The exclusive limitation in itself of dengue to the tropical regions and their vicinity and the restriction of epidemics to the hot season show that in the occurrence of an epidemic external exogenous phenomena play an important part. Were dengue as contagious as small pox, scarlet fever, measles, and the pandemic influenza, it would long ago, like them, have extended itself over the whole earth and over all the zones.
The local theory of dengue is supported by its limitation to the sea coast, to river banks, and to ships.
The observations that have been made in Cuba, Jamaica, Martinique. Reunion, and almost everywhere that dengue, in spite of active communication between the affected coast places and the interior, was not conveyed to the latter; that inhabitants of the interior frequently contracted dengue at the coast, but on their return home did not spread the disease (in the interior itself epidemics but rarely occur)-led the older observers to an anticontagionistic and strongly local view. They inferred that the occurrence of epidemics (both the endemics as well as those which occur after the importation of the germ) was dependent upon local conditions, and that the disease did not spread itself in an exclusively contagious, but chiefly in a miasmatic, manner, in that the exogenous germs having reached a place and there multiplied, stamped that place as a dengue spot, where any one might contract the disease without coming in contact with a dengue patient.
This same theory applied to yellow fever is the one at present most in favor for that disease. [Thus in 1895-but now, ten years later, transmission of yellow fever by the mosquito is generally accepted.-Ed.]
In favor of this opinion is the fact of sudden multiple infection, and. in possessing this characteristic, dengue, like influenza, differs markedly from other exclusively contagious diseases. It can hardly be denied that this fact is more easily explained by a local miasmatic hypothesis than by the assumption of dissemination of the disease exclusively by contagion from person to person.
Although we must admit that the suddenness of multiple simultaneous infection, as in mfluenza (compare p. 544), is frequently only apparent, because the first cases escape observation, nevertheless we must not- although very customary nowadays-regard this rapid affection of the masses simply as an error of observation. Are we to believe that the excellent observers of earlier dengue epidemics in the West Indies, and especially those in India. Egypt, and Rio de Janeiro, who give evidence of the rapid distribution of the pestilence, with drastic examples, were mistaken and overlooked the gradual development of the pestilence? We have already discussed this question in the article on Influenza, and have explained this rapid affection of the whole population-denied in error- by the supposition that the influenza germs had become aerodromal and had caused simultaneous infection in great numbers among the people. So in dengue a general vehicle (air, water) probably causes the distribution of germs at the site of the epidemic outbreak.
Examples of the behavior of dengue just discussed are found in literature in great numbers. We would refer to those of A. Hirsch,* and may mention, from the excellent reports of Lallemant (Rio de Janeiro), that the crews of ships, as a rule, were suddenly and simultaneously taken ill, and that frequently several laborers on the same job, either unloading or loading, became affected by dengue in the same manner.
In confirmation of the just mentioned "local" hypothesis is the fact that under unfavorable external conditions-e. g., outside of the tropical zone, in Philadelphia and Cadiz-dengue spreads but little. In the first mentioned instance the disease remained confined to a particularly dirty portion of the city (Front Street) before it became further distributed. It therefore obtained the name "Front Street fever." The "local" theory is further supported by the fact that dengue, even in its tropical home, frequently remains for a long time confined to certain areas-for instance, those near the ports or in localities with unfavorable hygienic conditions and in overpopulated portions of the city-before it becomes generally distributed (thus in St. Denis, Reunion, Calcutta, etc.).
In Constantinople, too, the disease first occurred in the dirtiest portions of the city-Kassin-Pacha, the chief focus of all epidemics and particularly of cholera. Mordtmann calls attention to the fact that numerous washerwomen live in this quarter who attend to the laundry of incoming ships. In Hong-Kong the disease prevailed in 1895, principally among the Chinese that were surrounded by unfavorable hygienic conditions. Exceptions to this rule are remarkably rare. Yet in Hong-Kong during the epidemic of 1876 it is alleged that the most favorably situated, sanitary, and new portions of the city were those chiefly affected.
Mordtmann in Constantinople reports that dengue prevailed most intensely in the most filthy quarters; in the summer quarters of the Bosphorus the cases that were imported frequently remained isolated. He calls attention to the analogy with cholera, and very correctly concludes that "for the occurrence of secondary outbreaks after the importation of a primary case certain favorable conditions are required."
The observers of earlier epidemics, from 1827 to about the beginning of the eighties, thought, indeed, that dengue was an "importable" disease, but that it was principally miasmatic-that is, an infectious disease dependent upon time and place. Later, and especially in the great epidemic of 1889, the opinions of strict contagion ists gained the upper hand (Ziilzer, de Brun, von During). Yet there were always objectors, like A. Hirsch and Wernich of earlier times, to the exclusive contagionistic teaching-e. g., Chrysochoos during 1889 in Smyrna, Gaskalis and Orenstein in Greece.
It is very interesting to note the opinions of those investigators who saw epidemics of dengue and influenza occurring in quick succession at the same place, and were thus able to draw comparisons between the manner of distribution of both diseases. These observers are of the unanimous opinion that contagious dengue adheres to the coast and penetrates with difficulty and slowly, or even not at all, under the ordinary conditions of little inland communication into the interior. Influenza, on the other hand, a miasmatic disease, carried by the air, spreads itself with the greatest rapidity equally over city and country, over a whole island or continent, without any difference between coast and interior.*