# Tropical Diseases/Introduction

INTRODUCTION
THE ETIOLOGY OF TROPICAL DISEASES
The title which I have elected to give to this work, Tropical Diseases, is more convenient than accurate. If by "tropical diseases" be meant diseases peculiar to the tropics, then half a dozen pages might have sufficed for their description; for, at most, only two or three comparatively unimportant diseases strictly deserve that title. If, on the other hand, the expression "tropical diseases" be held to include all diseases occurring in the tropics, then the work would require to cover almost the entire range of medicine; for the diseases of temperate climates are also, and in almost every instance, to be found in tropical climates.
I employ the term "tropical" in a meteorological, rather than in a geographical, sense, meaning by it sustained high atmospheric temperature; and by the term "tropical diseases" I wish to indicate diseases occurring only, or which from one circumstance or another are specially prevalent, in warm climates.
It must not be inferred from this, however, that high atmospheric temperature is the sole and direct cause of the bulk of tropical diseases. The physiological machinery of the human body is so adjusted that great variations of atmospheric temperature can be supported by man with impunity. Indeed, although temperature acts as an important pathogenic factor, it is very rarely that it does so directly. Extreme cold may cause frost-bite; exposure to the sun, sun erythema, sun headache, and symptomatic fever; a hot atmosphere, heat-exhaustion; prolonged residence in hot moist climates, vague, ill-defined conditions of debility; profuse sweating from heat of climate, ​prickly heat. But none of these states can with justice be regarded as disease.
This being so, it is natural to ask : In what way do tropical influences affect disease, as they undoubtedly do ; and why should it be that some diseases are peculiar to tropical climates, or are specially prevalent in such climates?
Speaking generally, the natives of tropical countries are not injuriously affected by the meteorological conditions of the climates they live in, any more than are the inhabitants of more temperate climates; their physiological activities are attuned by heredity and habit to the conditions they were born into. The European, it may be, on his first entering the tropics, and until his machinery has adjusted itself to the altered meteorological circumstances, is liable to slight physiological irregularities, and this more especially if he persist in the dietetic habits appropriate to his native land. A predisposition to certain diseases, and a tendency to degenerative changes, may be brought about in this way; but acute disease, with active tissue change, is not so caused. In the tropics, as in temperate climates, in the European and in the native alike, nearly all disease is of specific origin. It is in their specific causes that the difference between the diseases of temperate climates and those of tropical climates principally lies.
Modern science has clearly shown that nearly all diseases, directly or indirectly, are caused by germs. It must be confessed that although in many instances these germs have been discovered, in other instances they are yet to find ; nevertheless, their existence in the latter may be confidently postulated.
Germs are living organisms, and, like all living things, demand certain physical conditions for their well-being. One of these conditions is a certain temperature ; another is certain media; and a third is "certain opportunities.
In the majority of instances disease germs are true parasites, and therefore, to keep in existence as species, require to pass from host to host. If, during this passage from host to host, the temperature of the ​transmitting medium—be it air, water, or food—be too high or too low for the special requirements of the germ in question, that germ dies and ceases to be infective. In this way may be explained the absence from the tropics of a class of directly infectious diseases represented by scarlet fever, and the possible absence from temperate climates of a similar class of diseases. In the one case, during the short passage from one human being to another, tropical temperature is fatal to the air-borne germ; in the other it may be that the lower temperature of higher latitudes has the same effect.
In another type of disease, of which tropical scaly ringworm (tinea imbricate) is an excellent example, the germ vegetates on the surface of the body, and is thus exposed to the vicissitudes of climate. One of the requirements of the germ referred to is a high atmospheric temperature and a certain degree of moisture. Given these it nourishes; remove these and it dies out, just as a palm tree or a bird of paradise would die on being transferred to a cold climate.
Many diseases require for their transmission from one individual to another the services of a third and wholly different animal. The propagation and continued existence of a disease of this description will depend, therefore, on the presence of the third animal. If the latter be a tropical species, the disease for the transmission of which it is indispensable must necessarily be confined to the tropics. Thus the geographical range of malaria and of filariasis is determined by that of certain species of mosquito which ingest and act as alternative hosts to the respective germs, and, so to speak, prepare them for entrance into their human host. The distribution of a large number of animal parasitic diseases depends in this way on the distribution of these alternative hosts. When this animal happens to be a tropical species, the disease it subtends, so to speak, is, in natural conditions, necessarily tropical also.
Certain diseases are common to man and the lower animals. If these latter happen to be tropical species ​the opportunities for man to contract the common disease are most frequent, or are only found, in the tropics. Such, most probably, are some of the tropical ringworms.
Certain parasites are so organized that before re-entering man they must pass a part of their lives as free organisms in the outer world, where they require a relatively high temperature for their development. Such parasites, therefore, and the diseases they give rise to, must necessarily be tropical or sub-tropical. The Ankylostomum duodenale and ankylostomiasis are an instance in point.
There is a class of intoxication diseases which depend on toxins generated by germs whose habitat is the soil, water, or other external media, and whose germs do not enter the human body as a necessary feature in their life-histories, although their toxins may. The yeast plant 'and its toxin, alcohol, and the disease it causes, alcoholism, are the most familiar example of this. Such, too, are ergotism, atriplicism, and perhaps lathyrism. These germs require certain temperatures and certain media; consequently the diseases they produce have a corresponding geographical range. If one of these conditions be a high temperature, the disease is mainly a tropical one.
Lastly, I can conceive, and believe, that there is another and less directly-acting set of conditions influencing the distribution of disease conditions which as yet have been ignored by epidemiologists, but which, it seems to me, must have an important bearing on this subject. Disease germs, their transmitting agencies, or their intermediate hosts, being living organisms, are, during their extracorporeal phases, necessarily competing organisms, and therefore liable to be preyed upon or otherwise crushed out by other organisms in the struggle for existence. The malaria parasite is absent in many places in which, apparently, all the conditions favourable to its existence are to be found in perfection. Why is it not found there, seeing that it must certainly have been frequently introduced? I would suggest that in some instances this and other disease germs, or the organ​isms subtending them, are kept under by natural enemies which prey on them, just as fishes prey on and keep down water-haunting insects, or as mice do humble-bees. The geographical range of such disease germs, therefore, will depend, not only on the presence of favourable conditions, but, also, on the absence of unfavourable ones. Herein lies a vast field for study, and one which, as yet, has not been touched by epidemiologists.
In these and similar ways the peculiar distribution of tropical diseases is regulated. The more we learn about these diseases the less important in its bearing on their geographical distribution, and as a direct pathogenic agency, becomes the role of temperature per se, and the more important the influence of the tropical fauna.
Whatever may have been the original source of the pathogenic parasites of man, it is certain that many of those which have a wide distribution at the present day were much more restricted originally. The extension of many of them has occurred within historical, and of some even in recent, times. Thus, in the last century, cholera spread over a great part of the world from its reputed home in India. Small-pox and other Old- World diseases have crossed the Atlantic; and some originally American diseases, such as syphilis and the chigger, have appeared in the Old World; measles, whooping-cough, tuberculosis, and leprosy have been introduced into the Pacific Islands. The process of diffusion is still proceeding, assisted, doubtless, by the vastly increased rapidity and frequency of modern travel, and by the breaking down in recent times of social, political, and physical barriers that formerly isolated many communities, some of which had been from time immemorial the sole repositories of particular disease germs. Thus the sleeping sickness of West Africa is passing to East Africa, and thus, most probably, the yellow fever of America will pass to Asia.
There is one factor which undoubtedly has contributed powerfully to delay the diffusion of certain ​tropical diseases—the circumstance that most of them depend on protozoal or some other kind of animal germ requiring for its transmission an animal intermediary. Diseases which depend on bacterial germs, if their special bacterium be introduced, social and sanitary conditions being favourable, will spread in any country or climate, and thus it is that all bacterial diseases, with hardly an exception, are found, or are capable of existing, everywhere; in the passage from host to host their germs are not killed by ordinary atmospheric conditions, and they require no second intermediary. Diseases depending on protozoa or other animal germs, in many though not in all instances, will not establish themselves thus universally, because their germs in the passage from host to host demand, through their intermediaries or otherwise, very special and climatically restricted conditions. Tropical diseases belong for the most part to this category, and therefore their successful introduction and spread to new ground are attended with more difficulty than bacterial diseases, demanding, as the former generally do, the double condition of the successful introduction not only of the germ itself but also of the intermediary.
Although this double necessity has undoubtedly operated powerfully against the spread of certain tropical diseases, there is reason to believe that in time this difficulty will disappear; for, so far as we know, there is no reason why, if introduced into new places, these animal intermediaries should not obtain a permanent footing and spread.
There are many instances of exotic insects, for example, which have established themselves after either accidental or intentional introduction into new countries. There is no reason, therefore, for thinking that disease -germ insect intermediaries could not be similarly established in countries in which they are unknown at present. Thus, if the tse-tse flies were successfully introduced into India, sleeping sickness might appear there in due course; or, if appropriate anopheles were introduced into ​many at present malaria-free and salubrious Pacific islands, malaria would become established there. And thus, though certain tropical diseases have at present a limited range, there is great probability, unless measures are speedily set on foot to prevent such a calamity, that the swift and increasing intercourse of modern times, by facilitating the intentional or accidental introduction of their subserving intermediaries, will ere long enable them to extend their present geographical range.
It is evident from what has been advanced that the student of medicine must be a naturalist before he can hope to become a scientific epidemiologist, or pathologist, or a capable practitioner. The necessity for this in all departments of medicine is yearly becoming more apparent, but especially so in that section of medicine which relates to tropical disease. This is further accentuated if we reflect that, although we do know something about a few of the tropical diseases and their germs, there must be many more tropical diseases and tropical disease germs about which we know absolutely nothing. Who can doubt that just as the fauna and flora of the tropical world are infinitely richer in species than those of colder climates, so there is a corresponding distribution in the wealth and poverty of pathogenic organisms; and that many, if not most, of the tropical diseases have yet to be differentiated? The discoveries of the last few years show this. Opportunities and appliances for original pathological study are, from circumstances, too often wanting to the tropical practitioner; but in this matter of the etiology of disease he certainly enjoys opportunities for original research and discovery far superior in novelty and interest to those at the command of his fellow-inquirer in the well-worked field of European and American research.
In the following pages I have included certain cosmopolitan diseases, such as leprosy, plague, and beri-beri, diseases which, properly speaking, do not depend in any very special way, or necessarily, on climatic conditions. They have been practically ousted from ​Europe and the temperate parts of America by the spread of civilization and the improved hygiene that has followed in its train. They are now virtually confined to tropical and sub-tropical countries, where they still survive under those backward social and sanitary conditions which are necessary for their successful propagation, and which are more or less an indirect outcome of tropical climate.