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ASHBURN AND CRAIG. 



By reason of lack of suitable volunteers and the subsidence of the 

 epidemic we have been forced to bring our mosquito experiments to a 

 conclusion. We have been unable to investigate man}' interesting ques- 

 tions regarding the transmission of dengue by the mosquito, such as the 

 length of time the insect remains capable of transferring the infection, the 

 most infective period of the disease as regards transmission in this way, 

 and whether transmission is simply mechanical or depends upon the 

 development or multiplication of the parasite within the mosquito ; all of 

 these questions are of great importance to a correct conception of the 

 etiology of dengue and there would appear to be no good reason why, in 

 regions where the disease is common, they should not be thoroughly in- 

 vestigated. We realize that the work we have been able to do as regards 

 mosquito transmission is very incomplete and that much remains to be 

 done before this feature of the etiology of dengue is fully elucidated, but 

 we believe that we have confirmed Graham's results in this respect and 

 that we have proven experimentally that this disease may be transmitted 

 by the mosquito Culex fatigans Wied. We also believe that mosquito 

 transmission is the only natural method which has been proved by experi- 

 ment and that all the epidemiological data confirm such a method of 

 transmission. 



7. EXPERIMENTAL PERIOD OF INCUBATION IN DENGUE. 



As will be seen from a resume of the epidemiology of dengue, the 

 incubation period has been stated as varying from twenty-four hours to 

 ten days, the majority of observers regarding it to be from three to five 

 days. The following table gives the period of incubation in nine of our 

 experimental cases of the diseases : 



No. Of 

 case. 



How produced. 



Inoculation of unfiltered blood . 



do , 



do 



do 



do 



do 



Inoculation of filtered blood 



do 



9 



10 



11 By mosquito . 



Incubation period. 



3 days 18 hours. 

 2 days 19 hours. 

 2 days 18 hours. 



2 days 12 hours. 



4 days 4 hours. 

 7 days 



3 days 11 hours. 

 2 days 12 hours. 

 About 3 days 16 hours. 



From the above table it will be seen that the incubation period of 

 dengue in experimental cases of the disease varied from two and one-half 

 days to seven days, the average being about three days and fourteen hours. 

 This is practically the period of incubation stated as being most frequent 

 by clinicians. We have observed no case in which the incubation period 

 was as short as twenty-four hours, and from our experiments we very 

 much doubt the occurrence of such a short period of incubation. 



