TRANSMISSION OF DISEASE BY INSECTS

Until very recent years insects and their allies have been considered as of economic importance merely in so far as they are an annoyance or direct menace to man, or his flocks and herds, or are injurious to his crops. It is only within the past fifteen years that there has sprung into prominence the knowledge that in another and much more insiduous manner, they may be the enemy of mankind, that they may be among the most important of the disseminators of disease. In this brief period, such knowledge has completely revolutionized our methods of control of certain diseases, and has become an important weapon in the fight for the conservation of health.

It is nowhere truer than in the case under consideration that however abrupt may be their coming into prominence, great movements and great discoveries do not arise suddenly. Centuries ago there was suggested the possibility that insects were concerned with the spread of disease, and from time to time there have appeared keen suggestions and logical hypotheses along this line, that lead us to marvel that the establishment of the truths should have been so long delayed.

One of the earliest of these references is by the Italian physician, Mercurialis, who lived from 1530 to 1607, during a period when Europe was being ravaged by the dread "black death", or plague. Concerning its transmission he wrote: "There can be no doubt that flies feed on the internal secretions of the diseased and dying, then, flying away, they deposit their excretions on the food in neighboring dwellings, and persons who eat of it are thus infected."

It would be difficult to formulate more clearly this aspect of the facts as we know them to-day, though it must always be borne in mind that we are prone to interpret such statements in the light of present-day knowledge. Mercurialis had no conception of the animate nature of contagion, and his statement was little more than a lucky guess.

Much more worthy of consideration is the approval which was given to his view by the German Jesuit, Athanasius Kircher in 1658. One cannot read carefully his works without believing that long before Leeuwenhook's discovery, Kircher had seen the larger species of bacteria. Moreover, he attributed the production of disease to these organisms and formulated, vaguely, to be sure, a theory of the animate nature of contagion. It has taken two and a half centuries to accumulate the facts to prove his hypothesis.

The theory of Mercurialis was not wholly lost sight of, for in the medical literature of the eighteenth century there are scattered references to flies as carriers of disease. Such a view seems even to have been more or less popularly accepted, in some cases. Gudger (1910), has pointed out that, as far back as 1769, Edward Bancroft, in "An Essay on the Natural History of Guiana in South America," wrote concerning the contagious skin-disease known as "Yaws": "It is usually believed that this disorder is communicated by the flies who have been feasting on a diseased object, to those persons who have sores, or scratches, which are uncovered; and from many observations, I think this is not improbable, as none ever receive this disorder whose skins are whole."

Approaching more closely the present epoch, we find that in 1848, Dr. Josiah Nott, of Mobile, Alabama, published a remarkable article on the cause of yellow fever, in which he presented "reasons for supposing its specific cause to exist in some form of insect life." As a matter of fact, the bearing of Nott's work on present day ideas of the insect transmission of disease has been very curiously overrated. The common interpretation of his theory has been deduced from a few isolated sentences, but his argument appears quite differently when the entire article is studied. It must be remembered that he wrote at a period before the epoch-making discoveries of Pasteur and before the recognition of micro-organisms as factors in the cause of disease. His article is a masterly refutation of the theory of "malarial" origin of "all the fevers of hot climates," but he uses the term "insect" as applicable to the lower forms of life, and specific references to "mosquitoes," "aphids," "cotton-worms," and others, are merely in the way of similes.

But, while Nott's ideas regarding the relation of insects to yellow fever were vague and indefinite, it was almost contemporaneously that the French physician, Louis Daniel Beauperthuy argued in the most explicit possible manner, that yellow fever and various others are transmitted by mosquitoes. In the light of the data which were available when he wrote, in 1853, it is not surprising that he erred by thinking that the source of the virus was decomposing matter which the mosquito took up and accidentally inoculated into man. Beauperthuy not only discussed the rôle of mosquitoes in the transmission of disease, but he taught, less clearly, that house-flies scatter pathogenic organisms. It seems that Boyce (1909) who quotes extensively from this pioneer work, does not go too far when he says "It is Dr. Beauperthuy whom we must regard as the father of the doctrine of insect-borne disease."

In this connection, mention must be made of the scholarly article by the American physician, A. F. A. King who, in 1883, brought together an all but conclusive mass of argument in support of his belief that malaria was caused by mosquitoes. At about the same time, Finley, of Havana, was forcefully presenting his view that the mosquito played the chief rôle in the spread of yellow fever.

To enter more fully into the general historical discussion is beyond the scope of this book. We shall have occasion to make more explicit references in considering various insect-borne diseases. Enough has been said here to emphasize that the recognition of insects as factors in the spread of disease was long presaged, and that there were not wanting keen thinkers who, with a background of present-day conceptions of the nature of disease, might have been in the front rank of investigators along these lines.

THE WAYS IN WHICH ARTHROPODS MAY AFFECT THE HEALTH OF MAN

When we consider the ways in which insects and their allies may affect the health of man, we find that we may treat them under three main groups:

A. They may be directly poisonous. Such, for example, are the scorpions, certain spiders and mites, some of the predaceous bugs, and stinging insects. Even such forms as the mosquito deserve some consideration from this viewpoint.

B. They may be parasitic, living more or less permanently on or in the body and deriving their sustenance from it.

Of the parasitic arthropods we may distinguish, first, the true parasites, those which have adopted and become confirmed in the parasitic habit. Such are the itch mites, the lice, fleas, and the majority of the forms to be considered as parasitic.

In addition to these, we may distinguish a group of accidental, or facultative parasites, species which are normally free-living, feeding on decaying substances, but which when accidentally introduced into the alimentary canal or other cavities of man, may exist there for a greater or less period. For example, certain fly larvæ, or maggots, normally feeding in putrifying meat, have been known to occur as accidental or facultative parasites in the stomach of man.

C. Finally, and most important, arthropods may be transmitters and disseminators of disease. In this capacity they may function in one of three ways; as simple carriers, as direct inoculators, or as essential hosts of disease germs.

As simple carriers, they may, in a wholly incidental manner, transport from the diseased to the healthy, or from filth to food, pathogenic germs