Handbook of Medical Entomology

by William A. Riley; O. A. Johannsen

In the United States, one of the most important facts which has been brought out by the intensive studies of recent years is that certain species are migratory and that they can travel long distances and become an intolerable pest many miles from their breeding places. This was forcibly emphasized in Dr. Smith's work in New Jersey, when he found that migratory mosquitoes, developing in the salt marshes along the coast, are the dominant species largely responsible for the fame of the New Jersey mosquito. The species concerned are Aedes sollicitans, A. cantator and A. tæniorhynchus. Dr. Smith decided that the first of these might migrate at least forty miles inland. It is obvious that where such species are the dominant pest, local control measures are a useless waste of time and money. Such migratory habits are rare, however, and it is probable that the majority of mosquitoes do not fly any great distance from their breeding places.

While mosquitoes are thought of primarily as a pest of man, there are many species which have never been known to feed upon human or mammalian blood, no matter how favorable the opportunity. According to Howard, Dyar, and Knab, this is true of Culex territans, one of the common mosquitoes in the summer months in the Northern United States. There are some species, probably many, in which the females, like the males, are plant feeders. In experimental work, both sexes are often kept alive for long periods by feeding them upon ripe banana, dried fig, raisins, and the like, and in spite of sweeping assertions that mosquitoes must have a meal of blood in order to stimulate the ovaries to development, some of the common blood-sucking species, notably Culex pipiens, have been bred repeatedly without opportunity to feed upon blood.

The effect of the bite varies greatly with different species and depends upon the susceptibility of the individual bitten. Some persons are driven almost frantic by the attacks of the pests when their companions seem almost unconscious of any inconvenience. Usually, irritation and some degree of inflammation appear shortly following the bite. Not infrequently a hardened wheal or even a nodule forms, and sometimes scratching leads to secondary infection and serious results.

The source of the poison is usually supposed to be the salivary glands of the insect. As we have already pointed out, ([p. 34]), Macloskie believed that one lobe of the gland, on each side, was specialized for forming the poison, while a radically different view is that of Schaudinn, who believed that the irritation is due to the expelled contents of the œsophageal diverticula, which contain a gas and a peculiar type of fungi or bacteria. In numerous attempts, Schaudinn was unable to produce any irritation by applying the triturated salivary glands to a wound, but obtained the typical result when he used the isolated diverticula.

The irritation of the bite may be relieved to some extent by using ammonia water, a one per cent. alcoholic solution of menthol, or preparations of cresol, or carbolic acid. Dr. Howard recommends rubbing the bite gently with a piece of moist toilet soap. Castellani and Chalmers recommend cleansing inflamed bites with one in forty carbolic lotion, followed by dressing with boracic ointment. Of course, scratching should be avoided as much as possible.

Repellents of various kinds are used, for warding off the attacks of the insects. We have often used a mixture of equal parts of oil of pennyroyal and kerosene, applied to the hands and face. Oil of citronella is much used and is less objectionable to some persons. A recommended formula is, oil of citronella one ounce, spirits of camphor one ounce, oil of cedar one-half ounce. A last resort would seem to be the following mixture recommended by Howard, Dyar, and Knab for use by hunters and fishermen in badly infested regions, against mosquitoes and blackflies.

Take 2¼ lbs. of mutton tallow and strain it. While still hot add ½ lb. black tar (Canadian tar). Stir thoroughly and pour into the receptacle in which it is to be contained. When nearly cool stir in three ounces of oil of citronella and 1¼ oz. of pennyroyal.

At night the surest protection is a good bed net. There are many types of these in use, but in order to be serviceable and at the same time comfortable it should be roomy and hung in such a way as to be stretched tightly in every direction. We prefer one suspended from a broad, square frame, supported by a right-angled standard which is fastened to the head of the bed. It must be absolutely free from rents or holes and tucked in securely under the mattress or it will serve merely as a convenient cage to retain mosquitoes which gain an entrance. While such nets are a convenience in any mosquito ridden community, they are essential in regions where disease-carrying species abound. Screening of doors, windows and porches, against the pests is so commonly practiced in this country that its importance and convenience need hardly be urged.

Destruction of mosquitoes and prevention of breeding are of fundamental importance. Such measures demand first, as we have seen, the correct determination of the species which is to be dealt with, and a knowledge of its life-history and habits. If it prove to be one of the migratory forms, it is beyond mere local effort and becomes a problem demanding careful organization and state control. An excellent illustration of the importance and effectiveness of work along these lines is afforded by that in New Jersey, begun by the late Dr. John B. Smith and being pushed with vigor by his successor, Dr. Headlee.

In any case, there is necessity for community action. Even near the coast, where the migratory species are dominant, there are the local species which demand attention and which cannot be reached by any measures directed against the species of the salt marshes. The most important of local measures consist in the destruction of breeding places by filling or draining ponds and pools, clearing up of more temporary breeding places, such as cans, pails, water barrels and the like. Under conditions where complete drainage of swamps is impracticable or undesirable, judicious dredging may result in a pool or series of steep-sided pools deep enough to maintain a supply of fish, which will keep down the mosquito larvæ. Where water receptacles are needed for storage of rain water, they should be protected by careful screening or a film of kerosene over the top of the water, renewed every two weeks or so, so as to prevent mosquitoes from depositing their eggs. When kerosene is used, Water drawn from the bottom of the receptacle will not be contaminated by it to any injurious extent. Where ponds cannot be drained much good will be accomplished by spraying kerosene oil on the surface of the water, or by the introduction of fish which will feed on the larvæ.