Dengue fever ** has several epidemiologic and numerous semiotic characteristics in common with influenza, so that in former times the question was raised whether both diseases were not varieties of the same species.
In Paris, even at the time of the outbreak of the influenza pandemic in 1889, an animated discussion arose as to whether the disease were influenza or dengue. As a result of this numerous opinions were given which decided upon the identity or the close relation of both diseases, declaring that dengue was "atropical variety of influenza"-a great mistake, for only in a few symptoms is there any analogy; in well marked cases of both diseases they are throughout very different and do not give any occasion for confusion. Dengue is a specific infectious disease sui generis, which has nothing in common with influenza.
Since the influenza pandemic of 1889 the interest of the European physicians in dengue also has been aroused, so that a comprehensive review of this disease, especially upon the basis of our latest experiences, since it has been brought appreciably nearer to us by the epidemic of the year of 1889, will be opportune. Dengue, epidemi ologically considered, is one of the most interesting infectious diseases; for while it is generally conceded to be contagious in a high degree, communicable through commerce and especially through shipping, yet up to the present time it has been confined almost exclusively to tropical and subtropical regions. The principal area of distribution in which the disease has been observed lies between the tropic of Capricorn and the tropic of Cancer, and the contiguous districts of the temperate zones. Only transitionally has the disease appeared once as far north as 40° (1780, Philadelphia); in 1889, in Constantinople (41° N. Lat.), and perhaps in Varna (48.2° N. Lat.), while it hardly ever appears to have overstepped the equator in a southern direction further than the tropic of Capricorn (southern point, South Paolo in Brazil, 23.2° S. Lat.).
*It should be remembered, in reference to the remarks on the etiology of dengue in this article, that it was written before the publication of Ross's researches on malaria and the discoveries which have arisen therefrom.-Ed.
** The present description of dengue differs considerably from those usually found in the text books up to the present time. It makes a claim not only to have made use of the important and critical experiences of the latest epidemic, which occurred partly in Europe, in 1889, but also to give a complete clinical epidemiologic picture of the affection according to our present knowledge.
The remarkable progress which the pestilence made in the unusually hot summer of 1889 (Constantinople, Salonica) shows that the coast of the European Mediterranean is accessible for the entrance of dengue fever under favorable conditions. But when, on the strength of this fact, von During says, "the assumption that climate acts as a barrier to dengue is no longer tenable," we cannot agree with him. Dengue now, as before, still remains a disease of the tropical zones and the adjoining temperate areas, and only exceptionally has it extended further and reached Philadelphia in 1780, Cadiz and Seville in 1784, Constantinople and Varna in 1889.
These facts, like many others in geographic pathology, are of great general significance and should teach modesty to the extreme contagionists. I refer to those who think the course of epidemics can be explained by the increase of the germ in the body of the patient, their excretion, so to speak, by the patient and by the direct contagion from person to person-viz., by an exclusively endogenous hypothesis, while ignoring the exogenous temporal and localizing factors or depreciating their value. The last factors, as we shall see later, play an important role in dengue fever.