﻿ETIOLOGY OF DENGUE FEVER. 133 



8. IMMUNITY AND SUSCEPTIBILITY. 



There is considerable confusion existing in regard to these points, the 

 general trend of opinion being that almost everybody is susceptible, and. 

 that an attack of dengue produces immunity for a short time only. As 

 to the latter point — that is, the duration of acquired immunity — -we can 

 not express a very positive opinion, as we endeavored, except in one case, 

 in our experiments to avoid the use of men who had previously had 

 dengue. In the one case noted as an exception, dengue was induced 

 although the patieut said he had experienced three attacks, the last one 

 two and a half years ago. We have also known a few other cases in which 

 the disease developed naturally after a like period. The correctness of 

 reports of cases in which attacks have occurred a month apart we very 

 much doubt. We had about six patients sent back to us after such pe- 

 riods supposed to be suffering from second attacks, but in no case was 

 it so. The "second attack" was usually a malarial paroxysm. 



As to natural immunity, we know that it occurs, or at all events that 

 it may be temporarily present. We think it altogether probable that 

 it may be relative; that is, a small dose of virus may not be sufficient to 

 overcome it, but a large one may. In one of our cases (Case 8) we were 

 unable to decide positively whether an immunity which was present at the 

 time of the discharge of the patient was natural or was acquired from a 

 very light attack of the disease following inoculation with a half minim 

 of blood, though we incline to the latter belief. In at least one instance 

 immunity was apparent and not real ; that is, the patient did not develop 

 dengue when exposed to mosquitoes that had recently bitten other dengue 

 cases, but this was due to the fact that the subject was immune to mos- 

 quito bites. All the mosquitoes put in his net died after periods varying 

 from one to five or six days, and not one of them bit him. Later, when he 

 developed dengue from the intravenous injection of blood, mosquitoes bit 

 him freely. Fortunately, this characteristic so valuable in the Tropics, 

 was not permanently lost, for the patient now states that he is as free 

 from mosquito bites as before. 



Our knowledge as to natural immunity cost us rather dear, as we were 

 paying all our subjects of experiment, and did not relish exhausting the 

 funds at our disposal in payments to men not capable of developing the 

 disease. In the light of subsequent events we think that we made a 

 mistake in accepting volunteers from Fort McKinley, where an epidemic 

 of dengue had been and was prevailing ; because, while we did not begin 

 experiments upon the men until they had been under our observation and 

 free from exposure to dengue for periods varying from a week to three 

 weeks, and thus avoided the error of thinking the disease due to our in- 

 oculations when it was in reality due to other causes, we picked men, 

 some of whom had probably escaped natural infection because of their 

 natural immunity. 



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