Rural Hygiene

by Henry N. Ogden

Another pertinent fact discovered by scientific research is that the development of the malarial mosquito is confined to the vicinity of stagnant pools, because in fresh water, where fish are to be found, the eggs and larvæ of the mosquito are a most acceptable fish food. One of the most practical ways, therefore, of getting rid of possible mosquitoes is to make sure that the pond always contains a number of fish. Woods Hutchinson gives the following interesting description of the way this fact was discovered:—

"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 an 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."

Fig. 79.—Top view is of larva of Anopheles. Bottom view is of larva of Culex.

Another factor in the development of the mosquito from the egg to full-grown mosquitohood is that in the larvæ stage air must be supplied, curiously enough, through the tail which projects slightly above the surface of the water as the larvæ hang head downwards (see Fig. 79). If the surface of the water is covered with some impervious material, the mosquito larvæ will be suffocated, and it has been found that oil lends itself most readily to this desirable purpose, applied at the rate of one ounce per fifteen square feet of water surface. The oil spreads out over the surface in a very thin film, but persistent enough to keep off the air supply from the mosquito larvæ. This method, about which much has been written and said, is perhaps the one most commonly employed, and its results have been most satisfactory. In the vicinity of the city of Newark, New Jersey, for instance, is an area of about 3500 acres, 8 miles long and about 3 miles wide, practically all marshland. In 1903 ditches were dug throughout this marsh in such a way that the surface water was drained off, drying the ground so that hay can now be cut where formerly rubber boots were necessary to get onto the ground at all. The consequence has been that the mosquitoes have practically disappeared from this region, formerly frightfully infested, and the cost of the 70 miles of small ditches dug has been amply repaid by the freedom from malaria as well as from the nuisance of the ordinary mosquito.

Other campaigns have been waged, using kerosene or crude petroleum for the coating of ponds or pools. Wherever clear water exists the kerosene treatment is probably best. Where marshland is found, through which the kerosene penetrates with difficulty, drainage is a more useful method.

The size of the pools required for the development of the mosquito is very small. Thousands of mosquitoes may be formed in the amount of water contained in an old tomato can, and barrels half full of rain water or pools of water in the vicinity of an old pump or in the barnyard will afford golden opportunities for mosquitoes looking for a place to lay their eggs. While the ordinary culex requires from one to two weeks only for the complete transition from egg to mosquito, so that a pool filled with rain water and not dried up within that period will be sufficient to develop a brood, the malarial mosquito requires much longer—two or three months—for the full completion of her development. It is, therefore, a simple problem for an individual householder to search out the pools which remain filled with water for a period of two months, and either stock them with fish, drain them entirely, or coat them with kerosene. No hesitation need be felt about the result of this treatment. It will positively eliminate all malaria in the vicinity if the work is thoroughly done.

Limitation of mosquito infection.

The distance that the malarial mosquito can fly is of interest as indicating the distance which one must go from a house, hunting for available pools. All mosquitoes are unable to fly against the wind, so that, as already noted, one side of a swamp may be comparatively free from malaria, while the other side may be overrun with it, merely on account of the direction of the prevailing winds. Some mosquitoes that breed in salt marshes may be carried for miles, so that a land breeze will bring millions of the pests to seashore cottages which, with a sea breeze, are quite free from them. The anopheles has a habit of clinging to weeds, shrubs, and bushes when the wind blows, so that it is seldom carried more than about two hundred yards from the place where it is hatched. If all pools of water, therefore, within this radius are disposed of, the elimination of malaria will logically follow.

If one is obliged to be in a region where malaria is common, the disease can be avoided absolutely by protecting one's self from mosquitoes, and since the anopheles prefer the early morning and evening hours, it is at those times of the day particularly when precautions must be taken. It was once thought that the night air caused malaria, and this had some foundation in fact, because it is in the early evening that the anopheles is on the wing. By staying in the house after sundown and by carefully screening the doors and windows, one may live in a malarial country with perfect immunity. Volunteers have lived for months in the worst malarial regions in the world without a trace of the disease, the only precaution being to keep the doors and windows screened and to prevent mosquitoes from biting.