Epidemics occur indifferently during the dry or the rainy seasons, the hygrometric condition of the atmosphere being without manifest influence.
Usually a coast disease.— It would appear that dengue, like yellow fever, prefers the coast line and the deltas and valleys of great rivers to the interior of continents. There are many exceptions to this rule; in 1870-73 it spread all over India. The distribution and concentration of population on the seaboard and along rivers, the freedom of communication between communities that are so located, and the mosquito conditions probably determine this preference.
As a rule, elevated places enjoy a relative immunity; if the disease is introduced into such localities, it does not spread. To this, again, there are exceptions, for the Syrian epidemic referred to prevailed in certain spots 4,000 to 5,000 ft. above the sea. In the Mediterranean basin dengue is a disease of autumn months and generally follows the summer outbreak of phlebotomus fever.
The incubation period seems to be somewhat variable. It is certainly not a long one. I have seen a case in which it could not have exceeded twenty-four hours. Some observers place it at five and even seven days; this, I feel sure, is an over-estimate. One to three days seems to be near the truth.* [1]
Symptoms. Initial fever and eruption.— An attack of dengue may be preceded for a few hours by a feeling of malaise, or, perhaps, by painful rheumatic-like twinges in a limb, toe, finger, or joint. Usually it sets in quite suddenly. A patient, describing his experience, said that in the morning he got up feeling quite well, but before he could complete his dressing he was so prostrated by pain and fever that further exertion was impossible, and he had to crawl back to bed again. Similar stories, illustrative of the sudden incidence of the symptoms, circulate during every epidemic of dengue. Sometimes the fever is ushered in by a feeling of chilliness or even by a smart
- ↑ * The observations on which these erroneous conclusions were based were made principally on the crews of ships, and before the role of the mosquito in dengue was suspected.
