168 GNATS OR MOSQUITOES — CHAPTER VIII but it will not, I presume, be denied that the disease is everywhere in India accountable for a considerable proportion of the total mortality, or that any perceptible diminution in the prevalence of malaria could fail to make itself evident in the total death-rate.
It is inconceivable that the introduction of a pure water supply should raise the death-rate from cholera and bowel complaints, and as a matter of fact the figures of such of the towns as I have examined show improvement in this respect.
Nor can the enhancement of the death-rate be fairly ascribed to improved registration, as during the entire period this has been admittedly fairly accurate as to total mortality however worthless it may be as to detailed causes, nor is there any evidence to show that there has been any general improvement in this respect. Moreover it must be remembered that the dates of introduction of the regular water-supply differ widely in different places, and that the increased mortality has followed immediately in by far the majority of cases. Further, the increase though immediate is not progressive, as may be seen by running through the series of tables published up to the present date.
It is therefore undeniable that in the North West provinces at least, municipal malaria has increased and not diminished coincidentally with the introduction of filtered water-supplies.
CONDITIONS INFLUENCING PREVALENCE 169 For those who believe in the agency of the Mosquito in the propagation of malaria, the explanation of this unexpected and undesired result of modern sanitary enterprise is not difficult.
In by far the majority of cases no attempt at improved surface drainage has accompanied the spread of the waterpipe.
Financial tightness has necessitated that the essentials of sanitary reform should be taken in hand one by one ; and the effort to introduce a pure water-supply has so exhausted the resources of each municipality in which it has been carried out, that the proportion of cases in which the engineers have been able to so place their hydrants as to secure a ready flowing away of waste water has been perforce a very small one, and the result has been that often, each hydrant is the source of a string of puddles of constantly renewed, fresh cool water, and not unfrequently so placed as to be the greater part of the day in the shadow of tall buildings. In pools so fed and situated Anopheles larvse may be found at times of the year, when but for the hydrants, they would be rare as the dodo ; for these larvae do not appear to be able to develop in water as hot as that of the ordinary stagnant pool or tank in the hot dry weather. At any rate it is only in such exceptionally conditioned water that Anopheles larvse can be found in the N.W. provinces in March and April, for the ordinary garden tanks, which in the rains will harbour large numbers, are then full of Ctdex larvae only. In this way a piped water-supply extends the period of possible infections over several months, which ordinarily yield but few fresh cases. As even when confined to its normal times and seasons malaria is responsible for a larger share of the total mortahty than any other disease, the above explanation appears to me to adequately explain the apparent failure of pure water-supplies to improve the general health. It is obvious, however, that the increased sickliness and therefore presumably malariousness of these places are an indirect result only of the introduction of a water-supply ; as to the direct connection of which with malaria there exists no tittle of proof.
170 GNATS OR MOSQUITOES — CHAPTER VIII The case too of the French transport " Argo," which has so often been quoted as an example of malarial infection through the agency of water, has been conclusively shown by Celli (CM., p. 95) to have been probably an outbreak of acate poisoning, but certainly not malaria. Putting aside then the possibility of water acting as the direct vehicle of infection, let us see how far the prevalence of malaria is indirectly influenced by its agency.
The presence of water is absolutely essential to the multiplication of Mosquitoes, and further, it is now fully recognised that it is not large bodies of water, such as marshes, lakes and rivers, that form their favourite nurseries, but small stagnant collections such as puddles and pools of no great size that are favoured by them.
Further, these puddles must be sufficiently permanent to persist for at least ten days. It is therefore the surface distribution of water that is primarily of importance ; but as this is largely influenced by the disposition of the ground water the study of the depth and movements of the latter is of the greatest importance, and the prevalence of malaria is therefore influenced by this in exactly the same way as if it were a truly water-borne disease. Of the older generalisations on the subject, perhaps the only one that can be said to have stood the test of recent knowledge is the often demonstrated fact that localities where the ground water lies close to the surface are generally malarious. The reason of this is that in such places the surface is easily saturated and badly drained ; because, wherever the ground water lies high, it is usually also either sluggish or stagnant. Now a marsh is usually a place where the ground water comes 10 the surface, the level of the soil being below that of the ground water of the higher land around it ; but the marsh itself, per se, is rarely concerned in favouring the multiplication of Mosquitoes, the larvae of which are only very rarely to be found in such situations. It is the stretch of country immediately surrounding the marsh, where the ground water level is not belov/, but almost coincides with that of the soil, that is the true focus of " marsh miasmata," for in such places the smallest depression must needs remain a puddle, in spite of the most rapid evaporation.
CONDITIONS INFLUENCING PREVALENCE 171 It is obvious therefore that hydrauHc sanitation can do much to diminish malaria, though in some cases, as in the Eoman Campagna, the works required may, as pointed out by CeUi (CM., p. 126), be of such magnitude as to be beyond our present financial and engineering resources ; but there are many cases where much good can be effected. The level of the true subsoil water can as a rule be modified only by works of a public character, as the necessary works must usually be undertaken at a considerable distance from the properties they are intended to benefit. What is usually spoken of by agriculturalists as " subsoil drainage " only indirectly affects the ground water level, as the pipes or rubble drains, &c., employed are placed at but a little depth from the surface and nearly always much above the true subsoil water level. The only works that can directly affect the ground w^ater level are the straightening and regrading of the channels which form its natural outlets.
Harbour works for example, which involve the removal of tidal bars, may have an important influence of this kind, and there are other cases in which locks s"hutting off the flood tide from estuaries may have an equally good effect. It must be remembered in this connection that as a rule the surface of a river is the lowest point of a section of the ground water level across its basin at any given point of its course. Now, whatever may be the fall of a river bed, the main obstacle to its efficiency as a ground water effluent is friction, and the more tortuous and therefore longer its course, the flatter will be its gradient and the greater the total friction, which may also be greatly increased by the presence of " snags," boulders and similar obstructions, or by water plants, as in the instance of the " sudd " of the upper Nile. Fortunately, in many cases, works of this sort are not only of sanitary, but also of obviously commercially economic importance, and it is very possible that the desire to make "trade follow the flag" which actuates our attempts to cut a way through the " sudd" may result in a really far more economically important improvement in the health of the Soudan, whereby
172 GNATS OE MOSQUITOES — CHAPTER YIII spots like Fashoda may become valuable possessions, instead of merely affording worthless incitement to international strife and jealousy, and early graves for the Englishmen or Frenchmen who secure the pestiferous bone of contention.
Surface drainage on the other hand is more usually a matter of detailed small works, the collective effect of which, however, as in the case of our own Fen country, may have a most beneficial effect on the public health. Apart from mere gutters, the most effective means of drying the surface of the soil is the well-known agricultural system of " subsoil drainage." Somewhat similar to this, but less effective as a measure of malarial prophylaxis, is the system of drainage employed on Assamese tea gardens, in which the cultivation is divided into plots by straight drainage cuts some five or six feet deep, and these in their turn are divided and subdivided by progressively shallower cuttings. As a measure for drying the soil to suit the needs of the tea bush they are doubtless all that is required, but owing to the impossibihty of accurately grading simple cuttings in the soil, they are a fruitful source of puddles. Still they on the whole appear to be beneficial, as they have undoubtedly diminished the malariousness of certain estates. Influence of the Air. — On this point little need be said as its effects are always rather those of climate than composition. Its chemical composition, the relative proportion present of carbonic acid or ozone, &c., have nothing to do with the case ; but still the word malaria can hardly be said to be a complete misnomer, for it is the air that carries the Mosquito which is the actual vehicle of the malarial germ. The limits of distance to which they can be conveyed are, however, as we have seen very limited, and for practical purposes the quality of the air of a place may be left out of consideration.
Influence of Soil. — The comparative freedom of certain sites from malarial disease has been well known from the earliest times, though owing to ignorance of the actual underlying causes, all attempts at generalisation were rendered futile by the constant cropping up of perplexing exceptions.
CONDITIONS INFLUENCING PREVALENCE 173 Always assuming that there be no water-logging of the soil, it is obvious that the more porous it be, the less the probability of the formation of puddles of sufficient permanence to admit of the rearing of a brood of Mosquitoes. Sand for example, holds water so badly that short of complete water-logging the formation of puddles is an absolute impossibility ; but the deposition of a very thin layer of fine silt by flood water will suffice to render such soils sufficiently retentive. In the same way, rocky soils are usually healthy, because they are generally found in hilly regions, and are therefore associated with good surface drainage ; but if the peculiarities of the rock are such as to lead to the formation of basin-like depressions during the process of weathering, such a soil may be highly favourable to the development of malaria, always provided that the rock be of uniform and impervious texture, so that the water cannot soak away. For this reason the malariousness of Hong Kong was, in our older books on hygiene, ascribed to some mysterious influence of a soil consisting of decomposed granite.
In a certain sense, of course, it really is the decomposition of granite that is at the root of the mischief, but this is not because granite, whether intact or decomposed, can have anything to do with the effect ascribed to it, but because the denudation of such rocks leaves the undecomposed portion hollowed out into basins as watertight as the best porcelain, and moreover, where such rocks form the beds of water courses, they are particularly liable to be worn into " pot-holes" by the action of pebbles retained in some chance depression.
According to Drs. Stephens and Christophers, (B.S.M.C, July, 1900, p. 43), the persistence of Aiwpheles during the drier portions of the year in Freetown, Sierra Leone, is due to puddles retained in rock basins of this description, and similar conditions have been noted in Southern India, in addition to which I have myself met with Anopheles pools of this description at Jhansi.
Speaking generally, it is the character of the surface soil that is of the greatest importance, as however deep the
174 GNATS OR MOSQUITOES — CHAPTER VIII subsoil water may lie, puddles will necessarily form, provided only the surface layer be sufficiently retentive. Hence sites having a clayey surface are necessarily favourable to the development of malaria, and even if the actual surface be not of this character, it is quite possible for a thin intermediate stratum of such material to so hold up the surface water as to admit of the surface being practically waterlogged, in spite of the subsoil being quite pervious and the ground water deep. An impervious stratum of this sort is to be found in many parts of the Gangetic alluvium some four or five feet below the surface, as is shown by the fact that in such districts any attempt to increase water storage by the deepening of existing tanks merely results in their drying up.
In the Sitapur district (Oudh) for example, several tracts of country of this description are to be found, and though the subsoil water lies some thirty or forty feet from the surface they are notoriously malarious.
I am not aware that such an experiment has ever been tried, but in the case of limited areas, such as inhabited sites, it seems possible that their sanitary condition might in such cases be improved by the construction of a number of blind wells carried through the impervious layer so as to open up the porous subsoil beneath.
It would be impossible within any moderate limits to give any account of the various kinds of soil that are to be met with in India, as it is needless to say that in so vast a country almost every possible combination of soil and contour formation is to be met with, and malaria is rife more or less throughout ; but a few words with respect to the commoner formations may not be out of place. Between the foot of the Himalayas and the old island India of past geological times stretches an immense level plain reclaimed from the sea by the silt deposited by the mountain streams that now fall into the Ganges and Indus. After this old narrow sea had completely silted up, the rivers continued to raise their beds, wandering from side to side whenever the detritus deposited in their shoals accumulated sufficiently to raise them above the level of the land hard b3\
CONDITIONS INFLUENCING PREVALENCE 175 This process has been going on ever since, and is still in progress ; till to-day, this enormous area of alluvium forms an apparently level plain, stretching from sea to sea, and in most places some hundred miles wide. Though apparently as level as the ocean the imperceptible vi^atershed between the Ganges and Jumna flowing east and the westward bound rivers of the Punjab is really some 700 to 800 feet above the sea, and the depth of the alluvium is in some situations enormous. The whole of this area consists of sand and silt of various degrees of fineness, modified at the surface with a variable amount of decomposed organic matter. Saving where waters, rich with lime in solution, have matted together vegetable fibre and sand into " kunkar," nothing of the nature of a stone is to be found. At both the eastern and western limits of this wide alluvial area, the ground water is necessarily close to the surface, as hundreds of square miles of country are but a few feet above the level of the sea ; but speaking generally, as one travels further inland the level of the subsoil water gets deeper and deeper, and on the watersheds between the rivers may at times be as much as a hundred feet from the surface.
The eastern or Gangetic half of this area is for the most part naturally fertile, the natural rainfall being, in normal years, sufficient to water a sufficient crop to support a large population. In the easternmost portion indeed the normal rainfall may be said to be excessive, and malaria is necessarily rife and long continued.
On the other hand, once the country drained b}^ the Indus is reached the rainfall becomes scanty and precarious. Whether, however, we start from the Gangetic or Indus delta, the subsoil water becomes deeper and deeper as we travel inland so that in Oudh and the North-west Provinces, forty or fifty feet is no uncommon depth for a well, and in parts of the Punjab, the water may not be reached for twice that depth. On the western side the rainfall progressively diminishes, so that the country, even where the ground water is at no great depth, is a waterless desert ; and cultivation, apart from irrigation, an impossibility.