Insects and Diseases / A Popular Account of the Way in Which Insects may Spread or Cause some of our Common Diseases

Professor R.A. Cooley of Montana, from whose report the above quotation is taken, has also made studies of the habits of the tick and believes there can be no doubt that it is the disseminator of the disease.

Relapsing Fever. The relapsing fever is an infectious disease or possibly a group of closely related infectious diseases occurring in various parts of the world. Occasionally it is introduced into America, but it does not seem to spread here. It has been shown that the disease is communicated from one person to another by means of blood-sucking insects. In Central Africa where the disease is very prevalent a certain common tick (Ornithodoros moubata) (Fig. 18) is known to transmit the disease. This tick lives in the resting places and around the huts of the natives and has habits very similar to the bedbug of other climes, feeding at night and hiding during the day. It attacks both man and beast and is one of the most dreaded of all the African pests.

Nathan Bank, our foremost authority on ticks, in summing up the evidence against them says:

MITES

The mites are closely related to the ticks, and although none of them has yet been shown to be responsible for the spread of any disease their habits are such that it would be entirely possible for some to transmit certain diseases from one host to another, from animal to animal, from animal to man, or from man to man. A number of these mites produce certain serious diseases among various domestic animals and a few are responsible for certain diseases of men.

Face-mites. Living in the sweat-glands at the roots of hairs and in diseased follicles in the skin of man and some domestic animals are curious little parasites that look as much like worms as mites (Fig. 19). Such diseased follicles become filled with fatty matter, the upper end becomes hard and black and in man are known as blackheads. If one of these blackheads is forced out and the fatty substance dissolved with ether the mites may be found in all stages of development. The young have six legs, the adult eight. The body is elongated and transversely wrinkled. In man they are usually found about the nose and chin and neck where they do no particular harm except to mar the appearance of the host and to indicate that his skin has not had the care it should have. Very recently certain investigators have found that the lepræ bacilli are often closely associated with these face mites and believe that they may possibly aid in the dissemination of leprosy. It is also thought that they may sometimes be the cause of cancer, but as yet these theories have not been proven by any conclusive experiment.

In dogs and cats these same or very similar parasites cause great suffering. In bad cases the hair falls out and the skin becomes scabby. Horses, cattle and sheep are also attacked. The disease caused by these mites on domestic animals is not usually considered curable except in its very early stages when salves or ointments may help some.

Itch-mites. "As slow as the seven-years' itch" is an expression, the meaning of which many could appreciate from personal experience, for it certainly seemed to take no end of time to get rid of the itch once it was contracted. Just why seven years should have been set for the limit of the disease is not clear, for if the little roundish mites that cause the disease live for seven years on a host they are not going to move out voluntarily even if their seven-year lease has expired.

The minute whitish mites (Fig. 20) that cause this disgusting disease are barely visible to the naked eye. They are usually very sluggish but become more active when warmed. They live in burrows just beneath the outer layer of skin, sometimes extending deeper and causing most intense itching. As the female burrows, she lays her eggs from which come the young mites that are to spread the infection. Various sulphur ointments and washes are used as remedies. Cleanliness will prevent infection.

Closely related to the itch-mite of man (Sarcoptes scabiei) are several kinds attacking domestic animals, causing mange, scab, etc. The variety infesting horses burrows in the skin and produces sores and scabs, and is a source of very great annoyance. These mites may also migrate to man. Tobacco water and sulphur ointments are used as remedies.

Horses and cattle are also infested by other mites (Psoroptes communis) which cause the common mange. These do not burrow into the skin but live outside in colonies, feeding on the skin and causing crusts or scabs. The inflammation causes the animal to scratch and rub constantly and often causes the loss of much of the hair.

Harvest-mites. A score or more of different varieties of mites cause many other diseases of domestic animals, such as the scab of sheep and hogs and chickens, various other manges of the horses and cattle and dogs, etc. But we need to call attention to just one more example, that of the harvest-mites or jiggers (Fig. 21). Professor Otto Lugger, from whose report on the Parasites of Man and Domestic Animals most of these notes in regard to the mites are taken, thus feelingly refers to this pest.

Horses, cattle, dogs and cats and other animals suffer also. Again sulphur ointments are the best remedies.

CHAPTER IV

Not only do the insects preponderate in number of species, but the number of individuals belonging to many of the species is absolutely beyond our comprehension. Try to count the number of little green aphis on a single infested rose-bush, or on a cabbage plant; guess at the number of mosquitoes issuing each day from a good breeding-pond; estimate the number of scale insects on a single square inch of a tree badly infested with San José scale; then try to think how many more bushes or trees or ponds may be breeding their millions just as these and you will only begin to comprehend the meaning of this statement.

As long as these myriads of insects keep in what we are pleased to call their proper place we care not for their numbers and think little of them except as some student points out some wonderful thing about their structure, life-history or adaptations. But since the dawn of history we find accounts to show that insects have not always kept to their proper sphere but have insisted at various times and in various ways in interfering with man's plans and wishes, and on account of their excessive numbers the results have often been most disastrous.

Insects cause an annual loss to the people of the United States of over $1,000,000,000. Grain fields are devastated; orchards and gardens are destroyed or seriously affected; forests are made waste places and in scores of other ways these little pests which do not keep in their proper places are exacting this tremendous tax from our people.

These things have been known and recognized for centuries, and scores of volumes have been written about the insects and their ways and of methods of combating them.

But it is only in recent years that we have begun to realize the really important part that insects play in relation to the health of the people with whom they are associated. Dr. Howard estimates that the annual death rate in the United States from malaria is about twelve thousand, entailing an annual monetary loss of about $100,000,000, to say nothing of the suffering and misery endured by the afflicted. All this on account of two or three species of insects belonging to the mosquito genus Anopheles.

Yellow fever, while not so widespread, is more fatal and therefore more terrorizing. Its presence and spread are due entirely to a single species of mosquito. Flies, fleas, bedbugs, and many other insects have been shown to be intimately connected with the spread of several other most dreaded diseases, so it is no wonder that physicians, entomologists and biologists are studying with utmost zeal many of these forms that bear such a close relation not only to our welfare and comfort but to our lives as well.

It would be out of place to try to give here even a brief outline of the classification of insects, such as may be found in almost any of the many books devoted to their study.

The most generally accepted classification divides the insects into nineteen orders; as the Coleoptera, containing the beetles; the Lepidoptera, containing the butterflies and moths; the Hymenoptera containing the bees, ants and wasps, etc. Four or five of these orders will be of more or less interest to us.

The order Diptera, or two-winged flies, is the most important because to this belong the mosquitoes which transmit malaria and yellow fever, and the house-fly that has come into prominence since it has been found to be such an important factor in the distribution of typhoid and other diseases.

FLIES

The order Diptera is divided into sixty or more families, many of which contain species of considerable economic importance. For our present consideration the flies may be divided into two groups or sections: those with their mouth-parts fitted for piercing such as the mosquito and horse-fly, and those with sucking mouth-parts such as the house-fly, blow-fly and others.

Some of the species belonging to the first group are among the most troublesome pests not only of man but of our domestic animals as well. Next to the mosquitoes the horse-flies (Fig. 22) are perhaps the best known of these. There are several species known under various names, such as gad-fly, breeze-fly, etc. They are very serious pests of horses and cattle, sometimes also attacking man. Their strong, sharp, piercing stylets enable them to pierce through the toughest skin of animals and through the thin clothing of man. The bite is very severe and irritating, and as the flies sometimes occur in great numbers the annoyance that they cause is often very great indeed. It has often been claimed that these flies as well as the stable-fly and others carry the anthrax bacillus on their proboscis from one animal to another, and although this may not have been definitely proven the evidence is strong enough to make a very good case against the accused. It is interesting to note in this connection that anthrax, a very common disease among the domestic animals and one which may attack man also, was the first disease to be shown to be of bacterial origin. It was only about thirty-five years ago that the investigations of Koch and Pasteur demonstrated that the presence of this particular germ (Bacillus anthracis) was the cause of the disease, and it was early recognized that such biting flies may be important factors in the spread of the disease.

The stable-fly (Fig. 23) (Stomoxys calcitrans) which looks very much like the house-fly and, as will be noted later, frequently enters houses, is often an important pest of horses and cattle. Its blood-sucking habit makes it quite possible that it too may be concerned in carrying anthrax and other diseases.

In a later chapter it will be shown how the tsetse-fly, which is somewhat like the stable-fly, is responsible for the spread of the disease known as the sleeping sickness. This disease is caused by a Protozoan parasite, a trypanosome, which is transmitted from one host to another by the tsetse-fly.

In Southern Asia and in parts of Africa there is a very serious disease of horses known as surra which is caused by a similar parasite (Trypanosoma evansi). This parasite attacks horses, mules, camels, elephants, buffaloes and dogs, and has been recently imported into the Philippines. It is supposed that flies belonging to the same genus as the horse-fly (Tabanus and others), and the stable-fly (Stomoxys) and the horn-fly (Hæmatobia) are responsible for the spread of the disease.

Nagana is one of the most serious diseases of domestic animals in Central and Southern Africa. In some sections it is almost impossible to keep any kind of imported animals on account of this disease which is caused by a parasite (Trypanosoma brucei) similar to the one causing surra. This parasite is to be found in several different kinds of native animals which seem to be practically immune but are always a source of danger when other animals are introduced. Two or three species of tsetse-flies are responsible for the transmission of this disease.

Another group of flies much smaller but more numerous and much more insistent are the black-flies or buffalo-gnats (Fig. 24). For more than a century these little flies have been recognized as among the most serious pests of stock, particularly in the south where, besides the actual loss by death of many animals yearly, the annoyance is so great as to sometimes make it impossible to work in the field. Human beings are often attacked, and as the bite is poisonous and very painful great suffering may result and cases of deaths from such bites have been reported.

Belonging to another family, and smaller, but much like the buffalo-gnat in habits, are the minute little "punkies" or "no-see-ums" which sometimes occur in great swarms in certain regions where they make life a burden to man and beast. While it has not been shown that either the buffalo-gnats or the punkies are responsible for the transmission of any disease, their habits of feeding on so many different kinds of wild and domestic animals as well as on man makes it possible for them to act as carriers of parasites that might under proper conditions become of serious importance. Then, too, the irritation caused by the bites of these insects usually causes scratching which may result in abrasions of the skin that open the way for various harmful germs, particularly those causing skin diseases.

Coming now to the group containing the house-flies and related forms we find a number that are of interest on account of the suffering that they may cause, particularly in their larval stages.

The screw-worm flies (Chrysomyia macellaria) are among the most common and important of these (Fig. 25). These "gray flies," as they are sometimes called, lay a mass of three or four hundred eggs on the surface of wounds. The larvæ which in a few hours hatch from these make their way directly into the wound where they feed on the surrounding tissue until full grown when they wriggle out and drop to the ground where they transform to the pupa and later to the adult fly. Of course their presence in the wounds is very distressing to the infected animal, and great suffering results. Slight scratches that might otherwise quickly heal often become serious sores because of the presence of these larvæ.

Many cases are recorded of these flies laying their eggs in the ears or nose of children or of persons sleeping out of doors during the day. Especially is this apt to occur if there are offensive discharges which attract the fly. In such cases the larvæ burrow into the surrounding tissues, devouring the mucous membranes, the muscles and even the bones, causing terrible suffering and usually, death. The larvæ in such situations may be killed with chloroform and, if the case is attended to before they have destroyed too much of the tissues, recovery usually occurs.

The blow-flies (Fig. 26) (Calliphora vomitoria) and the blue-bottle flies (Fig. 27), (Lucilia spp.) and the flesh-flies (Fig. 28) (Sarcophaga spp.) all have habits somewhat like the screw-worm fly. Any of them may lay their eggs in wounds on man or animals with the same serious results.

The flesh-fly instead of laying eggs deposits the living larvæ upon meat wherever it is accessible, and as these develop with astonishing rapidity they are able to consume large quantities of flesh in a remarkably short time. In this way they may be of some importance as scavengers, but it is better to get rid of the waste in other ways than to leave it for a breeding-place for flies that are capable of causing so much damage and suffering.

Not infrequently the larvæ of certain flies are to be found in the alimentary canal where as a rule they do no particular damage. Altogether the larvæ of over twenty different species of flies have been found in or expelled from the human intestinal canal. In Europe, the majority of these larvæ belong to a fly which looks very much like the house-fly except that it is somewhat smaller and so is often known as "the little house-fly" (Fig. 29) (Homalomyia canicularis). The same species is very common in the United States, frequently occurring in houses. Under certain conditions it may even be more abundant than the house-fly. It is believed that the larvæ in the intestinal canal come from eggs that have been deposited on the victim while using an outdoor privy where the flies are often very abundant. Instances are also on record where these larvæ have been discharged from the urethra.

Another fly (Ochromyia anthropophaga) occurring in the Congo region has a blood-sucking larvæ which is known as the Congo floor-maggot. The fly which is itself not troublesome deposits its eggs in the cracks and crevices of the mud floors of the huts. The larvæ which hatch from these crawl out at night and suck the blood of the victim that may be sleeping on the floor or on a low bed.

BOT-FLIES

Another group of flies known as the bot-flies (Fig. 30) have their mouth-parts rudimentary or entirely wanting so of course they themselves cannot bite or pierce an animal. Nevertheless they are the source of an endless amount of trouble to stockmen and sometimes even attack man. Although these flies cannot bite, the presence of even a single individual may be enough to annoy a horse almost to the end of endurance. Horses seem to have an instinctive fear of them and will do all in their power to get rid of the annoying pests.

The eggs of the house bot-fly are laid on the hair of the legs or some other part of the body. The horse licks them off and they hatch and develop in the alimentary canal of their host. Sometimes the walls of the stomach may be almost covered with them thus of course seriously interfering with the functions of this organ. When full grown the larvæ pass from the host and complete their transformation in the ground.

The bot-flies of cattle or the oxwarbles (Fig. 31) gain an entrance into the alimentary canal in the same way, that is, by the eggs being licked from the hairs on the body where they have been laid by the adult fly. But instead of passing on into the stomach they collect in the esophagus and later make their way through the walls of this organ and through the tissues of the body until they at last reach a place along the back just under the skin. Here as they are completing their development they make more or less serious sores on the backs of the infested animals. The hides on such animals are rendered nearly valueless by the holes made by the larvæ. When fully mature they drop to the ground and complete their transformations.

The sheep bot-flies (Fig. 32) lay their eggs in the nostrils of sheep. The larvæ pass up into the frontal sinuses where they feed on the mucus, causing great suffering and loss. Many other species of animals are infested with their own particular species of bots. Several instances are recorded where the oxwarble has occurred in man, always causing much suffering and sometimes death.

One or more species of bot-flies occurring in the tropical parts of America frequently attack man. The early larval stage soon after it has entered the skin is known as the Ver macaque. Later stages as torcel or Berne. The presence of the larvæ produces very painful and troublesome sores. It is supposed that the adult flies (one species of which is Dermatobia cyaniventris) lay their eggs on the skin which the larvæ penetrate as soon as they hatch. It has also been suggested that they might reach the subcutaneous tissue by migrating from the alimentary canal as do some of the other bot-flies. A very serious eye disease, Egyptian opthalmia, is known to be spread by the house-flies and others. These flies are often abundant about the eyes, especially of children suffering from this disease. It is suspected that certain small flies (Oscinidæ) in the southern part of the United States are responsible for the spread of disease known as "sore eye."

FLEAS

The fleas used to be considered as degenerate Diptera and were placed with that group but they are now classed as a separate order (Siphonaptera). Within recent years these little pests have come into special prominence on account of their importance in connection with the spread of the plague. The fact that they are so abundant everywhere and that they will so readily pass from one host to another makes the possibility of their spreading infectious diseases very great. Besides the kinds that are concerned in the transmission of plague, which are discussed in another chapter, there are many other kinds infesting various wild and domesticated animals and a few attacking birds.

One of the most important of these is the jigger-flea or chigoe (Dermatophilus penetrans, Fig. 33). Various other names such as chigger-flea, sand-flea, jigger, chigger are also applied to this insect as well as to a minute red mite that burrows into the skin in much the same way as the female of the flea. So although they are entirely different creatures you can never tell from the common name, whether it is the flea or the mite that is being referred to. Both the male and female jigger-fleas feed on the host and hop on or off as do other fleas, but when the female is ready to lay eggs (Fig. 34), she burrows into the skin. Her presence there causes a swelling and usually an ulcer which often becomes very serious, especially if the insect should be crushed and the contents of the body escape into the surrounding tissue.

These little pests are found throughout tropical and subtropical America and have been introduced into Africa and from there have spread to India and elsewhere. They attack almost all kinds of animals as well as many birds, being of course a source of great annoyance and no inconsiderable loss. They are more apt to attack the feet of men, especially those who go barefooted. Sometimes they occur in such numbers as to make great masses of sores.

On account of being such general feeders they are difficult to control, but some relief may be obtained by keeping the houses and barns as free as possible from dirt and rubbish and by sprinkling the breeding-places of the pest with pyrethrum powder or carbolic water. Those that gain an entrance into the skin should be cut out, care being taken to remove the insect entire.

BEDBUGS

In the order Hemiptera, or the true "bugs" in an entomological sense, we find a few forms that may carry disease. The bedbug (Fig. 35) (Cimex lectularis) has been accused of transmitting plague, relapsing fever and other diseases. Very recent investigations show that the common bedbug of India (Cimex rotundatus) harbors the parasite that causes the disease known as kala azar, and there is no doubt that it transmits the disease.

LICE

The sucking lice (Fig. 36) which also belong to this order are suspected of carrying some of these same diseases. It is thought that the common louse on rats (Hæmatopinus spinulosus) is responsible for the spread from rat to rat of a certain parasite. (Trypanosoma lewisi), which, however, does not produce any disease in the rats, but if they are capable of acting as alternative hosts for such parasites, it is quite possible that they may also carry disease-producing forms.

HOW INSECTS MAY CARRY DISEASE GERMS

Insects may carry the germs or parasites which cause disease in a purely mechanical or accidental way, that is, the insect may in the course of its wanderings or its feeding get some of the germs on or in its body and may by chance carry these to the food, or water, or directly to some person who may become infected. Thus the house-fly may carry the typhoid germs on its feet or in its body and distribute them in places where they may enter the human body.

Several other flies as well as fleas, bedbugs, ticks, etc., may also carry disease germs in this mechanical way. While this method of transmission is just as dangerous as any other, and possibly more dangerous because more common, another method in which the insect is much more intimately concerned is more interesting from a biological standpoint at least and will be discussed more fully in the chapters on malaria, yellow fever and elephantiasis.

In these cases the insect is one of the necessary hosts of the parasite, which cannot go on with its development or pass from one patient to another unless it first enters the insect at a certain stage of its life-history.

CHAPTER V

he page shown in Fig. 37 was copied from one of our old second readers and shows something of the spirit in which we used to regard the house-fly. A few of them were nice things to have around to make things seem "homelike." Of course they sometimes became too friendly during the early morning hours when we were trying to take just one more little nap or they were sometimes too insistent for their portion of the dinner after it had been placed on the table, but a screen over the bed would help us out a little in the morning and a long fly-brush cut from a tree in the yard or made of strips of paper tacked to a stick or, still more fancy, made of long peacock plumes, would help to drive them from the table. Those that were knocked into the coffee or the cream could be fished out; those that went into the soup or the hash were never missed!

Not only were the flies regarded as splendid things with which to amuse the baby, but they were thought to be very useful as scavengers as they were often seen feeding on all kinds of refuse in the yard. Then, too, they seemed to be cleanly little things, for almost any time some of them could be seen brushing their heads and bodies with their legs and evidently having a good clean-up. More than that it never occurred to us that it would be possible to get rid of them even should it be thought advisable, for they came from "out doors," and who could kill all the flies "out doors"?

Fortunately, or otherwise, these halcyon days have gone by and the common, innocent, friendly little house-fly is now an outcast convicted of many crimes and accused of a long list of others (Fig. 38).

Its former friends have become its sworn enemies. The foremost entomologist of the land has suggested that we even change its name and give it one that would be more suggestive of the abhorence with which we now look upon it.

And all these changes have come about because science has turned the microscope on the house-fly and men have studied its habits. We know now that as the fly is "tickling baby's nose" it may be spreading there where they may be inhaled or where they may be taken into the baby's mouth thousands of germs some of which may cause some serious disease. We know that as they are buzzing about our faces while we are trying to sleep they may, unwittingly, be in the same nefarious business, and we know that as they sip from our cups with us or bathe in our coffee or our soup or walk daintily over our beefsteak or frosted cake they are leaving behind a trail of filth and bacteria, and we know that some of these germs may be and often are the cause of some of our common diseases. As the typhoid germs are very often distributed in this way, Dr. Howard has suggested that the house-fly shall be known in the future as the typhoid-fly, not because it is solely responsible for the spread of typhoid, but because it is such an important factor in it and is so dangerous from every point of view. The names "manure fly" and "privy fly" have also been suggested and would perhaps serve just as well, as the only object in giving it another name would be to find a more repulsive one to remind us constantly of the filthy and dangerous habits of the fly.

STRUCTURE

In order that we may better understand why it is that the house-fly is capable of so much mischief, let us consider briefly a few points in regard to its structure, its methods of feeding and its life-history.

The large compound eyes are the most conspicuous part of the head (Fig. 39). In front, between the eyes, are the three-jointed antennæ, the last joint bearing a short, feathery bristle. From the under side of the head arises the long, fleshy proboscis (Fig. 40). When this is fully extended it is somewhat longer than the head; when not distended and in use it is doubled back in the cavity on the under side of the head. About half-way between the base and the middle is a pair of unjointed mouth-feelers (maxillary palpi). At the tip are two membranous lobes (Fig. 41) closely united along their middle line. These are covered with many fine corrugated ridges, which under the microscope look like fine spirals and are known as pseudotracheæ. Thus it will be seen that the house-fly's mouth-parts are fitted for sucking and not for biting. Its food must be in a liquid or semi-liquid state before it can be sucked through the tube leading from the lobes at the tip up through the proboscis and on into the stomach. If the fly wishes to feed on any substance such as sugar, that is not liquid, it first pours out some saliva on it and then begins to rasp it with the rough terminal lobes of the proboscis, thus reducing the food to a consistency that will enable the fly to suck it up. Many people think that house-flies can bite and will tell you that they have been bitten by them. But a careful examination of the offender, in such instances, will show that it was not a house-fly but probably a stable-fly, which does have mouth-parts fitted for piercing.

The thorax bears the two rather broad, membranous wings (Fig. 42) which have characteristic venation. Three of these veins end rather close together just before the tip of the wing, the posterior one of the group being bent forward rather sharply a short distance from the tip. The stable-fly has this vein slightly curved forward but not nearly so conspicuously (Fig. 43).

Nearly all the other flies that are apt to be mistaken for the house-fly do not have this vein curved forward. The wings, although apparently bare, are covered with a fine microscopic pubescence. Among these fine hairs on the wing as well as among similar fine ones and coarser ones all over the body, particles of dust and dirt or filth (Fig. 44) or, what interests us more just now, thousands of germs may find a temporary lodgment and later be scattered through the air as the insect flies. Or they may get on our food as the fly feeds or while it rests and combs its body with the rows of coarse hairs on its legs.

The legs are rather thickly covered with coarse hairs or bristles and with a mat of fine, short hairs. On some of the segments the larger hairs are arranged in rows and are used as a sort of comb with which the fly combs the dirt from the rest of its body. The last segment (Fig. 45) of the leg bears at its tip a pair of large curved claws and a pair of membranous pads known as the pulvillæ. On the under side of the pulvillæ are innumerable minute secreting hairs (Fig. 46) by means of which the fly is able to walk on the wall or ceiling or in any position on highly-polished surfaces.

HOW THEY CARRY BACTERIA

These same little pads, with their covering of secreting hairs, are perhaps the most dangerous part of the insect for they cannot help but carry much of the filth over or through which the fly walks, and as this may be well stocked with germs the danger is at once apparent.

As the result of a series of carefully planned experiments it has been demonstrated that the number of bacteria on a single fly may range all the way from 550 to 6,600,000 with an average for the lot experimented with of about one and one-fourth million bacteria to each fly. Now where do all these bacteria come from? Necessarily from the place where the fly breeds or where it feeds.