Climate and Health in Hot Countries and the Outlines of Tropical Climatology / A Popular Treatise on Personal Hygiene in the Hotter Parts of the World, and on the Climates That Will Be Met Within Them.

by George Michael James Giles

In devising measures for the destruction of mosquitoes it is obvious that while they may be attacked either in the aquatic or aerial stages of their existence, the easily localised larvæ and pupæ will be far more easily dealt with than the elusive flying insect, and it will be therefore best to devote most attention to the destruction of the former.

There are two principal methods of dealing with the insects during their aquatic stage, i.e. (1) by doing away with their breeding places; (2) by poisoning them. Of the two it is clear that the former method is, where practicable, the more valuable, as in nearly all cases its effects are more or less permanent. With either object in view, however, the first step is to seek out the breeding places.

As a preliminary measure, all domestic rubbish capable of holding water, such as disused flower-pots, empty tins, &c., should be carefully sought out and destroyed or disposed of by throwing into some depression of the soil which it is desired to fill in. Next, the entire surface should be carefully inspected after a brisk shower of rain, and all such puddles as are of manageable dimensions carefully filled in and levelled.

The amount of labour that can be profitably afforded in work of this sort will depend a great deal on the probable duration of one’s stay in a place; as temporary residents, such as government officials, will find it much cheaper to employ temporary measures, such as the use of paraffin; while in the case of merchants and other permanent residents, the expenditure of a considerable sum on permanent measures will be more remunerative in the long run. The great difficulty often lies in finding spoil wherewith to fill in the depressions, as unless care be exercised, the only result will be to shift the site of the puddle. Sometimes, however, a bank of earth surrounds the compound by way of a hedge; and as in a level country the existence of such an obstruction to surface drainage is most objectionable such banks should always, if possible, be removed and a hedge of wire or bamboo substituted; when the spoil can be advantageously utilised for filling in hollows. Where no localised elevations are to be found, the material required should be obtained by a general very slight removal of the surface. In other cases it may be possible to drain a depression by cutting a shallow gutter to the nearest surface drain.

Another and very important class of breeding place consists of the comparatively large collections of water formed by the various reservoirs, channels, &c., constructed for obtaining and storing water for various domestic purposes, such as wells, tanks, &c., and especially the appliances for watering gardens. Of these the most harmful of all is canal irrigation, which should undoubtedly be never tolerated near a dwelling by any one valuing his health, malaria or no malaria; for the waterlogging of the soil, that is practically inseparable from the system, is either the exciting or predisposing cause of a variety of diseases, many of which, such as rheumatism, consumption and cancer, are perhaps more seriously dangerous than malaria. Sodden and waterlogged sites are notoriously unhealthy all over the world, and chronic ill-health is a high price to pay for a few flowers and vegetables; so that the settler will be well advised to leave irrigated cultivation to those who are constrained to adopt it for a livelihood, and banish it from his own premises; as much is gained by living on a comparatively dry site of however limited extent.

As a matter of fact, indeed, the possession of a garden of any sort is a more than doubtful benefit in malarious places, for trees and shrubs necessarily form lurking-places for mosquitoes; and even with the greatest care, it is difficult to carry out the necessary watering of the plants without giving rise to puddles; while the various reservoirs, &c., that are almost indispensable, are a constant source of danger unless constantly and minutely supervised.

In India, for example, gardens are commonly watered from wells by means of water lifts of various forms, which are worked either by means of bullocks or by manual labour. In order to facilitate the distribution of the water, masonry channels are usually constructed which carry the water from the well head to all parts of the garden, and as the lift cannot conveniently be worked continuously, a number of small tanks are arranged along the line of channels wherein water is stored, so that it can be dipped out and distributed by means of an ordinary watering can without the gardener ever having to go far to replenish it. Now these small tanks are par excellence the main source of supply of mosquitoes of all species, and therefore of malaria, to the houses to which they are appended. If, therefore, a garden be considered indispensable, all such tanks should be carefully emptied and all deposit cleaned out at least once a week; under which circumstances, though they will probably swarm with larvæ by the end of the time, it is impossible for any of the latter to complete their metamorphoses into adult insects. By far the least objectionable plan of relieving the dust and glare of a bare situation is the cultivation of a well-kept lawn, a few large trees being left and the area of flower beds strictly limited, for as flooding the grass is sure to spoil a lawn, success can only be ensured by careful and moderate waterings at frequent intervals.

Collections of water that are too large to be done away with by filling in or by draining, should be dealt with by oiling the water with paraffin. There are some other agents which may be employed for the purpose, but none of them are so cheap, efficient, and readily obtainable. As an additional advantage, though fatal to all kinds of insects, it is in the quantities employed, not only absolutely harmless to vegetation, but water so treated is actually a most valuable application, owing to its power of destroying other injurious insects. When sprinkled on water paraffin spreads out into an extremely thin film, so that a very small quantity will cover a considerable area. A three-gallon tin, for example, contains enough to cover an area of 100 yards each way, though it may be a day or two before the oil reaches all parts of such a space. It is important to remember that the cheaper and commoner the oil, the better it is for the purpose, and the addition of a little of common bazar ghi, or clarified butter, is said to make the oil spread better and render its action more lasting, though the writer has not personally experimented with the mixture.

In the quantities used there is no possible danger of fire, even though wooden structures, such as the piles of bridges, be immersed in the water, as the film is so thin that it is impossible to ignite it; a matter one would have thought sufficiently obvious, were it not that objections have been gravely raised to the use of paraffin on this score.

The method of application must be varied to suit the size and situation of the piece of water to be dealt with. Small puddles, such as those left in the course of roadside ditches, may most economically be dealt with by, as it were, lightly wiping over the surface with a wisp of rags dipped in the oil; filling in being in such cases out of the question, as to do so would be equivalent to obstructing the drainage; and it is quite impossible to maintain an exact and uniform gradient in an unrevetted channel.