Handbook of Medical Entomology Part 8

The representatives of the family TARSONEMIDae are minute mites, with the body divided into cephalothorax and abdomen. There is marked s.e.xual 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.

[Ill.u.s.tration: 52. Pediculoides ventricosus, female. After Webster.]

[Ill.u.s.tration: 53. Pediculoides ventricosus, gravid female. (80).

After Webster.]

_Pediculoides ventricosus_ (fig. 52 and 53) is, of all the Tarsonemidae 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).

[Ill.u.s.tration: 54. Pediculoides ventricosus, male. After Braun.]

There is a very striking s.e.xual dimorphism in this species. The non-gravid female is elongate, about 200 by 70 (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 larvae undergo their entire growth in the body of the mother. They emerge as s.e.xually mature males and females which soon pair. The male (fig. 54) is much smaller, reaching a length of only 320 but is relatively broad, 80, and angular. Its abdomen is very greatly reduced.

As far back as 1850 it was noted as causing serious outbreaks of peculiar dermat.i.tis 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 Trombiidae). 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.

[Ill.u.s.tration: 55. Lesions produced by the attacks of Pediculoides ventricosus. After Webster.]

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 const.i.tutional 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 alb.u.min 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 TYROGLYPHIDae (fig. 150_f_) may abound on dried fruits and other products and attacking persons handling them, may cause a severe dermat.i.tis, 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 dermat.i.tis, 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, b.l.o.o.d.y 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.

Sarcoptidae

The SARCOPTIDae are minute whitish mites, semi-globular in shape, with a delicate transversely striated cuticula. They lack eyes and tracheae. 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 cla.s.sification. 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 Sarcoptidae 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 striae which are partly interrupted on the back. There are transverse rows of scales, or pointed spines, and scattered bristles on the dorsum.

[Ill.u.s.tration: 56_a_. Sarcoptes scabiei, male. (100). After Furstenberg.]

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

[Ill.u.s.tration: 56_b_. Sarcoptes scabiei, female. (100.) After Furstenberg.]

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 150 100. Furstenberg 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 larvae. 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 s.e.xually mature males and the so-called p.u.b.escent 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.

[Ill.u.s.tration: 57. Sarcoptes scabiei. Diagrammatic representation of the course in the skin of man.]

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.

[Ill.u.s.tration: 58. Scabies on the hand. From portfolio of Dermochromes by permission of Rebman & Co., of New York. Publishers.]

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 cla.s.ses, 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 pa.s.sage, 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 princ.i.p.al 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.

[Ill.u.s.tration: 59. Scabies on the hand. After Duhring.]

[Ill.u.s.tration: 60. Generalized infection of Scabies. After Morrow.]

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 pharmacopia. 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 dermat.i.tis. 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 crustosae_, 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 215-230 long and 165-175 wide; the males 145-150 by 120-125. The most important character separating _Notoedres_ from _Sarcoptes_ is the position of the a.n.u.s, 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 circ.u.mstances it is better to kill them promptly, to avoid spread of the disease to children and others who may be exposed.

[Ill.u.s.tration: 61. Notoedres cati, male and female. After Railliet.]

Demodecidae

The DEMODECIDae 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_.

[Ill.u.s.tration: 62. Demodex folliculorum. (200) After Blanchard.]

_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 380-400 long by 45; the male 300 by 40. 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.

[Ill.u.s.tration: 63. Demodex folliculorum. Section through skin showing the mites in situ. Magnification of Nos. 1, 2, 6 and 7, 150; Nos, 3, 4, 5, 450. After Megnin.]

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 a.s.sumed.

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.