﻿134 ASHBURN AND CRAIG. 



As we have stated, three of our subjects were absolutely immune to 

 dengue. Our assumption, that failure to develop the disease after 

 inoculation with 20 minims of blood from a dengue case constitutes 

 absolute immunity, is arbitrary, but seems justified by the constancy and 

 severity of the symptoms produced in the successful cases. 



Three of the men possibly showed a relative immunity; that is, the 

 amount of virus transferred to them by mosquitoes was not sufficient 

 to cause the disease, although the intravenous injection of 20 minims of 

 dengue blood was sufficient to do so. Possibly this relative immunity 

 was only apparent, because we know that these men were not severely 

 bitten by the mosquitoes, and we do not know that the particular mos- 

 quitoes that did bite them might not have been laboring under some 

 disability that prevented their transmitting the disease. It is note- 

 worthy that two of these cases were very mild, and that the third, 

 while an ordinary one, presented an incubation period longer than the 

 average. 



Six cases, and if we count the doubtful one already described, seven, 

 presented no immunity; that is, they developed dengue following the 

 first attempt at inoculation. One case, immune to mosquito bites, 

 showed apparent immunity, but developed dengue after the first inoc- 

 ulation. 



Natural immunity and the practice of sleeping under mosquito nets 

 effectually protected a large proportion of healthy men against infection. 

 Thus, in the Fort McKinley epidemic, the highest percentage of infec- 

 tions occurring in any one company was 58, the next highest was 52, 

 and in the other companies it was lower. It must be remembered that 

 in this epidemic no special measures were adopted to prevent the spread 

 of the disease, and the mosquito protection consisted merely of the 

 ordinary routine use of nets during the sleeping hours. 



IMMUNITY AS SHOWN BY EXPEEIMENTS. 



The following cases whose clinical records are here given were proved 

 by experiment to be absolutely immune to dengue. The temperature 

 charts are not reproduced, as they contain no data of interest. 



Case 12. — W. H. 0., first-class private, Hospital Corps, United States Army. 

 This man was on duty at the Division Hospital at the time of experiments, and 

 had never had dengue. 



Experiment 1 : On the night of September 12 the subject slept under a mos- 

 quito net with mosquitoes that had bitten a dengue case the night before. No 

 symptoms of dengue developed. 



Experiment 2: On the night of September 28 the subject was again exposed 

 to mosquitoes that had bitten a dengue patient the night before. He was bitten 

 repeatedly during the next few nights, but no symptoms of dengue developed. 



Experiment 3 : On October 3 the subject was inoculated intravenously with 

 unfiltered dengue blood from Case 44 (Chart E). No symptoms of dengue 

 developed, and the man was returned to duty October 11, 1906. 



