﻿ETIOLOGY OF DENGUE FEVEE. 131 



Remarks. — This ease was in every way typical of a moderately severe attack of 

 dengue. The symptoms were fhose seen in the great majority of naturally 

 acquired infections and the temperature chart is a very characteristic one. This 

 man had not been exposed in our dengue camp before being bitten by the mosqui- 

 toes, and did not leave the mosquito-proof camp iintil after the onset of the disease. 



For reasons which have been stated, of the nine men exposed to the 

 bites of infected mosquitoes, only four can be considered in estimating 

 the results obtained. Of these, one, or 25 per cent, developed a typical 

 attack of dengue following the bites of infected mosquitoes ; but we do not 

 consider that the three negative cases are of much value, as the conditions 

 were such as to cause some doubt as to whether the men were bitten. 



It is obvious that many factors have to be considered in considering 

 mosquito experiments, and it is more than probable that in our negative 

 experiments we were unsuccessful in reproducing the favorable conditions 

 which must have been present in Experiment No. 11, or the mosquitoes, 

 if they became infected, may have perished before biting again. Schau- 

 dinn has recently called attention to some of the difficulties which may 

 be met with in attempting the experimental transmission of a disease by 

 mosquitoes. Thus, as he has shown, certain individuals of a species which 

 has been proven to transmit a certain disease are not able to transmit it, 

 and this may be due to the insect itself suffering from some other infec- 

 tion, to an inability to digest the ingested blood, to an acquired or 

 natural immunity resulting in the death of the specific parasite, or the 

 mosquito may die before it has bitten again. 



It is evident from the result of Experiment No. 11 that the parasite 

 causing dengue does not undergo any cycle of development within the 

 mosquito, unless it be a very short one; we are, therefore, of the belief 

 that the parasite of dengue is one capable of living in the stomach of the 

 mosquito for an unknown period of time, where it retains its virulence; 

 that infection may occur at any time after the insect has ingested blood 

 containing the parasite, and that it is introduced into man when the 

 insect bites, being regurgitated through the oesophagus and proboscis with 

 the fluid from the stomach. This theory is borne out by the results 

 recently obtained by the Indian Plague Commission in its remarkable 

 study of the transmission of plague from rat to rat by the flea, Pulex 

 cheopis Eothsch., and by the excessive rapidity of the diffusion of dengue, 

 which would be impossible were the parasite one which underwent a 

 prolonged -cycle of development in the tissues of the mosquito. We have 

 dissected and examined a large number of mosquitoes that had bitten 

 dengue patients, but have never found any organism either in the stomach 

 or tissues suggestive of a stage in the life cycle of a protozobn. We can 

 not confirm Graham's results in this respect, and we believe that in the 

 mosquito as well as in the blood of man, the dengue parasite is ultra- 

 microscopic in size. 



