicroscopic in size. This conclusion explains the uniformly 

 negative results obtained by nearly every trained observer in the search for 

 a dengue parasite. 



We conclude that an organism is present in the filtrate, rather than 

 a toxin, because of the length of the period intervening between inocula- 

 tion and the appearance of clinical symptoms, and also because we have 

 reproduced the disease by inoculation of the blood of experimental cases. 



There is one point of interest deserving of special consideration in 

 these two cases of dengue produced by filtered blood ; that is, the relatively 

 greater severity of the symptoms. In both these cases the symptoms 

 were more intense in almost every particular than in any of our exper- 

 imental cases, despite the fact that no greater amount of blood was 

 inoculated in these cases. This fact is very difficult of explanation, and 

 we must confess to our ignorance of the cause. It may be that the 

 admixture with salt solution or the time consumed in filtration, or both, 

 acts in some way to increase the virulence of the organism, or that condi- 

 tions favorable to its extra-corporeal development are present during the 

 process of preparing the filtrate which result in a more virulent form of 

 the organism, though we have no evidence to offer in this respect. 



6. EXPERIMENTAL TRANSMISSION OF DENGUE BT THE MOSQUITO. 



We have already mentioned the experiments of Graham (21), regarding 

 the transmission of dengue by the mosquito, in which he seems to have 

 proven conclusively that such transmission occurs; we have also noted 

 the negative results obtained by Carpenter and Sutton (22), Guiteras and 

 Cartaya (7), and Agramonte (19), all of whom believe, however, that the 

 mosquito is the active agent in the spread of the disease. To one who 

 carefully studies the epidemiology of dengue, the conclusion is almost 

 inevitable that this disease, which so closely resembles yellow fever and 

 malaria in this respect, must also be transmitted by some species of 

 mosquito. Its seasonal prevalence; its occurrence most frequently along 

 low-lying, moist, coast regions and in the valleys of rivers; its rapid 

 diffusion in certain localities, and its lack of diffusion in others; its 

 relation to changes in temperature and moisture; its manner of spread 

 from building to building in infected places ; the presence of multitudes 

 of mosquitoes wherever dengue occurs, and the absence of the disease in 

 regions where mosquitoes are few in number or absent, and the cessa- 

 tion of the epidemic in badly infected districts when conditions arise 

 unfavorable to the propagation of mosquitoes, all point to some species 

 of this insect as the transmitting agent. 



Accordingly, having demonstrated by the intravenous inoculation of 

 unfiltered dengue blood that the cause of the disease is present in the 



