Handbook of Medical Entomology

by William A. Riley; O. A. Johannsen

FOOTNOTES:

[A] This is diametrically opposed to the findings of Bordas (1905) in the case of the European Latrodectus 13-guttatus, whose glands are "much larger than those of other spiders." From a considerable comparative study, we should also unhesitatingly make this statement regarding the glands of our American species, L. mactans.

[B] Dr. E. H. Coleman (Kellogg, 1915) has demonstrated its virulence by a series of experiments comparable with those of Kobert.

[C] According to Stiles, the species occurring in the Northwest which is commonly identified as D. venustus should be called D. andersoni (see footnote, chapter 12).

[D] It should be remembered that in all the higher Hymenoptera the first abdominal segment is fused with the thorax and that what is apparently the sixth segment is, in reality, the seventh.

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CHAPTER III

PARASITIC ARTHROPODA AFFECTING MAN

The relation of insects to man as simple parasites has long been studied, and until very recent years the bulk of the literature of medical entomology referred to this phase of the subject. This is now completely overshadowed by the fact that so many of these parasitic forms are more than simple parasites, they are transmitters of other microscopic parasites which are pathogenic to man. Yet the importance of insects as parasites still remains and must be considered in a discussion of the relation of insects to the health of man. In taking up the subject we shall first consider some general features of the phenomenon of animal parasitism.

Parasitism is an adaptation which has originated very often among living organisms and in widely separated groups. It would seem simple to define what is meant by a "parasite" but, in reality, the term is not easily limited. It is often stated that a parasite is "An organism which lives at the expense of another," but this definition is applicable to a predatory species or, in its broadest sense, to all organisms. For our purpose we may say with Braun: "A parasite is an organism which, for the purpose of obtaining food, takes up its abode, temporarily or permanently, on or within another living organism".

Thus, parasitism is a phase of the broad biological phenomenon of symbiosis, or living together of organisms. It is distinguished from mutualism, or symbiosis in the narrow sense, by the fact that only one party to the arrangement obtains any advantage, while the other is to a greater or less extent injured.

Of parasites we may distinguish on the basis of their location on or in the host, ecto-parasites, which live outside of the body; and endo-parasites, which live within the body. On account of their method of breathing the parasitic arthropods belong almost exclusively to the first of these groups.

On the basis of relation to their host, we find temporary parasites, those which seek the host only occasionally, to obtain food; and the stationary or permanent parasites which, at least during certain stages, do not leave their host.

Facultative parasites are forms which are not normally parasitic, but which, when accidentally ingested, or otherwise brought into the body, are able to exist for a greater or less period of time in their unusual environment. These are generally called in the medical literature "pseudoparasites" but the term is an unfortunate one.

We shall now take up the different groups of arthropods, discussing the more important of the parasitic forms attacking man. The systematic relationship of these forms, and key for determining important species will be found in Chapter XII.

Acarina or Mites

The Acarina, or mites, form a fairly natural group of arachnids, characterized, in general, by a sac-like, unsegmented body which is generally fused with the cephalothorax. The mouth-parts have been united to form a beak or rostrum.

The representatives of this group undergo a marked metamorphosis. Commonly, the larvæ on hatching from the egg, possess but three pairs of legs, and hence are called hexapod larvæ. After a molt, they transform into nymphs which, like the adult, have four pairs of legs and are called octopod nymphs. These after a period of growth, molt one or more times and, acquiring external sexual organs, become adult.

Most of the mites are free-living, but there are many parasitic species and as these have originated in widely separated families, the Acarina form an especially favorable group for study of the origin of parasitism. Such a study has been made by Ewing (1911), who has reached the following conclusions:

"We have strong evidence indicating that the parasitic habit has originated independently at least eleven times in the phylogeny of the Ararina. Among the zoophagous parasites, the parasitic habit has been developed from three different types of free-living Acarina: (a) predaceous forms, (b) scavengers, (c) forms living upon the juices of plants."

Ewing also showed that among the living forms of Acarina we can trace out all the stages of advancing parasitism, semiparasitism, facultative parasitism, even to the fixed and permanent type, and finally to endoparasitism.

Of the many parasitic forms, there are several species which are serious parasites of man and we shall consider the more important of these. Infestation by mites is technically known as acariasis.

The Trombidiidæ, or Harvest Mites

In many parts of this country it is impossible for a visitor to go into the fields and, particularly, into berry patches and among tall weeds and grass in the summer or early fall without being affected by an intolerable itching, which is followed, later, by a breaking out of wheals, or papules, surrounded by a bright red or violaceous aureola, ([fig. 43]). It is often regarded as a urticaria or eczema, produced by change of climate, an error in diet, or some condition of general health.

Sooner or later, the victim finds that it is due to none of these, but to the attacks of an almost microscopic red mite, usually called "jigger" or "chigger" in this country. As the term "chigger" is applied to one of the true fleas, Dermatophilus penetrans, of the tropics, these forms are more correctly known as "harvest mites." Natives of an infested region may be so immune or accustomed to its attacks as to be unaware of its presence, though such immunity is by no means possessed by all who have been long exposed to the annoyance.

The harvest mites, or chiggers, attacking man are larval forms, possessing three pairs of legs ([fig. 44]). Their systematic position was at first unknown and they were classed under a special genus Leptus, a name which is very commonly still retained in the medical literature. It is now known that they are the larval forms of various species of the genus Trombidium, a group of predaceous forms, the adults of which feed primarily on insects and their eggs. In this country the species best known are those to be found late in summer, as larvæ at the base of the wings of houseflies or grasshoppers.

There is much uncertainty as to the species of the larvæ attacking man but it is clear that several are implicated. Bruyant has shown that in France the larvæ Trombidium inapinatum and Trombidium holosericeum are those most frequently found. The habit of attacking man is abnormal and the larvæ die after entering the skin. Normally they are parasitic on various insects.

Most recent writers agree that, on man, they do not bore into the skin, as is generally supposed, but enter a hair follicle or sebaceous gland and from the bottom of this, pierce the cutis with their elongate hypopharynx. According to Braun, there arises about the inserted hypopharynx a fibrous secretion—the so-called "beak" which is, in reality, a product of the host. Dr. J. C. Bradley, however, has made careful observations on their method of attack, and he assures us that the mite ordinarily remains for a long time feeding on the surface of the skin, where it produces the erythema above described. During this time it is not buried in the skin but is able to retreat rapidly into it through a hair follicle or sweat gland. The irritation from the mites ceases after a few days, but not infrequently the intolerable itching leads to so much scratching that secondary infection follows.

Relief from the irritation may be afforded by taking a warm salt bath as soon as possible after exposure or by killing the mites by application of benzine, sulphur ointment or carbolized vaseline. When they are few in number, they can be picked out with a sterile needle.

Much may be done in the way of warding off their attacks by wearing gaiters or close-woven stockings extending from ankle to the knee. Still more efficacious is the sprinkling of flowers of sulphur in the stockings and the underclothes from a little above the knee, down. The writers have known this to make it possible for persons who were especially susceptible to work with perfect comfort in badly infested regions. Powdered naphthalene is successfully used in the same way and as Chittenden (1906) points out, is a safeguard against various forms of man-infesting tropical insect pests.

The question of the destruction of the mites in the field is sometimes an important one, and under some conditions, is feasible. Chittenden states that much can be accomplished by keeping the grass, weeds, and useless herbage mowed closely, so as to expose the mites to the sun. He believes that in some cases good may be done by dusting the grass and other plants, after cutting, with flowers of sulphur or by spraying with dilute kerosene emulsion in which sulphur has been mixed. More recently (1914) he calls attention to the value of cattle, and more especially sheep, in destroying the pests by tramping on them and by keeping the grass and herbage closely cropped.

Ixodoidea or Ticks

Until recently, the ticks attracted comparatively little attention from entomologists. Since their importance as carriers of disease has been established, interest in the group has been enormously stimulated and now they rank second only to the mosquitoes in the amount of detailed study that has been devoted to them.

The ticks are the largest of the Acarina. They are characterized by the fact that the hypostome, or "tongue" ([fig. 45]) is large and file-like, roughened by sharp teeth. They possess a breathing pore on each side of the body, above the third or fourth coxæ ([fig. 45b]).

There are two distinct families of the Ixodoidea, differing greatly in structure, life-history and habits. These are the Argasidæ and the Ixodidæ. We shall follow Nuttall (1908) in characterizing these two families and in pointing out their biological differences, and shall discuss briefly the more important species which attack man. The consideration of the ticks as carriers of disease will be reserved for a later chapter.

Argasidæ

In the ticks belonging to the family Argasidæ, there is comparatively little sexual dimorphism, while this is very marked in the Ixodidæ. The capitulum, or so-called "head" is ventral, instead of terminal; the palpi are leg-like, with the segments subequal; the scutum, or dorsal shield, is absent; eyes, when present, are lateral, on supracoxal folds. The spiracles are very small; coxæ unarmed; tarsi without ventral spurs, and the pulvilli are absent or rudimentary.

In habits and life history the Argasidæ present striking characteristics. In the first place, they are long-lived, a factor which counts for much in the maintenance of the species. They are intermittent feeders, being comparable with the bed-bug in this respect. There are two or more nymphal stages, and they may molt after attaining maturity. The female lays comparatively few eggs in several small batches.

Nuttall (1911) concludes that "The Argasidæ represent the relatively primitive type of ticks because they are less constantly parasitic than are the Ixodidæ. Their nymphs and adults are rapid feeders and chiefly infest the habitat of their hosts. * * * Owing to the Argasidæ infesting the habitats of their hosts, their resistance to prolonged starvation and their rapid feeding habits, they do not need to bring forth a large progeny, because there is less loss of life in the various stages, as compared with the Ixodidæ, prior to their attaining maturity."

Of the Argasidæ, we have in the United States, several species which have been reported as attacking man.

Argas persicus, the famous "Miana bug" ([fig. 46]), is a very widely distributed species, being reported from Europe, Asia, Africa, and Australia. It is everywhere preeminently a parasite of fowls. According to Nuttall it is specifically identical with Argas americanus Packard or Argas miniatus Koch, which is commonly found on fowls in the United States, in the South and Southwest. Its habits are comparable to those of the bed-bug. It feeds intermittently, primarily at night, and instead of remaining on its host, it then retreats to cracks and crevices. Hunter and Hooker (1908) record that they have found the larva to remain attached for five or eight days before dropping. Unlike the Ixodidæ, the adults oviposit frequently.

The most remarkable feature of the biology of this species is the great longevity, especially of the adult. Hunter and Hooker report keeping larvæ confined in summer in pill boxes immediately after hatching for about two months while under similar conditions those of the Ixodid, Boophilus annulatus lived for but two or three days. Many writers have recorded keeping adults for long periods without food. We have kept specimens in a tin box for over a year and a half and at the end of that time a number were still alive. Laboulliene kept unfed adults for over three years. In view of the effectiveness of sulphur in warding off the attacks of Trombidiidæ, it is astonishing to find that Lounsbury has kept adults of Argas persicus for three months in a box nearly filled with flowers of sulphur, with no apparent effect on them.

We have already called attention to the occasional serious effects of the bites of this species. While such reports have been frequently discredited there can be no doubt that they have foundation in fact. The readiness with which this tick attacks man, and the extent to which old huts may be infested makes it especially troublesome.

Otiobius (Ornithodoros) megnini, the "spinose ear-tick" (figs. [47], [48]), first described from Mexico, as occurring in the ears of horses, is a common species in our Southwestern States and is recorded by Banks as occurring as far north as Iowa.

The species is remarkable for the great difference between the spiny nymph stage and the adult. The life history has been worked out by Hooker (1908). Seed ticks, having gained entrance to the ear, attach deeply down in the folds, engorge, and in about five days, molt; as nymphs with their spinose body they appear entirely unlike the larvæ. As nymphs they continue feeding sometimes for months. Finally the nymph leaves the host, molts to form the unspined adult, and without further feeding is fertilized and commences oviposition.

The common name is due to the fact that in the young stage the ticks occur in the ear of their hosts, usually horses or cattle. Not uncommonly it has been reported as occurring in the ear of man and causing very severe pain. Stiles recommends that it be removed by pouring some bland oil into the ear.

Banks (1908) reports three species of Ornithodoros—O. turicata, coriaceus and talaje—as occurring in the United States. All of these attack man and are capable of inflicting very painful bites.

Ixodidæ

The ticks belonging to the family Ixodidæ (figs. [49] and [50]) exhibit a marked sexual dimorphism. The capitulum is anterior, terminal, instead of ventral as in the Argasidæ; the palpi are relatively rigid (except in the subfamily Ixodinæ), with rudimentary fourth segment; scutum present; eyes, when present, dorsal, on side of scutum. The spiracles are generally large, situated well behind the fourth coxæ; coxæ generally with spurs; pulvilli always present.

In habits and life history the typical Ixodidæ differ greatly from the Argasidæ. They are relatively short-lived, though some recent work indicates that their longevity has been considerably under-estimated. Typically, they are permanent feeders, remaining on the host, or hosts, during the greater part of their life. They molt twice only, on leaving the larval and the nymphal stages. The adult female deposits a single, large batch of eggs. Contrasting the habits of the Ixodidæ to those of the Argasidæ, Nuttall (1911) emphasizes that the Ixodidæ are more highly specialized parasites. "The majority are parasitic on hosts having no fixed habitat and consequently all stages, as a rule, occur upon the host."

As mere parasites of man, apart from their power to transmit disease, the Ixodidæ are much less important than the Argasidæ. Many are reported as occasionally attacking man and of these the following native species may be mentioned.

Ixodes ricinus, the European castor bean tick (figs. [49], [50]), is a species which has been often reported from this country but Banks (1908) has shown that, though it does occur, practically all of the records apply to Ixodes scapularis or Ixodes cookei. In Europe, Ixodes ricinus is very abundant and very commonly attacks man. At the point of penetration of the hypostome there is more or less inflammation but serious injury does not occur unless there have been introduced pathogenic bacteria or, unless the tick has been abruptly removed, leaving the capitulum in the wound. Under the latter circumstances, there may be an abscess formed about the foreign body and occasionally, serious results have followed. Under certain conditions the tick, in various stages, may penetrate under the skin and produce a tumor, within which it may survive for a considerable period of time.

Ixodes cookei is given by Banks as "common on mammals in the Eastern States as far west as the Rockies." It is said to affect man severely.

Amblyomma americanum, ([fig. 158c]), the "lone star tick," is widely distributed in the United States. Its common name is derived from the single silvery spot on the scutum of the female. Hunter and Hooker regard this species as, next to Boophilus annulatus, the most important tick in the United States. Though more common on cattle, it appears to attack mammals generally, and "in portions of Louisiana and Texas it becomes a pest of considerable importance to moss gatherers and other persons who spend much time in the forests."

Amblyomma cajennense, noted as a pest of man in central and tropical America, is reported from various places in the south and southwestern United States.

Dermacentor variabilis is a common dog tick of the eastern United States. It frequently attacks man, but the direct effects of its bite are negligible.

The "Rocky Mountain spotted fever tick" (Dermacentor andersoni according to Stiles, D. venustus according to Banks) is, from the viewpoint of its effects on man, the most important of the ticks of the United States. This is because, as has been clearly established, it transmits the so-called "spotted fever" of man in our northwestern states. This phase of the subject will be discussed later and it need merely be mentioned here, that this species has been reported as causing painful injuries by its bites. Dr. Stiles states that he has seen cases of rather severe lymphangitis and various sores and swellings developing from this cause. In one case, of an individual bitten near the elbow, the arm became very much swollen and the patient was confined in bed for several days. The so-called tick paralysis produced by this species is discussed in a preceding chapter.

There are many other records of various species of ticks attacking man, but the above-mentioned will serve as typical and it is not necessary to enter into greater detail.

Treatment of Tick Bites—When a tick attaches to man the first thing to be done is to remove it without leaving the hypostome in the wound to fester and bring about secondary effects. This is best accomplished by applying to the tick's body some substance which will cause it to more readily loosen its hold. Gasoline or petroleum, oil or vaseline will serve. For removing the spinose ear-tick, Stiles recommends pouring some bland oil into the ear. Others have used effectively a pledget of cotton soaked in chloroform.

In general, the treatment recommended by Wellman for the bites of Ornithodoros moubata will prove helpful. It consists of prolonged bathing in very hot water, followed by the application of a strong solution of bicarbonate of soda, which is allowed to dry upon the skin. He states that this treatment is comforting. For severe itching he advises smearing the bites with vaseline, which is slightly impregnated with camphor or menthol. Medical aid should be sought when complications arise.

The Dermanyssidæ are Gamasid mites which differ from others of the group in that they are parasitic on vertebrates. None of the species normally attack man, but certain of them, especially the poultry mite, may be accidental annoyances.

Dermanyssus gallinæ ([fig. 51]), the red mite of poultry, is an exceedingly common and widespread parasite of fowls. During the day it lives in cracks and crevices of poultry houses, under supports of roosts, and in litter of the food and nests, coming out at night to feed. They often attack people working in poultry houses or handling and plucking infested fowls. They may cause an intense pruritis, but they do not produce a true dermatosis, for they do not find conditions favorable for multiplication on the skin of man.

Tarsonemidæ

The representatives of the family Tarsonemidæ are minute mites, with the body divided into cephalothorax and abdomen. There is marked sexual dimorphism. The females possess stigmata at the anterior part of the body, at the base of the rostrum, and differ from all other mites in having on each side, a prominent clavate organ between the first and second legs. The larva, when it exists, is hexapodous and resembles the adult. A number of the species are true parasites on insects, while others attack plants. Several of them may be accidental parasites of man.

Pediculoides ventricosus (fig. [52] and [53]) is, of all the Tarsonemidæ reported, the one which has proved most troublesome to man. It is a predaceous species which attacks a large number of insects but which has most commonly been met with by man through its fondness for certain grain-infesting insects, notably the Angoumois grain moth, Sitotroga cerealella, and the wheat straw-worm, Isosoma grande. In recent years it has attracted much attention in the United States and its distribution and habits have been the object of detailed study by Webster (1901).

There is a very striking sexual dimorphism in this species. The non-gravid female is elongate, about 200µ by 70µ ([fig. 52]), with the abdomen slightly striated longitudinally. The gravid female ([fig. 53]) has the abdomen enormously swollen, so that it is from twenty to a hundred times greater than the rest of the body. The species is viviparous and the larvæ undergo their entire growth in the body of the mother. They emerge as sexually mature males and females which soon pair. The male ([fig. 54]) is much smaller, reaching a length of only 320µ but is relatively broad, 80µ, and angular. Its abdomen is very greatly reduced.

As far back as 1850 it was noted as causing serious outbreaks of peculiar dermatitis among men handling infested grain. For some time the true source of the difficulty was unknown and it was even believed that the grain had been poisoned. Webster has shown that in this country (and probably in Europe as well) its attacks have been mistaken for those of the red bugs or "chiggers" (larval Trombiidæ). More recently a number of outbreaks of a mysterious "skin disease" were traced to the use of straw mattresses, which were found to be swarming with these almost microscopic forms which had turned their attentions to the occupants of the beds. Other cases cited were those of farmers running wheat through a fanning mill, and of thrashers engaged in feeding unthrashed grain into the cylinder of the machine.

The medical aspects of the question have been studied especially by Schamberg and Goldberger and from the latter's summary (1910) we derive the following data. Within twelve to sixteen hours after exposure, itching appears and in severe cases, especially where exposure is continued night after night by sleeping on an infested bed, the itching may become almost intolerable. Simultaneously, there appears an eruption which characteristically consists of wheals surrounded by a vesicle ([fig. 55]). The vesicle as a rule does not exceed a pin head in size but may become as large as a pea. Its contents rapidly become turbid and in a few hours it is converted into a pustule. The eruption is most abundant on the trunk, slight on the face and extremities and almost absent on the feet and hands. In severe cases there may be constitutional disturbances marked, at the outset, by chilliness, nausea, and vomiting, followed for a few days by a slight elevation of temperature, with the appearance of albumin in the urine. In some cases the eruption may simulate that of chicken-pox or small-pox.

Treatment for the purpose of killing the mites is hardly necessary as they attach feebly to the surface and are readily brushed off by friction of the clothes. "Antipruritic treatment is always called for; warm, mildly alkaline baths or some soothing ointment, such as zinc oxide will be found to fulfil this indication." Of course, reinfestation must be guarded against, by discarding, or thoroughly fumigating infested mattresses, or by avoiding other sources. Goldberger suggests that farm laborers who must work with infested wheat or straw might protect themselves by anointing the body freely with some bland oil or grease, followed by a change of clothes and bath as soon as their work is done. We are not aware of any experiments to determine the effect of flowers of sulphur, but their efficiency in the case of "red bugs" suggests that they are worth a trial against Pediculoides.

Various species of Tyroglyphidæ ([fig. 150f]) may abound on dried fruits and other products and attacking persons handling them, may cause a severe dermatitis, comparable to that described above for Pediculoides ventricosus. Many instances of their occurrence as such temporary ectoparasites are on record. Thus, workers who handle vanilla pods are subject to a severe dermatitis, known as vanillism, which is due to the attacks of Tyroglyphus siro, or a closely related species. The so-called "grocer's itch" is similarly caused by mites infesting various products. Castellani has shown that in Ceylon, workers employed in the copra mills, where dried cocoanut is ground up for export, are much annoyed by mites, which produce the so-called "copra itch." The skin of the hands, arms and legs, and sometimes of the whole body, except the face, is covered by fairly numerous, very pruriginous papules, often covered by small, bloody crusts due to scratching. The condition is readily mistaken for scabies. It is due to the attacks of Tyroglyphus longior castellanii which occur in enormous numbers in some samples of the copra.

Sarcoptidæ

The Sarcoptidæ are minute whitish mites, semi-globular in shape, with a delicate transversely striated cuticula. They lack eyes and tracheæ. The mouth-parts are fused at the base to form a cone which is usually designated as the head. The legs are short and stout, and composed of five segments. The tarsi may or may not possess a claw and may terminate in a pedunculated sucker, or simple long bristle, or both. The presence or absence of these structures and their distribution are much used in classification. The mites live on or under the skin of mammals and birds, where they produce the disease known as scabies, mange, or itch. Several species of the Sarcoptidæ attack man but the most important of these, and the one pre-eminent as the "itch mite" is Sarcoptes scabiei.

The female of Sarcoptes scabiei, of man, is oval and yellowish white; the male more rounded and of a somewhat reddish tinge, and much smaller. The body is marked by transverse striæ which are partly interrupted on the back. There are transverse rows of scales, or pointed spines, and scattered bristles on the dorsum.

The male ([fig. 56]) which is from 200-240µ in length, and 150-200µ in breadth, possesses pedunculated suckers on each pair of legs except the third, which bears, instead, a long bristle. The female ([fig. 56]) 300-450µ in length and 250-350µ in breadth, has the pedunculated suckers on the first and second pairs of legs, only, the third and fourth terminating in bristles.

The mite lives in irregular galleries from a few millimeters to several centimeters in length, which it excavates in the epidermis ([fig. 57]). It works especially where the skin is thin, such as between the fingers, in the bend of the elbows and knees, and in the groin, but it is by no means restricted to these localities. The female, alone, tunnels into the skin; the males remain under the superficial epidermal scales, and seldom are found, as they die soon after mating.

As she burrows into the skin the female deposits her eggs, which measure about 150 × 100µ. Fürstenberg says that each deposits an average of twenty-two to twenty-four eggs, though Gudden reports a single burrow as containing fifty-one. From these there develop after about seven days, the hexapod larvæ. These molt on the sixteenth day to form an octopod nymph, which molts again the twenty-first day. At the end of the fourth week the nymphs molt to form the sexually mature males and the so-called pubescent females. These pair, the males die, and the females again cast their skin, and become the oviparous females. Thus the life cycle is completed in about twenty-eight days.

The external temperature exercises a great influence on the development of the mites and thus, during the winter, the areas of infestation not only do not spread, but they become restricted. As soon as the temperature rises, the mites increase and the infestation becomes much more extensive.

In considering the possible sources of infestation, and the chances of reinfestation after treatment, the question of the ability of the mite to live apart from its host is a very important one. Unfortunately there are few reliable data on this subject. Gerlach found that, exposed in the dry, warm air of a room they became very inactive within twenty-four hours, that after two days they showed only slight movement, and that after three or four days they could not be revived by moisture and warming. The important fact was brought out that in moist air, in folded soiled underwear, they survived as long as ten days. Bourguignon found that under the most favorable conditions the mites of Sarcoptes scabiei equi would live for sixteen days.

The disease designated the "itch" or "scabies," in man has been known from time immemorial, but until within less than a hundred years it was almost universally attributed to malnutrition, errors of diet, or "bad blood." This was in spite of the fact that the mite was known to Mouffet and that Bonomo had figured both the adult and the egg and had declared the mite the sole cause of the disease. In 1834 the Corsican medical student, Francis Renucci, demonstrated the mite before a clinic in Saint Louis Hospital in Paris and soon thereafter there followed detailed studies of the life history of the various itch mites of man and animals.

The disease is a cosmopolitan one, being exceedingly abundant in some localities. Its spread is much favored where large numbers of people are crowded together under insanitary conditions and hence it increases greatly during wars and is widely disseminated and abundant immediately afterwards. Though more commonly to be met with among the lower classes, it not infrequently appears among those of the most cleanly, careful habits, and it is such cases that are most liable to wrong diagnosis by the physician.

Infection occurs solely through the passage, direct or indirect, of the young fertilized females to the skin of a healthy individual. The adult, oviparous females do not quit their galleries and hence do not serve to spread the disease. The young females move about more or less at night and thus the principal source of infestation is through sleeping in the same bed with an infested person, or indirectly through bedclothes, or even towels or clothing. Diurnal infestation through contact or clothing is exceptional. Many cases are known of the disease being contracted from animals suffering from scabies, or mange.

When a person is exposed to infestation, the trouble manifests itself after eight or ten days, though there usually elapses a period of twenty to thirty days before there is a suspicion of anything serious. The first symptom is an intense itching which increases when the patient is in bed. When the point of irritation is examined the galleries may usually be seen as characteristic sinuous lines, at first whitish in color but soon becoming blackish because of the contained eggs and excrement. The galleries, which may not be very distinct in some cases, may measure as much as four centimeters in length. Little vesicles, of the size of a pin head are produced by the secretions of the feeding mite; they are firm, and projecting, and contain a limpid fluid. Figures [58] and [59] show the typical appearance of scabies on the hands, while [figure 60] shows a severe general infestation. The intolerable itching induces scratching and through this various complications may arise. The lesions are not normally found on the face and scalp, and are rare on the back.

Formerly, scabies was considered a very serious disease, for its cause and method of treatment were unknown, and potentially it may continue indefinitely. Generation after generation of the mites may develop and finally their number become so great that the general health of the individual is seriously affected. Now that the true cause of the disease is known, it is easily controlled.

Treatment usually consists in softening the skin by friction with soap and warm water, followed by a warm bath, and then applying some substance to kill the mites. Stiles gives the following directions, modified from Bourguignon's, as "a rather radical guide, to be modified according to facilities and according to the delicacy of the skin or condition of the patient":

1. The patient, stripped naked, is energetically rubbed all over (except the head) for twenty minutes, with green soap and warm water. 2. He is then placed in a warm bath for thirty minutes, during which time the rubbing is continued. 3. The parasiticide is next rubbed in for twenty minutes and is allowed to remain on the body for four or five hours; in the meantime the patient's clothes are sterilized, to kill the eggs or mites attached to them. 4. A final bath is taken to remove the parasiticide.

The parasiticide usually relied on is the officinal sulphur ointment of the United States pharmacopœia. When infestation is severe it is necessary to repeat treatment after three or four days in order to kill mites which have hatched from the eggs.

The above treatment is too severe for some individuals and may, of itself, produce a troublesome dermatitis. We have seen cases where the treatment was persisted in and aggravated the condition because it was supposed to be due to the parasite. For delicate-skinned patients the use of balsam of Peru is very satisfactory, and usually causes no irritation whatever. Of course, sources of reinfection should be carefully guarded against.

Sarcoptes scabiei crustosæ, which is a distinct variety, if not species, of the human itch mite, is the cause of so-called Norwegian itch. This disease is very contagious, and is much more resistant than the ordinary scabies. Unlike the latter, it may occur on the face and scalp.

Sarcoptes scabiei not only attacks man but also occurs on a large number of mammals. Many species, based on choice of host, and minute differences in size and secondary characters, have been established, but most students of the subject relegate these to varietal rank. Many of them readily attack man, but they have become sufficiently adapted to their normal host so that they are usually less persistent on man.

Notoedres cati (usually known as Sarcoptes minor) is a species of itch mites which produce an often fatal disease of cats. The body is rounded and it is considerably smaller than Sarcoptes scabiei, the female ([fig. 61]) measuring 215-230µ long and 165-175µ wide; the males 145-150µ by 120-125µ. The most important character separating Notoedres from Sarcoptes is the position of the anus, which is dorsal instead of terminal. The mite readily transfers to man but does not persist, the infestation usually disappearing spontaneously in about two weeks. Infested cats are very difficult to cure, unless treatment is begun at the very inception of the outbreak, and under ordinary circumstances it is better to kill them promptly, to avoid spread of the disease to children and others who may be exposed.

Demodecidæ

The Demodecidæ are small, elongate, vermiform mites which live in the hair follicles of mammals. The family characteristics will be brought out in the discussion of the species infesting man, Demodex folliculorum.

Demodex folliculorum ([fig. 62]) is to be found very commonly in the hair follicles and sebaceous glands of man. It is vermiform in appearance, and with the elongate abdomen transversely striated so as to give it the appearance of segmentation. The female is 380-400µ long by 45µ; the male 300µ by 40µ. The three-jointed legs, eight in number, are reduced to mere stubs in the adult. The larval form is hexopod. These mites thus show in their form a striking adaptation to their environment. In the sebaceous glands and hair follicles they lie with their heads down ([fig. 63]). Usually there are only a few in a gland, but Gruby has counted as many as two hundred.

The frequency with which they occur in man is surprising. According to European statistics they are found in 50 per cent to 60 per cent or even more. Gruby found them in forty out of sixty persons examined. These figures are very commonly quoted, but reliable data for the United States seem to be lacking. Our studies indicate that it is very much less common in this country than is generally assumed.

The Demodex in man does not, as a rule, cause the slightest inconvenience to its host. It is often stated that they give rise to comedons or "black-heads" but there is no clear evidence that they are ever implicated. Certain it is that they are not the usual cause. A variety of the same, or a very closely related species of Demodex, on the dog gives rise to the very resistant and often fatal follicular mange.

Hexapoda or True Insects

The Hexapoda, or true insects, are characterized by the fact that the adult possesses three pairs of legs. The body is distinctly segmented and is divided into head, thorax, and abdomen.

The mouth-parts in a generalized form, consist of an upper lip, or labrum, which is a part of the head capsule, and a central unpaired hypopharynx, two mandibles, two maxillæ and a lower lip, or labium, made up of the fused pair of second maxillæ. These parts may be greatly modified, dependent upon whether they are used for biting, sucking, piercing and sucking, or a combination of biting and sucking.

Roughly speaking, insects may be grouped into those which undergo complete metamorphosis and those which have incomplete metamorphosis. They are said to undergo complete metamorphosis when the young form, as it leaves the egg, bears no resemblance to the adult. For example, the maggot changes to a quiescent pupa and from this emerges the winged active fly. They undergo incomplete metamorphosis, when the young insect, as it leaves the egg, resembles the adult to a greater or less extent, and after undergoing a certain number of molts becomes sexually mature.

Representatives of several orders have been reported as accidental or faculative parasites of man, but the true parasites are restricted to four orders. These are the Siphunculata; the Hemiptera, the Diptera and the Siphonaptera.

Siphunculata

The order Siphunculata was established by Meinert to include the true sucking lice. These are small wingless insects, with reduced mouth-parts, adapted for sucking; thorax apparently a single piece due to indistinct separation of its three segments: the compound eyes reduced to a single ommatidium on each side. The short, powerful legs are terminated by a single long claw. Metamorphosis incomplete.

There has been a great deal of discussion regarding the structure of the mouth-parts, and the relationships of the sucking lice, and the questions cannot yet be regarded as settled. The conflicting views are well represented by Cholodkovsky (1904 and 1905) and by Enderlein (1904).

Following Graber, it is generally stated that the mouth-parts consist of a short tube furnished with hooks in front, which constitutes the lower lip, and that within this is a delicate sucking tube derived from the fusion of the labrum and the mandibles. Opposed to this, Cholodkovsky and, more recently, Pawlowsky, (1906), have shown that the piercing apparatus lies in a blind sac under the pharynx and opening into the mouth cavity ([fig. 64]). It does not form a true tube but a furrow with its open surface uppermost. Eysell has shown that, in addition, there is a pair of chitinous rods which he regards as the homologues of the maxillæ.

When the louse feeds, it everts the anterior part of the mouth cavity, with its circle of hooks. The latter serve for anchoring the bug, and the piercing apparatus is then pushed out.

Most writers have classed the sucking lice as a sub-order of the Hemiptera, but the more recent anatomical and developmental studies render this grouping untenable. An important fact, bearing on the question, is that, as shown by Gross, (1905), the structure of the ovaries is radically different from that of the Hemiptera.

Lice infestation and its effects are known medically as pediculosis. Though their continued presence is the result of the grossest neglect and filthiness, the original infestation may be innocently obtained and by people of the most careful habits.

Three species commonly attack man. Strangely enough, there are very few accurate data regarding their life history.

Pediculus humanus ([fig. 65]), the head louse, is the most widely distributed. It is usually referred to in medical literature as Pediculus capitis, but the Linnean specific name has priority. In color it is of a pale gray, blackish on the margins. It is claimed by some authors that the color varies according to the color of the skin of the host. The abdomen is composed of seven distinct segments, bearing spiracles laterally. There is considerable variation in size. The males average 1.8 mm. and the females 2.7 mm. in length.

The eggs, fifty to sixty in number, stick firmly to the hairs of the host and are known as nits. They are large and conspicuous, especially on dark hair and are provided with an operculum, or cap, at the free end, where the nymphs emerge. They hatch in about six days and about the eighteenth day the young lice are sexually mature.

The head lice live by preference on the scalp of their host but occasionally they are found on the eyelashes and beard, or in the pubic region. They may also occur elsewhere on the body. The penetration of the rostrum into the skin and the discharge of an irritating saliva produce a severe itching, accompanied by the formation of an eczema-like eruption ([fig. 66]). When the infestation is severe, the discharge from the pustules mats down the hair, and scabs are formed, under which the insects swarm. "If allowed to run, a regular carapace may form, called trichoma, and the head exudes a fœtid odor. Various low plants may grow in the trichoma, the whole being known as plica palonica."—Stiles.

Sources of infestation are various. School children may obtain the lice from seatmates, by wearing the hats or caps of infested mates, or by the use, in common, of brushes and combs. They may be obtained from infested beds or sleeper berths. Stiles reports an instance in which a large number of girls in a fashionable boarding school developed lousiness a short time after traveling in a sleeping car.

Treatment is simple, for the parasites may readily be controlled by cleanliness and washing the head with a two per cent solution of carbolic acid or even kerosene. The latter is better used mixed with equal parts of olive oil, to avoid irritation. The treatment should be applied at night and followed the next morning by a shampoo with soap and warm water. It is necessary to repeat the operation in a few days. Xylol, used pure, or with the addition of five per cent of vaseline, is also very efficacious. Of course, the patient must be cautioned to stay away from a lighted lamp or fire while using either the kerosene or xylol. While these treatments will kill the eggs or nits, they will not remove them from the hairs. Pusey recommends repeated washings with vinegar or 25 per cent of acetic acid in water, for the purpose of loosening and removing the nits.

Treatment of severe infestations in females is often troublesome on account of long hair. For such cases the following method recommended by Whitfield (1912) is especially applicable:

The patient is laid on her back on the bed with her head over the edge, and beneath the head is placed a basin on a chair so that the hair lies in the basin. A solution of 1 in 40 carbolic acid is then poured over the hair into the basin and sluiced backwards and forwards until the whole of the hair is thoroughly soaked with it. It is especially necessary that care should be taken to secure thorough saturation of the hair over the ears and at the nape of the neck, since these parts are not only the sites of predilection of the parasites but they are apt to escape the solution. This sluicing is carried out for ten minutes by the clock. At the end of ten minutes the hair is lifted from the basin and allowed to drain, but is not dried or even thoroughly wrung out. The whole head is then swathed with a thick towel or better, a large piece of common house flannel, which is fastened up to form a sort of turban, and is allowed to remain thus for an hour. It can then be washed or simply allowed to dry, as the carbolic quickly disperses. At the end of this period every pediculus and what is better, every ovum is dead and no relapse will occur unless there is exposure to fresh contagion. Whitfield states that there seem to be no disadvantages in this method, which he has used for years. He has never seen carboluria result from it, but would advise first cutting the hair of children under five years of age.

Pediculus corporis (= P. vestimenti) the body louse, is larger than the preceding species, the female measuring 3.3 mm., and the male 3 mm. in length. The color is a dirty white, or grayish. P. corporis has been regarded by some authorities as merely a variety of P. humanus but Piaget maintains there are good characters separating the two species.

The body louse lives in the folds and seams of the clothing of its host, passing to the skin only when it wishes to feed. Brumpt states that he has found enormous numbers of them in the collars of glass-ware or grains worn by certain naked tribes in Africa.

Exact data regarding the life-history of this species have been supplied, in part, by the work of Warburton (1910), cited by Nuttall. He found that Pediculus corporis lives longer than P. humanus under adverse conditions. This is doubtless due to its living habitually on the clothing, whereas humanus lives upon the head, where it has more frequent opportunities of feeding. He reared a single female upon his own person, keeping the louse enclosed in a cotton-plugged tube with a particle of cloth to which it could cling. The tube was kept next to his body, thus simulating the natural conditions of warmth and moisture under which the lice thrive. The specimen was fed twice daily, while it clung to the cloth upon which it rested. Under these conditions she lived for one month. Copulation commenced five days after the female had hatched and was repeated a number of times, sexual union lasting for hours. The female laid one hundred and twenty-four eggs within twenty-five days.

The eggs hatched after eight days, under favorable conditions, such as those under which the female was kept. They did not hatch in the cold. Eggs kept near the person during the day and hung in clothing by the bedside at night, during the winter, in a cold room, did not hatch until the thirty-fifth day. When the nymphs emerge from the eggs, they feed at once, if given a chance to do so. They are prone to scatter about the person and abandon the fragment of cloth to which the adult clings.

The adult stage is reached on the eleventh day, after three molts, about four days apart. Adults enter into copulation about the fifth day and as the eggs require eight days for development, the total cycle, under favorable conditions, is about twenty-four days. Warburton's data differ considerably from those commonly quoted and serve to emphasize the necessity for detailed studies of some of the commonest of parasitic insects.

Body lice are voracious feeders, producing by their bites and the irritating saliva which they inject, rosy elevations and papules which become covered with a brownish crust. The intense itching provokes scratching, and characteristic white scars ([fig. 67]) surrounded by brownish pigment ([fig. 68]) are formed. The skin may become thickened and take on a bronze tinge. This melanoderma is especially marked in the region between the shoulders but it may become generalized, a prominent characteristic of "vagabond's disease." According to Dubre and Beille, this melanoderma is due to a toxic substance secreted by the lice, which indirectly provokes the formation of pigment.

Control measures, in the case of the body louse, consist in boiling or steaming the clothes or in some cases, sterilizing by dry heat. The dermatitis may be relieved by the use of zinc-oxide ointment, to which Pusey recommends that there be added, on account of their parasiticidal properties, sulphur and balsam of Peru, equal parts, 15 to 30 grains to the ounce.

Phthirius pubis (= P. inguinalis), the pubic louse, or so-called "crab louse," differs greatly from the preceding in appearance. It is characterized by its relatively short head which fits into a broad depression in the thorax. The latter is broad and flat and merges into the abdomen. The first pair of legs is slender and terminated by a straight claw. The second and third pairs of legs are thicker and are provided with powerful claws fitted for clinging to hairs. The females ([fig. 69]) measure 1.5 to 2 mm. in length by 1.5 mm. in breadth. The male averages a little over half as large. The eggs, or nits, are fixed at the base of the hairs. Only a few, ten to fifteen are deposited by a single female, and they hatch in about a week's time. The young lice mature in two weeks.

The pubic louse usually infests the hairs of the pubis and the perineal region. It may pass to the arm pits or even to the beard or moustache. Rarely, it occurs on the eyelids, and it has even been found, in a very few instances, occurring in all stages, on the scalp. Infestation may be contracted from beds or even from badly infested persons in a crowd. We have seen several cases which undoubtedly were due to the use of public water closets. It produces papular eruption and an intense pruritis. When abundant, there occurs a grayish discoloration of the skin which Duguet has shown is due to a poisonous saliva injected by the louse, as is the melanoderma caused by the body louse.

The pubic louse may be exterminated by the measures recommended for the head louse, or by the use of officinal mercurial ointment.