The importance of swamps in the origin of malaria has been proved many times by the disappearance of the disease on draining them.
Still there are numerous examples of swamps which, on account of their situation in tropical and subtropical countries, seem especially adapted to breed malaria and yet are free from it; for instance, the swamps of New Caledonia, in which work may be undertaken without danger (here even the brackish water swamps are found free from the disease); the pampas of the Rio de la Plata, etc.
The reverse-namely, the occurrence of severe malaria in places having no swamp characteristics-may be found quite as frequently. In the first place, there is the Roman Campagna (Agra Romano), for the characteristics of which we must thank the searching investigations of Colin; again, the Punjab, where, according to Fayrer, the soil is dry, and water lies 50 to 100 feet below the surface; but where numerous sources of water occur, not to mention the irrigation canals, prolific sources of anophelines; finally, the dry, barren plateau of New Castile and the plains of Iran.
Malaria is a disease the origin of which is associated with the soil, and the proposal has been made to designate it as a telluric disease.
It has been frequently confirmed that ships' crews are safe, so long as they remain on board (certain exceptions are mentioned below), even when the vessel lies near such an infected coast that a few hours' sojourn on land is sufficient to contract the disease. Vincent and Burot, for example, reported from the campaign in Madagascar (1895) that while the land troops were decimated by malaria , the marines and sailors on the ships, scarcely 300 meters from shore, remained unaffected throughout months.
The influence of the soil is further evidenced by the well known fact that excavations in malarial districts are exceedingly dangerous, and that even in otherwise non infected localities they are frequently followed by epidemics in the laborers or residents. The building of railroads, canals, fortifications, and streets, the cultivation of virgin soil, and the clearing of woods in malarial regions have cost innumerable human lives. These and other well attested examples have now received their explanation from the occurrence of anopheles (infected) under such conditions.
Experience teaches that the soil is the more insalubrious the less it is cultivated. A striking example of this is the Roman Campagna. In ancient times this was well populated and cultivated, as is proved by numerous monuments and history, but with barbarian invasion and social decline the population disappeared and the ground was neglected.
In the cultivation of the soil we include several factors-namely, the regulation of the ground moisture by canalization, drainage, etc., the growing of plants, which, by using up the water, contributes to the drying of the soil, and finally the number of the population, etc.
The geologic characteristics of the soil come into consideration only in so far as they are connected with ground moisture and surface water-that is, the mineral composition is more or less irrelevant, while the porosity is important.
Malaria thrives least on a soil from which the moisture quickly runs or is quickly absorbed; best on a soil which retains its surface water in pools, etc., and only gradually gets rid of it by evaporation. Accordingly, barren rock soils, except where they retain water in hollows and pockets, and deep sandy soils are immune; but a basalt or granite soil covered by a layer of clay or other porous earth (even sand) is dangerous.
The configuration of the ground is important, since on it depends the regulation of the surface water. Ditches, troughs, and similar excavations which possess no outlet are, in connection with other conditions, very favorable to the disease; while a cone shaped elevation from which the rain flows quickly is least favorable.
There are in the literature several instances of malaria among the crews of ships, the holds, storerooms, etc., of which were dirty or poorly ventilated, so that molds developed in the drains and elsewhere (Holden, Siciliano, and others). The principal thing to be said is that the clinical descriptions of these cases are by no means convincing that they were malaria and not other infectious diseases.
Van den Korput and Hammon were infected with malaria in a region free from disease through, so they think, having in their sleeping room an aquarium for the breeding of certain algae. Van den Korput further affirms that his instructor, Morren, in Liege, had warned him against keeping the aquarium in the sleeping room, since he (Morren) had already observed that malaria developed at the time of the fructification of the spores. Similar assertions were made by Schiirzf, Zwickau, and others. Unfortunately, we must say again the proofs are insufficient to establish the presence of malaria absolutely. In similar cases blood examinations would be of the greatest interest.
The elevation of the ground is of unmistakable influence, since with increasing height cases usually decrease in number and severity, though there are not a few highly situated infected places. This influence is probably exercised by the absence of the factors mentioned previously as the chief ones, namely, warmth and surface water, together with the requisite anophelines. Hirsch gives for the Alpine regions of Germany, 400 to 500 m. elevation as the boundary of malaria ; in Italy it rises to 1000 m. On the slopes of the Himalayas, on the elevated plains of Ceylon, on the eastern inclination of the Rocky Mountains, the disease is found here and there at a height of 2000 m., and on the Peruvian Andes at even 2500 m. and over. These figures need no further explanation.
If the corresponding requirements are present, especially if a dry hill rises from a moist warm valley, the difference as regards local conditions of breeding places of anophelines between two places lying close to each other may be striking.
Laveran mentions that the inhabitants of Constantine (Algeria), a city situated on rock ground 600 m. high, almost never suffer from malaria , while in the valley of the Rummel, about 130 m. lower, almost no one escapes. The same is true of Bona and numerous other places. It is consequently natural that in infected regions the more highly situated places should be chosen for settlement, and especially in summer should be made serve as resorts of refuge.
Moreover, height exercises an influence for other reasons, for experience teaches that the higher the elevation from a malarial soil, the less is the clanger of infection-i. e., from the bite of anophelines.
From innumerable observations it appears that the "malarial virus" can rise only an inconsiderable distance above the ground, so that residence at a certain elevation above the insalubrious soil diminishes the danger of infection. This knowledge has been put into practice, in that laborers working in malarial regions build their huts on piles several meters high in order to be out of danger during the night.
From what has been said it is evident that certain factors, like warmth, moisture, and soil, may be necessary to the development of malaria , yet these factors are by no means sufficient to produce the disease. They are simply factors which influence, in one way or another, anophelines and the parasites which they convey. It is thus clear that two places may exist under exactly similar telluric and climatic conditions, yet one be healthy, and the other severely infected. In the former case all the external requirements for the development of the parasite are present except the anopheline, indispensable for its transmission. Again a region may seem unsuspicious on account of its telluric and climatic conditions, and still be infected, owing to the presence of anophelines (and cases of malaria, active or latent).
In this way only can we explain the malarial epidemics that occasionally spread over districts at other times immune, and the becoming healthy of insalubrious districts.