It will be remembered that in the nine experiments as to dengue transmission, conducted by Ashburn and Craig, the authors threw out five of the cases for such reasons as previous immunity or refusal of the experimental mosquitoes to bite. Of the four remaining volunteers only one developed dengue. This man, however, had been on duty at the Division Hospital in Manila and the statement is made that he had not been exposed to the disease so far as could be determined. This of course rather militates against the value of this isolated experiment and furthermore the mosquitoes which bit him had fed on the blood of a dengue patient only two nights previously. If this is to be considered as a valid experiment, we must believe that only a short sojourn of the virus in the mosquito is requisite, which is rather at variance with the twelve days for the yellow fever virus and eight days for that of pappataci fever.
In the recent Australian epidemic (1916) experiments failed to show C. fatigans capable of transmitting the disease. Stegomyia, however, gave success in 4 out of 7 cases, the volunteers developing dengue in from six to nine days after being bitten.
Stegomyia mosquitoes are often termed the domesticated ones, since they are observed to breed and pass their lives in the immediate environment of man and further to be distinctly urban, rather than rural, in their distribution. For their breeding places they choose artificial collections of water, such as cisterns, barrels, pails, bottles and cans, in or near dwellings.
These mosquitoes are small in size, silver-striped, vicious feeders and very alert. The female alone bites, blood apparently being necessary for ovulation. It feeds especially during the morning and afternoon hours,—much less commonly at night unless there is a light. The life history of Stegomyia is discussed more fully under yellow fever.
As regards the transmission of the disease by blood filtered through a diatomaceous filter it will be remembered that Ashburn and Craig, by proving this fact, placed the dengue virus in the same category with the filterable viruses of the two diseases just considered. Cleveland found the virus in the washed cells as well as in the serum. The virus maintains its potency for several days outside the body it being present in the blood at periods of from 18 to 90 hours.
Lavinder injected dengue blood from cases in the second to fifth day into rhesus monkeys without noting any variation in their temperature or blood findings.
Graham in Beirut carried out some experiments, one of which would seem almost positively to demonstrate mosquito transmission. He took mosquitoes which had fed on dengue patients, to a village in the mountains where no case of dengue existed. He caused these mosquitoes to feed on two natives of the village and both men became sick with dengue four and five days respectively after being bitten by the mosquitoes. Graham’s claims to have noted piroplasma-like organisms in dengue blood have not been verified and do not receive credence.
The most convincing evidence as to mosquito transmission of dengue is that afforded by the absence of dengue in Port Said during the years 1906 and 1907 notwithstanding the prevalence of the disease in adjacent parts of Egypt. This was attributed to the absence of mosquitoes, these having been destroyed in the fight to make Port Said malaria-free. This campaign was commenced in May, 1906.
Other species of culicine mosquitoes, among which may be noted Stegomyia, have been incriminated. In the Philippines I was convinced that Culex microannulatus might transmit the disease as well as C. fatigans. In one of his experiments Graham claimed to have produced dengue by injecting an emulsion of the salivary glands of a mosquito which had fed on a dengue patient one or two days previously.