Preventable Diseases

by Woods Hutchinson

It was also a widespread belief in malarial regions that the hours when you are most likely to "git mylary inter yer system" were those just before and just after sundown; and now entomologists inform us that these are precisely the hours at which the Anopheles mosquito, the only genus that carries malaria, flies abroad.

Of course, a number of popular causes, such as bad drainage, the drinking of water from shallow surface wells, damp subsoils under the houses, and especially that peculiarly widespread and firmly held article of belief that new settlements, where large areas of prairie sod were being freshly upturned by the plough, were peculiarly liable to the attack and spread of malaria, had to go by the board,—with this important reservation, however, that almost every one of these alleged causes either implied or was pretty safe to be associated with pools or swamps of stagnant water in the neighborhood, which would furnish breeding-spots for the mosquitoes.

The discovery explains at once a score of hitherto puzzling facts as to the distribution of malaria. Why, for instance, in all tropical or other malarious countries, those who slept in second and third story bedrooms were less likely to contract the disease, supposedly because "bad air didn't rise to that height," is clearly seen to be due to the fact that the mosquito seldom flies more than ten or twelve feet above the level of the ground or marsh in which he breeds, except when swept by prevailing winds. It also explained why in our Western and Southwestern states the inhabitants of the houses situated on the south bank of a river, though but a short distance back from the stream, would suffer very slightly from malaria, while those living upon the north bank, half a mile back, or even upon bluffs fifteen or twenty feet above the water level, were simply plagued with it. The prevailing winds during the summer are from the south and mosquitoes cannot fly a foot against the wind, but will fly hundreds of yards, and even the best part of a mile, with it. The well-known seasonal preference of the disease for warm spring and summer months, and its prompt subsidence after a killing frost, were seen simply to be due to the influence of the weather upon the flight of mosquitoes. Shakespeare's favorite reference to "the sun of March that breedeth agues" has been placed upon a solid entomological basis by the discovery that, like his pious little brother insect, the bee, the one converted and church-going member of a large criminal family, the mosquito hies himself abroad on his affairs at the very first gleam of spring sunshine, and will even reappear upon a warm, sunny day in November or December. Perhaps even some of the popular prejudice against "unseasonable weather" in winter may be traceable to this fact.

Granted that mosquitoes do cause and are the only cause of malaria, what are you going to do about it? At first sight any campaign against malaria which involves the extermination of the mosquito would appear about as hopeless as Mrs. Partington's attempt to sweep back the rising Atlantic tide with her broom. But a little further investigation showed that it is not only within the limits of possibility, but perfectly feasible, to exterminate malaria absolutely from the mosquito end. In the first place, it was quickly found that by a most merciful squeamishness on the part of the plasmodium, it could live only in the juices of one particular genus of mosquito, the Anopheles; and as nowhere, not even in the most benighted regions of Jersey, has this genus been found to form more than about four or five per cent of the total mosquito population, this cuts down our problem to one-twentieth of its apparent original dimensions at once. The ordinary mosquito of commerce (known as Culex) is any number of different kinds of a nuisance, but she does not carry malaria.

Here the trails of the extermination party fork, one of them taking the perfectly obvious but rather troublesome direction of protecting houses and particularly bedrooms with suitable screens and keeping the inhabitants safely behind them from about an hour before sundown on. By this simple method alone, parties of explorers, of campers, of railroad-builders going through swamps, of the laborers on our Panama Canal, have been enabled to live for weeks and months in the most malarious regions with perfect impunity, so long as these precautions were strictly observed. The first experiment of this sort was carried out by Bignami upon a group of laborers in the famous, or rather infamous, Roman Campagna, whose deadly malarial fevers have a classic reputation, and has achieved its latest triumphs in the superb success of Colonel Gorgas at Panama. While this procedure should never be neglected, it is obvious that it involves a good deal of irksome confinement and interferes with freedom of movement, and it will probably be carried out completely only under military or official discipline, or in tropical regions where the risks are so great that its observance is literally a matter of life or death.

The other division of malaria-hunters pursued the trail of the Anopheles to her lair. There they discovered facts which give us practically the whip-hand over malarial and other tropical fevers whenever we choose to exercise it. It had long been known that the breeding-place of mosquitoes was in water; that their eggs when deposited in water floated upon the surface like tiny boats, usually glued together into a raft; that they then turned into larvæ, of which the well-known "wigglers" in the water-butt or the rain-barrel are familiar examples; and that they finally hatched into the complete insect and rose into the air.

Obviously, there were two points at which the destroyers might strike, the egg and the larvæ. It was first found that, while the eggs required no air for their development, the larvæ wiggled up to the surface and inhaled it through curious little tubes developed for this purpose, oddly enough from their tail-ends. If some kind of film could be spread over the surface of the water, through which the larvæ could not obtain air, they would suffocate. The well-known property of oil in "scumming over" water was recalled, two or three stagnant pools were treated with it, and to the delight of the experimenters, not a single larva was able to develop under the circumstances. Here was insecticide number one. The cheapest of oils, crude petroleum, if applied to the pool or marsh in which mosquitoes breed, will almost completely exterminate them. Scores of regions and areas to-day, which were once almost uninhabitable on account of the plague of mosquitoes, are now nearly completely free from these pests by this simple means. An ounce to each fifteen square feet of water-surface is all that is required, though the oiling needs to be repeated carefully several times during the season.

But what of the eggs? They require no air, and it was found impossible to poison them without simply saturating the water with powerful poisons; but an unexpected ally was at our hand. It was early noted that mosquitoes would not breed freely in open rivers or in large ponds or lakes, but why this should be the case was a puzzle. One day an enthusiastic mosquito-student brought home a number of eggs of different species, which he had collected from the neighboring marshes, and put them into his laboratory aquarium for the sake of watching them develop and identifying their species. The next morning, when he went to look at them, they had totally disappeared. Thinking that perhaps the laboratory cat had taken them, and overlooking a most contented twinkle in the corner of the eyes of the minnows that inhabited the aquarium, he went out and collected another series. This time the minnows were ready for him, and before his astonished eyes promptly pounced on the raft of eggs and swallowed them whole. Here was the answer at once: mosquitoes would not develop freely where fish had free access; and this fact is our second most important weapon in the crusade for their extermination. If the pond be large enough, all that is necessary is simply to stock it with any of the local fish, minnows, killies, perch, dace, bass,—and presto! the mosquitoes practically disappear. If it be near some larger lake or river containing fish, then a channel connecting the two, to allow of its stocking, is all that is required.

On the Hackensack marshes to-day trenches are cut to let the water out of the tidal pools; while in low-lying areas, which cannot be thus drained, the central lowest spot is selected, a barrel is sunk at this spot, and four or five "killie" fish are placed in it. Trenches are cut converging into this barrel from the whole of the area to be drained, and behold, no more mosquitoes can breed in that area, and, in the language of the day, "get away with it."

Finally, most consoling of all, it was discovered that, while the ordinary Culex mosquito can breed, going through all the stages from the egg to the complete insect, in about fourteen days, so that any puddle which will remain wet for that length of time, or even such exceedingly temporary collections of water as the rain caught in a tomato-can, in an old rubber boot, in broken crockery, etc., will serve her for a breeding-place, the Anopheles on the other hand takes nearly three months for the completion of her development. So that, while a region might be simply swarming with ordinary mosquitoes, it would frequently be found that the only places which fulfilled all the requirements for breeding-homes for the Anopheles, that is, isolation from running water or larger streams, absence of fish, and persistence for at least three months continuously, would not exceed five or six to the square mile. Drain, fill up, or kerosene these puddles,—for they are often little more than that,—and you put a stop to the malarial infection of that particular region. Incredible as it may seem, places in such a hotbed of fevers as the west coast of Africa, which have been thoroughly investigated, drained, and cleaned up by mosquito-brigades, have actually been freed from further attacks of fever by draining and filling not to exceed twenty or thirty of these breeding-pools.