Handbook of Medical Entomology Part 1

Handbook of Medical Entomology.

by William Albert Riley and Oskar Augustus Johanssen.

PREFACE

The Handbook of Medical Entomology is the outgrowth of a course of lectures along the lines of insect transmission and dissemination of diseases of man given by the senior author in the Department of Entomology of Cornell University during the past six years. More specifically it is an ill.u.s.trated revision and elaboration of his "Notes on the Relation of Insects to Disease" published January, 1912.

Its object is to afford a general survey of the field, and primarily to put the student of medicine and entomology in touch with the discoveries and theories which underlie some of the most important modern work in preventive medicine. At the same time the older phases of the subject--the consideration of poisonous and parasitic forms--have not been ignored.

Considering the rapid s.h.i.+fts in viewpoint, and the development of the subject within recent years, the authors do not indulge in any hopes that the present text will exactly meet the needs of every one specializing in the field,--still less do they regard it as complete or final. The fact that the enormous literature of isolated articles is to be found princ.i.p.ally in foreign periodicals and is therefore difficult of access to many American workers, has led the authors to hope that a summary of the important advances, in the form of a reference book may not prove unwelcome to physicians, sanitarians and working entomologists, and to teachers as a text supplementing lecture work in the subject.

Lengthy as is the bibliography, it covers but a very small fraction of the important contributions to the subject. It will serve only to put those interested in touch with original sources and to open up the field. Of the more general works, special acknowledgment should be made to those of Banks, Brumpt, Castellani and Chalmers, Comstock, Hewitt, Howard, Manson, Mense, Neveau-Lemaire, Nuttall, and Stiles.

To the many who have aided the authors in the years past, by suggestions and by sending specimens and other materials, sincerest thanks is tendered. This is especially due to their colleagues in the Department of Entomology of Cornell University, and to Professor Charles W. Howard, Dr. John Uri Lloyd, Mr. A. H. Ritchie, Dr. I. M. Unger, and Dr. Luzerne Coville.

They wish to express indebtedness to the authors and publishers who have so willingly given permission to use certain ill.u.s.trations. Especially is this acknowledgment due to Professor John Henry Comstock, Dr. L. O.

Howard, Dr. Graham-Smith, and Professor G. H. T. Nuttall. Professor Comstock not only authorized the use of departmental negatives by the late Professor M. V. Slingerland (credited as M. V. S.), but generously put at their disposal the ill.u.s.trations from the MANUAL FOR THE STUDY OF INSECTS and from the SPIDER BOOK. Figures 5 and 111 are from Peter's "Der Arzt und die Heilkunst in der deutschen Vergangenheit." It should be noted that on examining the original, it is found that Gottfried's figure relates to an event antedating the typical epidemic of dancing mania.

WM. A. RILEY.

O. A. JOHANNSEN.

CORNELL UNIVERSITY, January, 1915.

CHAPTER I.

INTRODUCTION

EARLY SUGGESTIONS REGARDING THE 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 p.r.o.ne 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 acc.u.mulate 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 role 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 ma.s.s 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 role 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 ca.n.a.l or other cavities of man, may exist there for a greater or less period. For example, certain fly larvae, 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 which cling to their bodies or appendages. Such, for instance, is the relation of the house-fly to the dissemination of typhoid.

As direct inoculators, biting or piercing species may take up from a diseased man or animal, germs which, clinging to the mouth parts, are inoculated directly into the blood of the insect's next victim. It it thus that horse-flies may occasionally transmit anthrax. Similarly, species of spiders and other forms which are ordinarily perfectly harmless, may accidentally convey and inoculate pyogenic bacteria.

It is as essential hosts of disease germs that arthropods play their most important role. In such cases an essential part of the life cycle of the pathogenic organism is undergone in the insect. In other words, without the arthropod host the disease-producing organism cannot complete its development. As ill.u.s.trations may be cited the relation of the Anopheles mosquito to the malarial parasite, and the relation of the cattle tick to Texas fever.

A little consideration will show that this is the most important of the group. Typhoid fever is carried by water or by contaminated milk, and in various other ways, as well as by the house-fly. Kill all the house-flies and typhoid would still exist. On the other hand, malaria is carried only by the mosquito, because an essential part of the development of the malarial parasite is undergone in this insect.

Exterminate all of the mosquitoes of certain species and the dissemination of human malaria is absolutely prevented.

Once an arthropod becomes an essential host for a given parasite it may disseminate infection in three different ways:

1. By infecting man or animals who ingest it. It is thus, for example, that man, dog, or cat, becomes infected with the double-pored dog tapeworm, _Dipylidium caninum_. The cysticercoid stage occurs in the dog louse, or in the dog or cat fleas, and by accidentally ingesting the infested insect the vertebrate becomes infested. Similarly, _Hymenolepis diminuta_, a common tapeworm of rats and mice, and occasional in man, undergoes part of its life cycle in various meal-infesting insects, and is accidentally taken up by its definitive host. It is very probable that man becomes infested with _Dracunculus (Filaria) medinensis_ through swallowing in drinking water, the crustacean, _Cyclops_, containing the larvae of this worm.

2. By infecting man or animals on whose skin or mucous membranes the insect host may be crushed or may deposit its excrement. The pathogenic organism may then actively penetrate, or may be inoculated by scratching. The causative organism of typhus fever is thus transmitted by the body louse.

3. By direct inoculation by its bite, the insect host may transfer the parasite which has undergone development within it. The malarial parasite is thus transferred by mosquitoes; the Texas fever parasite by cattle ticks.

CHAPTER II.

ARTHROPODS WHICH ARE DIRECTLY POISONOUS

Of all the myriads of insects and related forms, a very few are of direct use to man, some few others have forced his approbation on account of their wonderful beauty, but the great hordes of them are loathed or regarded as directly dangerous. As a matter of fact, only a very small number are in the slightest degree poisonous to man or to the higher animals. The result is that entomologists and lovers of nature, intent upon dissipating the foolish dread of insects, are sometimes inclined to go to the extreme of discrediting all statements of serious injury from the bites or stings of any species.