192 GNATS OR MOSQUITOES— CHAPTER VIII malaria is that they carry and distribute over the surface of the land an enormous amount of fine silt, the deposition of which renders the surface of the soil impervious to water, and so favours the permanence of puddles. The proportion of solid matter suspended in the water at the intake of some inundation canals may reach 3^^ of its weight, but so high a proportion indicates a high velocity in the supplying river, and therefore a coarser deposit, most of which will fall in the earlier reaches of the canal before the water is distributed on the land. In spite of this, however, the amount of fine deposit may be very large, especially in rivers such as the Indus, which flood mainly owing to the melting of the snows supplying the glaciers of the far distant mountains in which they take their source ; and this really dangerously fine, clogging silt, is naturally the most difficult to get rid of. Col. Tremenhern, R.E. (" Roorkee Professional Papers," first series, vol. iii., p. 25), states that " Those inundation canals in Sind which draw their supply from branches separated from the main river by islands covered with brushwood and long grass, contain a comparatively small amount of material in suspension. The brushwood and grass impede the velocity of the water and clarify it," and the selection of a swampy tract of this sort, at a distance from dwellings and cultivation, as the site for the intake of a canal of this sort, could hardly fail to be useful in diminishing its malarious tendencies.
I do not think that the importance of this waterproofing effect of fine silt deposited by floods is sufficiently appreciated as a malaria favouring agency ; but it appears to me to be the true explanation of the way in which a previously healthy place may be rendered permanently malarious by a flood of but short duration. Anyone who has been concerned in the management of large municipal filter beds knows well how rapid and efficient is this " staunching " action of fine silt. In the Calcutta waterworks, for example, after about a fortnight, the filters at certain seasons become practically watertight to a head of 24 inches or so of water.
All canals, of course, carry more or less silt, but the
CONDITIONS INFLUENCING PREVALENCE 193 amount brought down by perennial canals is comparatively insignificant.
It is, I believe, this staunching action of silt that renders places liable to flooding so notoriously malarious. The actual floodings are too occasional and temporary to exercise much direct influence ; but the waterproofing of the surface enormously favours the production of fairly permanent puddles. The obvious remedy lies in the digging into the soil of some more coarsely grained material, and in many cases this would be not only practicable, but actually beneficial to cultivation, for soil of this kind is necessarily impervious not only to water but to air, and is therefore unfavourable to the growth of plants. Not unfrequently the thickness of this silt-choked layer of soil is but small, and is immediately underlaid by coarse, pervious sand, so that there is no need to bring material from a distance, and that all that is required is digging, or even ploughing, to a suflicient depth to break up the thin impervious superficial layer, and to mix with it the abundant more coarsely grained substratum.
Whatever the system employed, however, the extent to which cultivation, whether irrigated or otherwise, favours malaria depends almost entirely on the habits of the cultivators. Where the agriculturist is neat and systematic in his operations, and especially when he is anxious to utihse every available foot of land, cultivation is harmless because the waste of space and of water implied by the presence of puddles is carefully avoided, while on the other hand untidy cultivation necessarily spells opportunity for the entire race of Mosquitoes.
It is for this reason that garden cultivation is as a rule harmless, unless there be some special feature in the plan of operations that provides the necessary collections of water in some other way, and such is unfortunately the case with the gardens that commonly surround our bungalows in Northern India.
For the greater part of the year artificial watering is an absolute necessity, and the plan employed is usually not irrigation in the usual sense of the word, but watering by 13
194 GNATS OE MOSQUITOES — CHAPTER VIII hand in the same way as is done in England. The water is usually raised from a well by the agency of bullocks, and as the gardens are generally tolerably large, is generally, to save labour, distributed by masonry channels to a number of small cemented tanks, such as that illustrated below, fmm which the water is dipped up by the gardeners. Fig. 33. — Typical garden tauk (one uf a dozeu) iu the gaideu of European residence in yhahjahaupur, N.W.P., India. Now as far as the Anglo-Indian resident of upper India is concerned these tanks are par excellence the most fruitful of breeding places for Mosquitoes of all kinds. During the hot dry weather, they teem with the larvae of Ciilex fatigans, whose imagines render life in the attached bungalow well nigh unbearable ; when the rains come this species is associated with Stegomyia fasciata, AnopheleteSy and other Mosquitoes appertaining to the season ; and all through the cold weather they serve as the last vantage ground of the few Mosquitoes that remain active at that time of the year.
To those who have lived only in Europe the remedy
CONDITIONS INFLUENCING PREVALENCE 195 appears simple and obvious :- Construct a single covered tank at the well head, and distribute the water by means of pipes provided with taps. But in practice the expense, and the difficulty of obtaining sufficiently skilled workmen to construct such an installation and keep it in order are prohibitory. What might, however, be done, would be to insist on each tank being carefully cleaned out and left dry for a few hours once in every week ; but it is difficult to persuade even Europeans to do this, and though in smaller stations, where the houses are scattered, a good deal of personal immunity might be secured by attention to this point, in the larger places one would gain but little unless one's neighbours could be induced to do the same. In such places, however, the weekly cleaning out of all such tanks should be made compulsory by municipal bye-law, for in view of our present knowledge it is no more justifiable to foster the multiphcation of Mosquitoes than to permit the maintenance of the germs of cholera or other zymotic disease.
How far care and neatness in cultivation can go to diminish malaria is well shown in the case of Egypt. The cHmate is well suited to the spread of malaria, the ground water nowhere far from the surface, and the annual inundation of the entire inhabited area would, one would think, render the country a perfect hot-bed of the disease. During the actual rise of the Nile, when the whole country becomes a shallow sea, the conditions are no doubt not really favourable to the multiplication of Mosquitoes, but as the waters recede ; but for the fact that the restricted area available for cultivation renders every square foot of value, the conditions are naturally ideally favourable to the spread of the disease. Nevertheless, Dr. Sandwith, of Cairo^ assures me that, although present, malarial disease is neither common nor serious in the Delta, and a few walks among the fields near Cairo, showed me that the entire cultivation was really of the best garden type, and that land and water were afike regarded by the fellaheen as far too precious to be wasted as sites and material for puddles. Celli regards meadow land as unfavourable to the develop-
19G GNATS OR MOSQUITOES— CHAPTER VIII ment of malaria. In the European sense of the word cultivation of this description is so exceptional in the East that it can have but little influence, but in Europe level pieces of land with a high ground water level are commonly chosen for pastoral purposes, and the careful levelling of the fields, together with the thick covering of grass, must necessarily go far to prevent the formation of permanent puddles ; and the disappearance of malaria from England is probably largely due to the substitution of pastoral for arable agriculture throughout the country. This disappearance, it will be noticed, dates from the time when the improvement of means of transport so cheapened imported corn as to render the cultivation of cereals in England unprofitable. At that time quinine was so expensive, and its proper administration so little understood, that it was administered in doses so small that it could have had but little effect in checking the disease which, nevertheless, had practically disappeared from the country before the great fall in the price of the drug rendered its full administration practicable.
I have been told by a druggist who was in business in the fen country while it was still malarious, that the popular remedy was then the prophylactic use of opium. It was quite common for a farmer's wife to carry home along with her other weekly market day purchases, a half pint of laudanum ; but he was but rarely asked for quinine. Now while there is a certain amount of evidence tending to show that the habitual use of opium has some protective influence against malaria, it is certainly incapable of destroying the parasite, and it appears on the whole unlikely that the use of drugs can have had any appreciable effect in stamping out the disease, which is more probably due entirely to alterations and improvements in agriculture. The Effects of Human Occupation. — Given the necessary climatic conditions, it is man himself who is the most efficient of all agencies in bringing about the conditions that favour malaria. The human being is essentially a digging animal, and, whether it be the savage, who scoops up a few handfuls of mud wherewith to smear the wattles of his hut
CONDITIONS INFLUENCING PREVALENCE 197 or the modern engineer with his steam navy, the result isthe same less or more, viz., depressions of the surface, and hence puddles, and it cannot be too often insisted upon that it is the small, unnoticed puddle close to the dwelling, and not the large, but distant marsh or pond, that, for practical purposes, is the real nursery of malaria. The large tank or tanks that are to be found in almost every Indian village are generally far too dirty for the taste of even the none too particular Indian species of Anopheles, but with us, as with most semi-civilised communities, mud forms not only the main material of domestic architecture, but also of house decoration. At frequent intervals the careful Indian housewife smears the interior of her home and the little platform before it with a mixture of mud and the dung of the sacred cow, and the result is certainly to give an appearance of cleanliness that could hardly be otherwise obtained for so small an expenditure. In Zululand, and, I believe, generally among African tribes, the same mixture is employed. But though the Hindu lady likes to keep her house neat in this way, both she and the goodman are tolerant of untidy surroundings to an extent that will be scarcely believed in Europe, and the result is that the ground round the house soon becomes honeycombed with small excavations, which in the rains get filled with fairly clean water. Nor is the European one whit less blameworthy. Apart from the special malaria-brewing tanks already described, with which most of our gardens are furnished, there will be found close to every building, whether it be the cottage of the Eurasian clerk or the imposing barrack or law court, an excavation of corresponding dimensions, from which has been taken the earth for its plinth ; and rarely or never is any attempt made to fill up or drain the hollow so formed.
Further, as the surroundings of European dwellings and official buildings are usually kept fairly clean, the results to the inmates, as far as malaria is concerned, are even worse than those of the tanks that have originated in the same way round a native village.
The results of public works are equally disastrous on a
198 GNATS OR MOSQUITOES — CHAPTER VIII Fig. 34. — View in an Indian cantonment, showing, in the foreground, a pond resulting from the excavation made to furnish the spoil required for the plinth of a bungalow, which lies just behind the trees. Fig, 35. — " Borrow pits " beside an Indian railway.
CONDITIONS INFLUENCING PREVALENCE 199 larger scale. Every road and railway has, on either side of it, a continuous chain of " borrow pits," which, in rainy weather, form simply ideal nurseries for mosquitoes of all sorts.
It is obvious that, in many cases, these pits might easily be converted into excellent surface drains, but unfortunately, partly to facilitate the measurement of the work done by the excavating gangs, and partly to prevent the scouring effect that might be exercised by a continuous channel, the engineers carefully avoid doing so, and leave them as a chain of pools, which remain continuously full of water for months together. The results, especially where a road or line passes close to habitations, are so serious that, at very least in such situations, the making of such undrained hollows should be prohibited by departmental regulation.
The results even of avowedly sanitary works are unfortunately too often no better. The untoward effects of pure municipal water supplies, on modern lines, have already been adverted to, and very often those of attempts at surface drainage are no more fortunate.
On the next page are four photographs of pools in the course of the local surface-drainage system, all taken within a few hundred yards of my bungalow, and it would have been perfectly easy to fill a large scrap-book with similar prints. In hill stations, such as Naini Tal, the small masonry tanks, such as are shown in the two upper illustrations, form almost the only nurseries for Mosquitoes to be found in such places, as the precipitous lay of the ground is very unfavourable to the formation of natural collections of water.
Celli (C. M., p. 147), also emphasises these undesired effects of otherwise invaluable public works, and is particularly emphatic on those of railways. Railways, and necessarily also ordinary roads, when embanked, may also often increase the malariousness of a locality in another manner ; for when, as is often the case, they chance to be carried across the natural lines of drainage, they inevitably bank up the drainage of the land lying above them ; and this, too, in spite of an apparently liberal allowance of culverts.