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Entered according to Act of Congress, A. D. 1901, by A. N. BELL, in the office
of the Librarian of Congress, at Washington.
E. H. B
INSECTS AS DISSEMINATORS OF DISEASE.*
By CH. WARDELL STILES, Ph. D., Washington, D. C.,
It is sometimes charged that scientists make public too many facts concerning microbes, and that, at the present rate, the laity will soon either become disgusted with hygienic research or will come to the belief that death lurks in everything we eat, drink, touch, or breathe.
That wild hygienic theories and speculations have at times been launched upon the public cannot be denied, and some men have undoubtedly laid themselves open to censure by rushing into print with unjustified conclusions based upon scientific dreams. Let us recall, however, ihat until the cause and method of dissemination of a given disease are known, the hygienist whose duty it is to guard against that malady must take into consideration all imaginable possibilities. When, on the other hand, the cause and source are once matters of positive knowledge, effort may be intelligently concentrated, and the preventive measure given a definite direction.
The case is much the same as in inilitary operations. If you know, either from information obtained by your scouts or from a technical military or geographic knowledge, that your enemy must take a certain route, your plan of campaign is much easier than if the opposing force may arrive by any one of twenty different roads.
Individual workers in the medical sciences are the nation's scouts in hygienic matters, to determine by exactly what route our microbic enemies arrive, just as military scouts learn the enemy's movements or as the traveling salesmen and government
* Annual Toner Lecture, delivered at Georgetown University, April 12, 1901.
consuls are the scouts for a nation's trade. Some erroneous reports are occasionally brought in, and not every Funston captures his Aguinaldo, but boards of health and other sanitary organizations form clearing houses for the scattered investigations; the reports are tested; some are verified, others disproved, and the general plan of campaign against the disease is determined.
When it is definitely proved by what route our microbic enemies arrive, it is clearly proper to lay the matter before the public ---not for the purpose of causing alarm, but in order to enlist cooperation in fighting disease, thus protecting human life and both local and national prosperity.
That some of the hygienic measures recommended to the public do not meet with immediate popular approval or even credulity need not cause surprise, for many of the views advanced are som shall I say—romantic in character, that even experienced scientists call for a confirmatory demonstration of the claims advanced before they are willing to accept them.
This evening I invite your attention to a few of these biologic romances in connection with the subject of disease.
Insects must be looked upon as among our best friends and worst enemies. Some insects are instrumental in the spread of the poilen by which certain plants are fertilized, and they are thus important factors, not only in beautifying our surroundings, but also in the economics of the country; destroy all of the bees and you will destroy one of the iniportant forage plants—namely, clover. Other insects are injurious to plants or animals; it is estimated, for instance, that certain flies cause an annual loss of about $50,000,000 to the cattle industry of the United States. Still other insects are active in the dissemination of disease germs; a certain mosquito in Italy is the normal carrier of a malady which causes that country 15,000 deaths and about 3,000,000 cases of disease per year. When we consider the aggregate human suffering involved and the economic loss brought about by this mosquito, our imagination is almost staggered. Assume, for instance, that these 3,000,000 patients average a loss of one dollar-surely a very low estimate-in time taken from work, in doctors' bills, in drugs, funeral expenses, etc., etc., and we obtain the total of $3,000,000, in addition to the pain endured, and the economic loss through death.
The insects which come more especially into consideration in connection with the diseases of man are mosquitoes, Aies and
fleas; and when we turn to the diseases of animals, we must add to this list the ticks and crustaceans, two groups of arthropods closely related to insects.
Let us now distinguish between two general classes of diseases ---first, those which must necessarily be transmitted by insects; and, second, those in which insects are only accidental, though in many cases important, factors. Thus, malaria is normally transmitted by mosquitoes, and is dependent upon them; kill off mosquitoes and malaria will disappear. Typhoid fever, on the cther hand, may be transmitted by flies, yet it is not dependent upon them; kill off all flies and you will undoubtedly decrease typhoid; but since insects are only accidental and not necessary spreaders of this malady, such measures will not eradicate it.
We may lay down two general biologic rules, which, I believe, are enunciated to-night for the first time: The first rule, to which at present a few exceptions are known, is that diseases which are accidentally spread by insects are caused by parasitic plants, particularly by bacicria; the second, to which no erceptions are as yet known, is that those diseases which are dependent upon insects or other arthropods for their dissemination and transmission are caused by parasitic animals, particularly by sporocoa and
Let us now turn to the germs which cause certain diseases, and follow their transmission.
You have all heard that during the recent war with Spain many cases of typhoid fever occurred among our troops, particularly in certain camps, and you have read that tie germs were spread by flies. For nearly thirteen years it has been known that if the typhoid bacillus is fed to flies, virulent germs can be found in the fly specks. This fact has been demonstrated by experiment. We do not, however, have to assume that the insects must swallow germs in order to disseminate them, for by simply lighting upon typhoid discharges they soil their feet and mouth parts with the bacilli; then fying to the kitchen, the pantry, or diningroom, and lighting upon food, insects can spread typhoid to articles of diet, and tlius disseminate the disease. There can, in fact, be scarcely any reasonable room for doubt that typhoid was spread among the troops in this manner.
Likewise the germs of tuberculosis have been fed to flies, and twenty-four hours later the bacilli have been found in the fly specks. Insects can also carry the bacilli on their feet and mouth parts after feeciing on tubercular sputum.