"Medical ENTOMOLOGY. - What used to be called comprehensively" economic "entomology is now more conveniently divided into two distinct branches - economic (see Economic Entomology) and medical. In its medical bearings the scope of entomology comprehends not insects only but arthropoda of every kind directly hurtful to health. Until about 1880 the fact that many arthropoda can bite and sting, and that some are troublesome parasites of man, was not thought important enough in medical practice to require special attention in their entomological aspect; but when in the last quarter of the 19th century the startling announcement was made that one of the commonest diseases of the tropics is propagated by a bloodsucking insect it was soon realized that light might be thrown on many obscure problems in the causation of infective disease by a precise knowledge of the arthropoda that come into contact with man. This illuminating doctrine was first opened out in 1879, when Sir Patrick Manson, at that time a plain medical practitioner in China, published the essential parts of an experimental proof that the parasitic worm which causes the disfiguring febrile and subfebrile diseases known collectively as filariasis is communicated from man to man by a common house-haunting mosquito. It is true that long before 1879 seasonal fevers in several parts of the world had been ascribed, not only in popular tradition but also by plausible argument, to the bites of insects, but these vague glimmerings of the truth had not been regarded very seriously, and Manson was the first to demonstrate, by verifiable experiment, that in such cases the insect plays an indispensable triple part, namely, (1) after infecting itself with parasites responsible for the fever by imbibing the blood of a human being infected with them, (2) to provide in its own organs and tissues the appropriate medium for the growth and development of the parasites so absorbed and finally (3) to carry the parasites so fostered to fresh human victims. The precise manner in which the filariated mosquito passes on its acquired infection was not settled by Manson, but his wonderful discovery originated that new branch of study where a full and exact knowledge of all the arthropoda that touch man in his person, his dwellings, and his food and drink, finds, as Medical Entomology, a rational application in the control of disease.
Subject-matter
In dealing with the subject-matter of Medical Entomology it is convenient rather than severely logical to group the arthropoda inimical to health in five categories, according to the nature, the direct effects and the after consequences of their results, namely as (1) simply parasitic, (2) simply pre daceous, (3) specifically infective, (4) contaminative and (5) venomous, the third and fourth being far the most important.
Simple Parasites
Here are included all those arthropod parasites that are hurtful solely by their presence on or in the body and not by any subsequently disclosed effects. Such are the itch-mites (Sarcoptes) specific to man, and the numerous food-mites, animal-mites, bird-mites, insect-mites, harvest-mites, etc., which in certain circumstances or at particular seasons may attack man, though not normally parasitic on him or in every case parasitic at all in normal habit. Though they usually attack the epidermis these mites may get into the bowel, or the lung, or the bladder. Such also are those muscoid flies which in their maggot stage are constantly and entirely dependent on warm-blooded animals for their existence: two such species of maggots are notorious subcutaneous parasites of man, namely the ' macaw worm" (Dermatobia hominis) in tropical America, and the "tumbu" (Cordylobia anthropophaga) in tropical Africa. Bluebottles, flesh-flies and other domestic flies, which normally deposit their eggs or larval progeny in decomposing meat and carrion, may be attracted for this purpose to foul and neglected wounds, or to the eyes, nostrils and other natural orifices of unclean or incapable humanity, with dire results: the "screw-worm" flies (Cochliomyia in America and Chrysomyia in India) are said to be particularly prone to this deplorably mistaken instinct. Living maggots of many species of flies may find a congenial abode in the human bowel, as also may larvae of other orders of insects. Other simple parasites of man are the "Chigger" flea (Dermatophilus penetrans) common in certain parts of tropical America and Africa, the pregnant female of which embeds herself in the skin; and the "Congo floor-maggot" (Auchmeromyia) of tropical Africa, a maggot which infests native huts like a bedbug and sucks the blood of sleeping people, though the parent fly is harmless. An abnormal and misdirected parasite that, in places where large snakes are numerous, may get encysted in the human viscera, is the larva of the extraordinary wormlike arthropod Porocephalus, the adult of which is a bloodthirsty inhabitant of the lungs of snakes.
Predaceous Arthropoda
To this category may be relegated a miscellany of blood-sucking insects, etc., the bite of which may be painful or may even cause severe inflammation, but is not known to be followed by any specific infection apart from a chance of ordinary septic contamination as a result of scratching. Though there are times and places when these insects may be an intolerable burden to travellers or to country folk, they have no preference for human blood and no particular affection for houses. They cannot therefore be regarded as a standing menace to the public health, although they may be treated with a reasonable amount of suspicion, because some of them are known to spread specific infections among domestic stock, and also because they may be chance mechanical carriers of some individual contamination, or may have parasites proper to themselves which might possibly under certain circumstances be transferred to man. This suspicious assemblage includes many species of mosquitos and gadflies, a lot of midges (Simulium, Psychoda, Culicoides, etc.), and numerous species of muscoid flies (Stomox y s, Haematobia, Lyperosia, etc.) that usually feed on cattle; also various flies (Leplidae, Asilidae, etc.) that normally feed on other insects; and some kinds of plant bugs and leaf hoppers in which the thirst for blood is an aberration.
Specifically Infective Arthropoda
The parasitic and predaceous forms so far enumerated are for the most part obnoxious rather than greatly dangerous, and the trouble they cause does not linger after their disappearance; but we have now to deal with an assortment of parasitic and predaceous arthropoda, which, by propagating microparasites pathogenic to man, cause ill effects that outweigh and outlast any direct damage that they themselves can inflict. Such arthropoda, having severally a definite biological association with a particular disease, and being specific "nurses" or intermediary hosts of the parasite actually causing the disease, are known as "carriers" or "vectors." Their function is beautifully exemplified in Sir Ronald Ross's great discovery of the way by which the parasites of malarial fevers are maintained and disseminated - a discovery which must here have precedence, not that it was historically the first of its kind, but because it was originally presented in a complete form and best illustrates the emergence of Medical Entomology as a distinct study.
Independently of Ross's investigations, it was known that the parasites multiplied periodically by non-sexual fission (schizogony) in the blood of malaria patients, and that they also gave issue to prospectively sexual bodies (gametocytes), male and female, which did not become ripe until after the blood had been withdrawn from the blood vessels; and it had been discovered by MacCallum that when the ripe sexual elements (gametes) did become revealed in the drawn blood the males and females paired together to form united couples or zygotes; it was known, in short, that while the parasites multiply non-sexually in the blood of the individual human sufferer, the process by which they are maintained as specific parasites of the human race must take place, somehow, outside the individual human host. Manson inferred, from his own earlier investigations of the filaria parasite, that it must be in the interior of some blood-sucking arthropod, probably a mosquito, and the arduous proof of this hypothesis was unravelled by Ross. Ross followed the parasites from the vertebrate host into the stomach of the mosquito, and thence into the insect's stomach wall, where the zygotes encyst and establish themselves as parasitic oocysts. Under favourable conditions the oocysts increase in size by the internal proliferation of their contents, and in about ten days ripen into sporocysts, full of minute spore elements or sporozoites. By the rupture of the sporocysts the sporozoites escape into the body cavity of the mosquito and accumulate considerably in its salivary glands, which also lie in the body cavity, so that when the now infective insect bites a healthy person the sporozoites are injected with its saliva into that person's blood, where they grow and multiply non-sexually, and after a definite interval usually reveal their presence in a paroxysm of malarial fever.
The mosquito thus is the medium for the necessary sexual reproduction of the malaria parasites; eventually the insect not only becomes a kind of supersporocyst at whose expense the actual sporocysts and their sporozoites are developed and nourished, but also acts as the locomotor instrument for distributing the sporozoites and planting them in their future sphere of action.
So far as is known, the only mosquitos that serve as nurses for the malaria parasites of man are mosquitos of the genus Anopheles. It can hardly be supposed that these parasites originated at the outset either in man or in Anopheles mosquitos. Kindred parasites inhabit the blood of other mammals as well as of birds and reptiles and are distributed by other kinds of blood-sucking insects, and it is reasonable to believe that all may have had a common origin and, up to a certain point, a common line of evolution, and that the species now adapted to man were derived, through the intermediation of Anopheles mosquitos, from forms whose evolution had been hammered out quite independently of the human race. Not every one of the many species of Anopheles is susceptible to infection, nor of the susceptible species are all equally so, or indifferently so under all conditions. Again, not all species of Anopheles are disposed to attack man, or to shelter in houses. Furthermore, although under favourable conditions the sexual development of the parasites in the mosquito is completed in about ten days, and although under ordinary circumstances the sporozoites would be discharged from the salivary glands of the infected insect in the course of a few bites, yet the sexual development may be much delayed by cold and other unfavourable conditions, and the salivary glands might remain infective for a long time if the insect remained inactive or torpid.
Ross's great work, which was experimentally elucidated with a common malaria parasite of birds, was the outcome of Manson's illuminative discovery of the necessary intermediation of mosquitos in propagating lymphatic filariasis, a disease, common in most tropical countries, caused by the invasion of the lymphatic system by the parasitic worm Filaria bancrofti. In certain phases of this disease the female worm gives issue to swarms of minute embryos, or microfilariae, which are carried into the patient's blood and are usually found in the superficial blood-vessels at night. Manson, in 1878, persuaded a Chinaman with microfilariae in his blood to suffer himself to be bitten by some common house mesquites during sleep, and on examining the replete insects at intervals during a number of days following their infective repast he traced the ingested microfilariae through definite stages of larval development in the insects' stomach and muscles. Subsequent observers, of whom Dr. G. C. Low was the first to publish his investigations, discovered that the larval filariae find their way into the proboscis of the mosquito, whence on occasion offered they escape into the skin of a fresh human victim. More than a dozen species of mosquitos are now known to be capable of acting as intermediary host to the larvae of the worm of lymphatic filariasis.
Yellow fever is another disease which, so far as is known, is communicated from man to man exclusively by mosquitos of one species, namely, the notorious Stegomyia fasciata. The connexion between this characteristically house-haunting insect and the disease was inferred by Dr. Finlay of Havana in 1881, and was experimentally demonstrated by the U.S. Commission under Dr. Walter Reed in 1900. Here again, as in malaria and filariasis, the specific parasite of the disease must undergo a definite development within the insect, since the insect can become infected only if it feeds on a patient during the first three days of fever, and does not become infective until at least 12 days after so feeding.
Another disease spread in this way by mosquitoes is dengue fever. The actual virus of the disease has not yet been demonstrated, but there is good proof that it is transmitted from sick to healthy persons by two of the commonest house-haunting species of the tropics, namely Stegomyia fasciata and Cu'ex fatigans. Mosquitos may, possibly, spread other infections mechanically. Indeed they are known to be one of several kinds of flies that mechanically transport the eggs of the "macaw worm" (Dermatobia hominis) to its host. But such mechanical work is something different from the constant and indispensable biological accommodation which mosquitos alone provide for the parasites of filariasis, malarial fevers, and yellow fever, outside the human body.
Almost as notorious in recent history as the Stegomyia and Anopheles that at one time threatened to stop the construction of the Panama Canal are the tsetse flies, which, as intermediary hosts of the trypanosome parasites of sleeping-sickness, have depopulated certain parts of tropical Africa. These bloodthirsty insects, which constitute the genus Glossina, are akin to the stable-fly (Stomoxys), the house-fly, and the blow-fly, all being included in the great family of Muscidae. Except for one species, Glossina tachinoides, which ranges into the extreme south-western corner of Arabia, the entire genus Glossina is restricted to the torrid regions of the African continent. The trypanosome animalcules, which, so far as the species that cause disease in man and domestic animals in Africa are concerned, are associated mainly with tsetse flies, occur in the state of nature in the blood of all classes of vertebrate animals in most parts of the world; many, if not all, of the trypanosomes living naturally in the blood of wild animals appear to be harmless to their hosts, and it is perhaps because the trypanosomes that get into man and domestic animals, by the agency mainly of biting insects, are trypanosomes out of their proper place, that they are hurtful.
Recent investigations, beginning with the discovery made by Sir David Bruce in 1895 that the destructive "tsetse-fly disease" of domestic animals is due to a trypanosome parasite in the blood, have established the facts that there are two varieties of the human trypanosome disease known as sleeping-sickness, one predominant in equatorial Africa and transmitted mainly by Glossina palpalis, the other predominant in south-eastern tropical Africa and transmitted mainly by Glossina morsitans; that in both cases the respective trypanosome undergoes definite stages of development in the fly's gut and at last settles itself and continues to proliferate in the fly's salivary glands; and that a fly once infected remains infective by its saliva for the rest of its life. Thus in a general way the part taken by tsetse flies in spreading sleeping-sickness is similar to that taken by Anophe l es in spreading malaria. But there is this difference: first, that the development of the trypanosome parasite in the tsetse fly is not actually known to have any sexual interpretation; and secondly, that although in epidemics of sleeping-sickness the fly derives its infection from human sufferers, yet in other circumstances it may possibly become infected from a wild animal, since trypanosomes morphologically indistinguishable from those that cause disease in man in Africa have been observed living naturally in some of the big game animals and also in the wild tsetse flies. The existence of these natural foci of possible - though not demonstrated - infection must enormously increase the difficulty of controlling sleeping-sickness. On the other hand, the laboratory experiments of Bruce and others have shown that only a small percentage of tsetse flies fed on infected blood take the infection. Finall y it should be mentioned that there are some who still believe that in certain circumstances a tsetse fly may transfer infection from man to man mechanically on its proboscis.
In the instances reviewed so far a blood-sucking insect - mosquito or tsetse fly - acquires a specific infection while feeding, incubates it for a definite term during which infectivity is latent, and then at length transmits it, in the act of feeding, to another susceptible host. In this manner also Conorhinus inegistus and other Reduviid bugs have been shown to transmit the trypanosome of Chagas's disease in South America, and in like manner the midge Phlebotomus papatasii is believed to transmit the virus of phlebotomus fever. In the same way, probably, as Manson suggested, the Tabanid flies Chrysops dimidiata and salacea in West Africa foster and transmit a filarial worm, Filaria loa, that is troublesome to man there. It is possible, too, that the Leishmania parasite of kalaazar is communicated from man to man in India, as W. S. Patton thinks, by the bedbug, and in the Mediterranean region, as others suppose, by fleas.
A specific infection acquired by a blood-sucking arthropod in feeding is not always passed on by means of the infected saliva: it may be communicated in its juices and excreta. It is in this way, as numerous observers have ascertained, that body lice infected by imbibing the blood of sufferers from typhus fever, relapsing fever, and trench fever usually transmit the respective virus to a fresh host who, in seeking relief from their presence by scratching, may crush them or rub their infected excrement into his broken skin; in the case of typhus the insect is said to transmit infection by its bite also. The part played by the fleas of rats and other rodents in epidemics of bubonic plague is similar, except that the original foci of infection are infected rodents. From these the fleas absorb the plague bacillus, which, as shown by the Indian Plague Commission, multiplies in their stomach and is voided with undiminished virulence in their excreta. Bacot and Martin have also shown that a flea may sometimes get its stomach so stuffed with the prolific bacilli that it may regurgitate some of them when biting a fresh victim.
A specific infection acquired by a bloodsucking arthropod is not always terminated in the individual: it may be bequeathed to the offspring. This has been shown by Dutton and Todd to be the course of events with the tick Ornithodorus moubata, which propagates the spirillum of the African variety of relapsing fever. Hereditary transmission of an acquired infection is said by Nicolle and other authorities to take place in lice with the spirillum of relapsing fever. Another aspect of the same phenomenon occurs in the case of the tick, Dermatocentor venustus, that serves as intermediary for the virus of the typhus-like disease known as Rocky Mountain spotted fever. Here, according to Ricketts, it is the minute larval tick that becomes infected, perhaps from natural foci of the infection in ground squirrels and various other local rodents, perhaps by inheritance. The larva, however, has to undergo many vicissitudes before it is transformed into an adult; accommodation has to be found on three different hosts, and in the case of Dermatocentor venustus these vicissitudes may be spun out for two years before the larva reaches the adult stage in which the virus subsisting in it can become infective to man. These inherited and postponed infections render preventive measures correspondingly tedious and difficult, even when there are no natural foci of infection to be reckoned with. The noxa of the exanthematous disease known as Japanese river fever, attributed to the bite of the "harvest-mite" larva of a velvet mite, is thought to be inherited by that larva.
Finally, a specific infection can be transmitted from man to man by the intermediation of an arthropod that is neither parasitic nor predaceous. This was shown by Fedshenko to be the case with the guinea-worm (Dracunculus medinensis), a subcutaneous parasite of man (and other animals) in certain warm parts of the globe. The embryos of the worm after being set free are ingested by and undergo a definite larval development in fresh-water copepod crustacea of the genus Cyclops, and man is infected by swallowing the infected Cyclops in unfiltered water. The ribbon-worm (Dibothriocephalus latus) is also thus fostered and disseminated by copepod crustacea, as has recently been explained by Janicki and Rosen; only in this ease the infected copepods are first swallowed by a fresh-water fish in which the larval ribbon-worms develop still further before they get to man. In the Far East several species of fresh-water crabs harbour the cercaria stage of the lung fluke.
Contaminative Arthropoda
It is necessary to discriminate between insects which, like Anopheles and Glossina, supply the indispensable frame for a particular pathogenic microorganism, and insects which, like house-flies, are casual but not necessary agents in the promiscuous dispersal of microbes of many kinds. Among insects of the latter sort, indiscriminately pollutive but not specifically infective, all common house pests, such as cockroaches, crickets, ants, domestic beetles and flies, must be included. By far the most dangerous of them - perhaps the most insidiously dangerous of all the insects that come into the medical purview - is the common house-fly, Alusca domestica, by reason of its ubiquity, its fecundity, its persistent activity, and, above all, by the profane impartiality with which it distributes its attentions, now upon the purgamenta and faeculenta of the community, and now upon the kitchen, the larder and the refreshment-table. A house-fly may carry contamination on its feet, and so is said to spread purulent ophthalmia, anthrax and perhaps other infections. Or, after feeding upon infected excrement it may void the contents of its crop or its intestine into food or drink, and in this way house-flies have played their part in outbreaks of epidemic diarrhoea, typhoid fevers, bacillary dysentery and, perhaps, cholera, and may also disseminate eggs of intestinal worms and cysts of intestinal protozoa. It must not, however, be supposed that house-flies are always common carriers of all the germs they may pick up: much, fortunately, depends upon coexistent circumstances, and all germs cannot stand such rough and precarious transport.
It is possible that the beetles, moths, mites, etc., which in all parts of the world infest grain, meal, biscuit and dried provisions of all kinds, may at times so befoul and vitiate those commodities as to make them harmful as food though not actually spoiling them for certain markets. Beyond the fact that larvae of the meal moth and the meal beetle may be infected with the larvae of a tapeworm, nothing very definite is known at present on this point.
Venomous Arthropoda
Some of the insects and ticks mentioned on other grounds might be included here also. The bite of ticks in particular is notoriously venomous, and may cause fever and temporary paralysis by the toxic properties of the saliva; so also may that of some of the gadflies, particularly those of the genus Chrysops. Scorpions, bees, wasps, etc., all have special venom glands, the secretion of which is variously neurotoxic and haemolytic like that of snakes, and have special organs for injecting the secretion. The venom of some spiders is known to be seriously toxic even to man: that of Lathrodectes is particularly so, and that of the South American "Podadora" (Glyptocranium gastracanthoides) is said by Escomel to be sometimes fatal to man. In West Africa the larva (known locally as "Fura") of a tiger-beetle is said by Pollard to inflict a bite having effects almost as severe as the sting of a scorpion. Many species of lepidopterous caterpillars are liberally provided with spines or finely barbed hairs having venomous properties, and numerous kinds of insects, besides the well-known blister beetles, can eject irritant and vesicant secretions. But from the entomological standpoint these venomous arthropoda are not important.
Process of Research
Having surveyed the field, it remains to consider the economic aspects of medical entomology. It is plain that the actual discovery of the pathogenic capabilities of any particular arthropod is most likely to be made by the medical or pathological specialist - the history of the subject entirely confirms this assumption. The treatment of pathological effects is even more plainly and exclusively a medical matter. But, once the pathogenetic significance of a species has been established, it becomes the first concern of medical entomology to unravel the biological history of that species in every detail, however apparently trivial, and to investigate every circumstance that may be supposed to influence its noxious powers, with the object of circumventing its activity or of restricting its existence in propinquity to man.
Where the harmful species is a specific parasite of man, or - like the yellow-fever mosquito - constantly haunts domiciles, its biology is usually easy to follow, and its control, in an educated and convinced community, should not be difficult. But where it is a free ranger, like the tsetse fly and many species of Anopheles, the investigation of its biology may be extremely difficult.
An entomological investigation must comprehend every stage of the creature's existence, from the egg to the procreant adult. It must include not only its natural affinities, specific characters and anatomical structure, but also its distribution and seasonal prevalence, 1L. 7. abits, hiding-places and hours of work and rest; its powers and usual range of locomotion, and its propensity to extend its range at any season; its fecundity, sexual instincts and manner of reproduction; its times and places of breeding, method of dispensing its eggs and providing for its larvae; and its length of life in every stage of its existence. It must also follow up, in each separate stage, the general conditions of existence, such as food preferences, meteorological requirements and means of withstanding vicissitudes of season and climate, adaptations for transport and dispersal, and all the circumstances of the organic environment - natural shelters, direct and indirect help-givers in the struggle for existence, parasites, enemies and rivals; for, as Darwin explained so well, the species of a fauna do not stand alone in nature, but all hang together in most complicated interdependence. Furthermore, the investigation must embrace the varying circumstances - meteorological, seasonal, etc. - which in the case of a specifically infective species influence its reception and retention of infection.
With some assured knowledge of the bionomy of a harmful species, it becomes the practical work of medical entomology to consider how its harmful activities can be forestalled or it itself entirely banished from the vicinity of man - whether by clearing off everything that can shelter the adult; or by abolishing or restricting or periodically devastating its breeding-places; or by cultivating its parasites and natural enemies; or by depriving it of its ultimate food resources; or by direct attack with insecticides and other destructive appliances; or by screens and defensive apparatus; or by educational propaganda. All these principles have their application, which must be decided with regard to local conditions and resources.
As a general proposition it may be maintained that, although there are many occasions when systematic attack with destructive appliances is advisable, and several instances where it has been eminently successful - the Herculean cleansing of the yellow-fever and malaria stricken Panama Canal Zone from infective mosquitos by Gen. Gorgas and his staff being the crown and garland of them all - yet the only permanently successful procedure against a harmful arthropod is to upset its environment by steady perseverance in the ordinary principles of hygiene.
The truth of this proposition is illustrated by the history of malaria in England. Not so very long ago malarial fevers were quite common in many parts of that country: in 1657 John Evelyn's son Richard died at Deptford "after six fits of a quartan ague": in Shakespeare's time ague must have been an every-day affair, for it is a familiar word in the mouth of his people, from homely persons like Mistress Quickly to great personages like Hotspur and Richard II.: Sir John Falstaff died of a "burning quotidian tertian," old John of Gaunt expired in an ague fit with "frozen admonitions" on his lips, and Sir Andrew Aguecheek is a catch-name for a shivering coward. Yet, except in a few water-logged spots where it may still linger, ague quietly disappeared from England ere ever any connexion with Anopheles mosquitos was dreamt of, or ever any malaria parasite had been discovered by a Laveran, although the species of Anopheles that used to spread it in England are still familiar British insects to those who know where to look for them. Anopheles mosquitos prefer to shelter in dark damp habitations, and they do not fly far from the waters where they are bred; and what seems to have happened in England is that, in the general improvement of drainage and sanitation, conditions have changed so completely that the population no longer lives among the breeding-haunts of Anopheles in houses that offer any attraction to those insects, and that thus the old communion between man and Anopheles, by which malaria was fostered, has gradually been dissolved. Something of the same sort has happened in the well-kept European quarters of some of the large towns of India.
It follows that medical entomology is really a branch of hygiene - a branch that finds its fullest application not so much in settled countries where man has long since set a bound to organic nature, as in those vast imperfectly developed tropical territories where sanitary arrangements are crude or non-existent and man is struggling with his organic environment.
As a branch of hygiene medical entomology should also be concerned with questions of the unwitting dissemination of pathogenic arthropoda in the intercourse of trade and travel, and in the special circumstances of war; and also with difficult problems relating to the destruction of such elements of the fauna of a particular territory as are believed to foster some local noxious arthropod, and to the introduction of such exotic species and parasites as are thought likely to destroy some local noxious arthropod, as sanitary measures; for such measures, if lightly undertaken, may start new mischiefs without mitigating those they are intended to check. (A. A.)
