216 GNATS OE MOSQUITOES— CHAPTER VIII covered in all, and is in a number of instances already known. The older frame of mind saved all trouble in thinking, and therefore, quite unconsciously, the modern standpoint is repugnant to them, and they resent being told that if they plan their gardens so as to be perfect incubators for Mosquitoes and refuse to adopt the most simple precautions against being bitten, they have only themselves to thank if they get fever. That though now and again the insect may have been reared elsewhere, the chances are nine to one that the gnat that inoculated them would never have seen the light had not they themselves provided his parents with every possible convenience for rearing their family. As a matter of fact, every naturalist would scout the idea that we can hope to perceptibly diminish the number of Mosquitoes in tropical regions ; and can probably advance better grounds for the faith that is in him than those who picture him as proposing to root out the race ; but he does not share in popular notions as to these insects "appearing" from no ascertainable origin, or that they can migrate or be carried long distances by the wind. He knows on the contrary that for all practical purposes every locality breeds its own Mosquitoes, and that whenever he sees one of these insects he may be sure that the water it was reared in is not a quarter of a mile off. Given that a place be destitute of collections of water suitable for rearing the larvae and it will be necessarily free from Mosquitoes, even if there be places that swarm with them within a mile's walk. From this it follows that to secure a local immunity from them is almost always within the range of possibility, and may in a few cases be a quite simple matter, and resolves itself essentially into a question of expense. Given that we have free access to the entire area, and liberty to deal with every breeding pool we discover as we think best, there are very few collections of water that cannot be drained, filled in, or in some other way rendered unfit for the rearing of Mosquito larvse; but in many cases the cost would be prohibitory, as for example where the insects larvate in rice fields, and if we desire to prevent the multiplication of Anopheles, we must make, up our minds to prohibit the cultivation of the staple food of the population — too
CONDITIONS INFLUENCING PREVALENCE 217 high a price to pay, even for immunity from fever. On the other hand there are many places where the number of possible breeding places are small in number and limited in extent, and where accordingly immunity can be secured at very trifling trouble or expense. The instance of the successful use of kerosme as a larvicide thirty years ago, in America, quoted in the first edition, must have been an extreme example of this. In this case, L. 0. Howard ("Insect Life," vi, p. 90) describes how a residence was freed from Mosquitoes by killing the larvae in a single pool some 4,000 feet square. In by far the majority of places the experiment would, at best, have proved but partially successful, owing to the existence of alternative, but unsuspected breeding places.
To mention another instance : There is, I have no hesitation in stating, no good reason why the European residents of most stations in Upper India should be much troubled by Mosquitoes in the hot weather. The fierce, dry heat has dried up every possible natural breeding place, and the swarms of Mosquitoes that render life a burden are bred exclusively in the garden tanks described above, and in other easily discoverable domestic collections of water. If each resident would devote a few minutes once a week to seeing that his servants emptied these and carefully cleaned them out, the nuisance would be reduced to a minimum. Moreover, in many places the individual houses are so scattered that a considerable degree of personal immunity may be secured by attention to one's own bungalow alone.
As the quarter in which they reside is usually quite isolated from the houses of natives other than their own domestic servants, the area is entirely under their own control, and all that is required is a little friendly cooperation.
From what has been said it will, I think, be clear that it is impossible to prescribe any generally applicable method of dealing with malaria by radical measures intended to diminish the prevalence of Mosquitoes, and success can only be hoped for by a careful study of local conditions, guided
218 GNATS OR MOSQUITOES — CHAPTER VIII by a clear understanding of the conditions that favour the multiplication of these troublesome insects. The first step is to discover the pools where they breed and the seasons at which they do so. This done, we have to consider why these pools exist, and if possible to remove the cause. Unfortunately in many cases the breeding places, although small, are so numerous that the task of dealing with them in detail is almost hopeless, and would involve heavy and continuous expense. This may be due to the character of the soil or to waterlogging, and in such cases only extensive drainage works, surface and deep, can be expected to afford any permanent benefit. As a rule it is too costly to attempt anything but surface drainage, and even this is by no means an easy matter, for unless the drains be most carefully levelled and paved, they generally during the rains becomQ chains of small puddles, which form the favourite nurseries for the noxious "dapple-wing" Mosquitoes. But the paving of drains on any adequate scale is a most costly business, and except in closely populated municipal areas out of the question on the score of expense. In most cases, therefore, they are better restricted to a few deep cuttings, and to straightening and clearing the natural outfalls, for I am convinced that the multiplication of what are called in India " kachcha " drains, i.e., shallow unpaved gutters, does more harm than good, and that in cantonments and in other places where sickliness is so costly to the tax-payer as to justify any reasonable expense on sanitation, it would be better to systematically drain the area by means of agricultural " subsoil " drains. As already pointed out, it is a misnomer to speak of these by their usual name, as they are placed immediately beneath the surface, and so cannot directly affect the level of the subsoil water, and they really drain only the surface ; but as its contour remains unaltered they cannot lead to the production of puddles in the same way as mere open cuttings in the soil. Deep cuttings, on the other hand, though equally irregular, do not for some reason so often harbour Anopheles larv«. It may be that this is because the water is hidden from the female Mosquitoes when seeking for a place wherein to lay
CONDITIONS INFLUENCING PREVALENCE 219 their eggs, but a more probable suggestion, for which I am indebted to Dr. St. George Gray, of St. Lucia, is that the larvae require plenty of light.
It is in level country such as the Indian alluvia that conditions such as the above are generally found. On midulating ground surface drainage may usually mainly be left to care for itself, and in such a place as Freetown, Sierra Leone, to judge from the reports of the two Malaria Commissions, no extensive drainage measures can be expected to be of any proportionate value. Here the breeding pools are basins of solid rock which necessarily cannot be drained, and as far as one can judge, the measures that suggest themselves are the regrading of the banks of the rocky watercourses, and the improvement of the roads and other places where the rock lies bare, and the formation of these peculiar puddles thereby becomes possible. Possibly, where not exposed to traffic, many of these pools might be rendered innocuous for a considerable period by filling them with sand ; but this aside, the main reliance must be placed on larvicides, and I hardly understand why Drs. Stephens and Christophers should be disappointed in the results of their employment (K.S.M.C, p. 43) on the ground that the larvas reappeared as soon as the use of the larvicides was discontinued. The effect of such agents is at most a matter of days, and there is no possible reason for the Mosquitoes not returning to pools so treated the moment their effects have disappeared. The main objection to the use of these agents is not that they are wanting in efficiency, but that success can only be obtained by continuous trouble and expense, but where the breeding pools are in manageable numbers, and they cannot be done away with except at prohibitory expense, it is as reasonable to keep up a staff of puddle oilers as of scavengers, for the result of the labours of the street sweepers is no more permanent than that of the larvae destroyers, and it is as fair to object on this score to the continuous employment of the one municipal servant as the other. I feel perfectly sure that Eoss never expected to obtain any advantage from the use of such agents without their
220 GNATS OR MOSQUITOES— CHAPTER VIII employment being continuously and systematically kept up, and for the purposes of an initial experiment such as his, or in any case where immediate results are demanded, no other plan is applicable. Sanitation against malaria on radical lines will everywhere require years of continuous and progressive effort, and as long as it remains uncompleted we must necessarily depend upon hand to mouth expedients, such as the use of larvicides. But whatever plan be adopted those who hold the purse strings must reconcile themselves to the fact that the prevention of malaria is impossible without considerable outlay, and they must further be prepared to see a great deal of it expended, though not really wasted, in futile attempts, for we shall no more immediately hit upon the best method of dealing with this difiiculty than we have in the deodorisation of sewage or any other sanitary problem. For the present, while radical measures should always be kept in view and undertaken as funds and opportunity permit, we must content ourselves with temporary methods, and it is well to remember that though we may not be able even locally to " exterminate" the carriers of disease, it is always worth while to diminish their numbers. Every breeding pool filled in or otherwise dealt with, means one possible focus for dissemination the less, and a great deal might be soon accomplished by the systematic attention to the immediate neighbourhood of barracks and other dwellings.
Turning then to measures of this class, it is clear that the insects might be attacked either in the aquatic or the aerial stage of their existence, but that they may be far more easily got at m the former. During their adult life, the only period during which much advantage is likely to be gained by attempts to destroy them is that of hybernation, and it has been shown that, in uniformly warm chmates, they cannot really be said to hybernate at all. Where, however, they do so, their destruction is of the first importance, and, owing to the sluggish condition of the insects, not so impracticable as might be imagined. The impregnated hybernating females are, it must be remembered, the main hope of the race for the generation of the coming year, and
CONDITIONS INFLUENCING PREVALENCE 221 their destruction means that of their posterity. Their numbers, moreover, are comparatively small, and the situations in which they are likely to be found can be predicated with tolerable certainty. I do not, however, propose that one should actually search for them, for the insects understand too well their business to make any search profitable ; but it may be taken as certain that the nooks and corners of every room and native hut harbour numbers of them, and it is perfectly easy to destroy them by fumigation with sulphur employed in the same way as in ordinary disinfecting operations. The room to be dealt with should of course be closed up as closely as possible, but no elaborate arrangements for closing all crannies are necessary, as they succumb to a proportion of the vapour in the air that would be perfectly harmless to bacteria. It is very little use to fumigate with pure sulphur, as it burns so uncertainly that it generally goes out before half the material is consumed, and it is even more futile to burn the sulphur by throwing it on a charcoal braaier as is often done, because the result is mainly vaporised sulphur, which is quite useless. A very short exposure to the fumes suffices to kill the stoutest Mosquito that ever buzzed, but as the buildings to be dealt with are mostly extremely pervious, it is of the first importance that the sulphur should be burned very quickly, so as to produce a volume of sulphurous anhydride large enough to be lethal before the product of the portion first burned has time to escape. What is wanted, in fact, is a mild firework which will burn out in a very short time, but yet will not flare up to a dangerous extent. About one part each of nitre and charcoal to eight of sulphur answers well, the mixture being made up into pastilles weighing 4 oz. each, by means of a little gum-water, and dried in the sun. Pastilles of this description were made up for me by Messrs. Waldie, of Cawnpore, and I found that even in thatched buildings not a single insect was left alive. The floor of a bath-room, in which hardly any Mosquitoes could be found by any ordinary search, was found after fumigating in this way covered with dead Mosquitoes, a circumstance which gives a good idea of the effectual way in which they hide
222 GNATS OR MOSQUITOES — CHAPTER VIII themselves. One pastille should be allowed for every thousand cubic feet of space. Merely for the sake of personal comfort, it is well worth while occasionally burning these pastilles in the hot season in bath-rooms and other favourite lurking places, as to do so costs but little either in cash or trouble, and the smell of the fumes disappears as soon as the place has been aired ; but it is absolutely useless to do so if active breeding places are allowed to exist close by. To be effectual, fumigating should be done towards the close of the hybernating season, and should be performed during the heat of the day, when even the least sluggish are sure to be under shelter. I trust that no playful critic will indulge us with an arithmetical dissertation on the number of tons of sulphur required for the annual fumigation of every native home in India, on the gratuitous assumption that I propose to enforce some such measure at the point of the bayonet, for rudimentary statistical exercises of this sort are within the powers of the most modest mathematician, and it is needless to say that everyone who has served in the sanitary department in India knows far better than any others that it is quite useless to try to force or even persuade the native to adopt sanitary precautions of any sort within the precincts of his own house. The caution is hardly needless, as, if I remember rightly, some wit went to the pains of calculating the annual enhancement of taxation per household necessary to provide all native babies with a mosquito net, on no better grounds than because some one of us had been emphasising the importance of Europeans protecting themselves by their use. But though we cannot fumigate the Indian continent, there is no reason why the plan should not be adopted in the case of all barracks and other quarters provided by Government, and in cantonments where such natives as are permitted to reside within their limits, do so on the distinct understanding that their status is more or less one of sufferance, and on the understanding that they are willing to submit to the sanitary whims of those for whose use the area has been set aside, however unreasonable they may appear to people in their particular stage of civilisation ; it would be not
CONDITIONS INFLUENCING PREVALENCE 223 onl}^ justifiable, but feasible to insist on such a measure being carried out.
As each pastille costs no more than a Lee-Metford cartridge, the cost would not be prohibitory, and the annual bill for invaliding the men who have been educated to use the latter is so heavy that it would pay well to adopt any measure likely to diminish it ; for the lessening of the annual invaliding roll by one or two names would more than meet the cost. In most parts of India the month of February would be the best time to apply such a measure. With this exception we possess no means likely to be of service in destroying the adult insect, and therefore must in the main confine our efforts to attacking the more vulnerable larvee. In attempting this the species found in a pool, or even their presence or absence, is a matter of little importance, for almost any collection of water is a possible breeding place ; and the rule should be to leave as few as possible available. All small pools are certainly best dealt with by filling them up ; care being of course taken not to obtain the necessary material by excavating a new hollow. In most cases sand from the nearest river bed will be the best thing to employ, as it refuses to hold water, and no harm is likely to result from digging it. A large proportion of Anopheles pools are, however, so small that a dozen or so may be done away with with a single cart-load of sand, and as the results would be fairly permanent this would be far more economical than the use of larvicides. For larger collections of water, pending the adoption of more radical measures, the use of the latter is the only feasible plan, and the only question is as to what is the best material to employ. Unfortunately the larvae are singularly resistant to ordinary poisons, and are capable of disporting themselves for hours in such energetic fluids as the Liq. hyd. bichlor. B.P., or in Fowler's solution.
Celli (C. M., p. 19G, et. seq.) gives the results of an extensive series of experiments as to the quantity required to kill the larvae of a large number of agents, which may be consulted by those who desire more detailed information on the subject ; but their practical outcome is that with the