Table of Contents   This Chapter   All Chapters

INFESTATIONS

Scabies

Etiology and pathogenesis

Scabies represents infestation of the skin by the mite Sarcoptes scabiei . This mite (Fig.27.1) burrows in the skin, where the gravid female lays eggs and deposits feces (Figs 27.2 and 27.3). The host will not notice the mite immediately, as it takes several weeks for the immune system to recognize the mite as foreign. Once this occurs, however, intense itching ensues.

    Prolonged physical contact is necessary for transmitting the mite, which can only live on humans. Children (Fig.27.4) and the elderly are particularly prone to this infection. Any one patient may have 5–20 mites on their body at one time. The patient will be unable to see the mite, but the trained clinician can detect a black dot at the end of a burrow (Fig.27.5), which represents the mite. The areas of the body that are best for viewing the mite and its burrows are the sides of the fingers, the sides of the feet, and the inner wrists.

Figure 27.1 Microscopic examination of the scabies mite. One method of collecting a specimen for viewing is to put several drops of mineral oil on a slide and moisten a no. 15 blade with it. Then, scrape the suspected burrows to remove sufficient tissue for examination (often slight bleeding results). The classic mite is shown here. Often, it is still alive and may be seen moving.

Figure 27.2 Scabies burrow viewed intact under the microscope. Note the clustered eggs at one end.

Figure 27.3 Microscopic examination of scabies. Note the two oval eggs and the multiple brown feces (scybala) in this figure. One excellent method for diagnosing scabies is to extract the mite on a needle.

Figure 27.4 Scabies in an infant. Diffuse pruritic, eczematous lesions on an infant are often confused with eczema.

Figure 27.5 Scabies burrow. The needle points to the burrow.

Clinical

All patients whose predominant complaint is itching should be closely examined for burrows (Fig.27.6). These thread-like structures are often V-shaped at one end, corresponding to the external opening of the burrow in the stratum corneum. At the other, a black dot representing the mite is seen (Fig.27.7). The patient should fully disrobe to allow examination of the web spaces, sides of fingers (Fig.27.8), wrists, axillae, trunk, waist (Fig.27.9), groin (Fig.27.10), and feet (Fig.27.11). Linear, thread-like burrows, often with a black dot at one end (the mite), are virtually diagnostic. A mineral oil examination (see Figs 27.1 and 27.3) is diagnostic if the mite, eggs, or feces are seen.

    However, the dominant rash is usually that which occurs secondary to itch and consequent scratching; there may be widespread rash, often with a non-specific ‘buckshot' pattern, even though the number of mites present is only small. The rash often has mixed eczematous and urticarial features. Over time, lichenification, excoriations, scabetic nodules, and secondary bacterial infection may develop. Note that the latter may obscure the diagnosis. Small vesicles or even bullae may occur rarely (Fig.27.12a). Occasionally, the patient with scabies will complain only of a hand dermatitis (Fig. 27.12b) or a penile eruption. Pustules of the hands and feet are common in infants (Fig.27.13), and the scalp may also be involved, although this is uncommon in older children and adults living in temperate climates. Multiple red-brown papulonodules may develop in the patient who has had scabies for several months (Figs 27.14 and 27.15).

Figure 27.6 Scabies burrows are white, thread-like structures best seen in areas of thicker skin (e.g. sides of feet, sides of fingers, and wrists).

Figure 27.7 The black speck at the end of the burrow is the mite.

Figure 27.8 The sides of the fingers are excellent places to look for the burrows of scabies.

Figure 27.9 Lesions about the umbilicus. The lower abdomen and umbilicus are commonly affected by nondescript, excoriated lesions in scabies.

Figure 27.10 Scabies of the penis. Very few conditions cause diffuse itching and red nodules of the penis. This clinical presentation is nearly diagnostic of scabies.

Figure 27.11 Scabies burrow on the side of the foot.

Figure 27.12 (a) Small vesicles and even bullae may rarely occur in scabies. They result from the patient’s inflammatory response to the mite and its eggs and feces. (b) Scabies causing hand dermatitis. Any patient who presents with hand dermatitis should have at least a brief examination of the sides of the fingers and the wrists for burrows.

Figure 27.13 Scabies in an infant. A close inspection of the palms and soles often reveals the characteristic burrows. Often, vesicles and pustules occur in an infant. One should always consider infantile acropustulosis (see Fig. 19.88).

Figure 27.14 Nodular scabies. (a) Multiple red-brown lesions are shown here on the inner thigh. The groin is a characteristic place for nodular scabies. (b) Some lesions of nodular scabies possess a burrow running across the top, as shown here. The patient is developing an intense immune reaction to the mite’s presence.

Figure 27.15 Chinese writing scabies. In some immunocompetent patients with prolonged scabies infestation, hundreds of burrows may occur. This differs from crusted (Norwegian) scabies in that each of the burrows is individually identifiable, forming a pattern that has been likened to Chinese writing.

Differential diagnosis

The broad differential here is that of significant pruritus without much of a rash, with scattered non-specific rash, or with a rash that is mainly secondary to scratching (Table 27.1). Remember that the rash of scabies:

Treatment

External treatment with permethrin is safe and effective; this is a synthetic compound derived from pyrethrin but is more potent than its parent compound. Note that it must be the preparation designed to treat scabies, not the lower-concentration rinse preparation used for head lice. The patient, all household contacts, intimate contacts, and any other people who have had physical contact of a repeated or prolonged nature with the patient since the time that they have been itching should be treated. Treating only the patient and not everyone in the household is a common mistake. This will often lead to recurrence. It is important to remember that it may take a full month of the mite burrowing on a person before they start to itch, as it may take that long for the allergy (the cause of the itching) to develop; contacts who are asymptomatic are therefore likely to be in this incubation phase.

    The cream should be applied from the neck down and left on overnight. Every nook and cranny of the patient (e.g. web spaces and under every nail) must be covered, not just the area of the rash. Also, all clothing must be washed in warm or hot water and then dried in a dryer. Cloth items that cannot be laundered may be stored for several days, which is a period thought sufficient to allow the natural death of the mite. In the morning, all individuals put their linen and pajamas in the wash, and shower. If done correctly, this should be curative in over 90% of cases. However, if one person leaves even one mite under one nail, the infestation will recur. Patients should be informed that itching will continue for 1–4 weeks after successful treatment, as the dead mites, eggs, and scybala (feces) to which they are still allergic will be on them for that time. A medium topical steroid (e.g. triamcinolone 0.1% cream) can be applied during that time. Intralesional triamcinolone (5–10 mg/mL) may be helpful for nodular lesions.

    Recently, oral ivermectin has been shown useful in treatment of scabies. In one study comparing the efficacy of ivermectin versus topical permethrin cream, a single application of permethrin was superior to a single dose of ivermectin (200 m g/kg). Two doses of ivermectin, however, were as effective as a single application of permethrin. For the 60-kg adult, two 6-mg tablets once and repeated in 7 days is appropriate. Ivermectin should not be used in pregnancy, while breast-feeding, or in children less than 15 kg (where the blood–brain barrier is less effective and the risk of seizures is higher).

    Very young children and pregnant women need special attention. For the pregnant or nursing woman, or the infant less than 2 months of age, crotamiton or precipitated sulfur can be used safely, although the cure rate is not high. Use of permethrin in pregnancy is much more likely to be successful, and the risk of side effects is low. Treatment failure may occur secondary to reinfection, failure to apply the scabicide to infected areas (e.g. under the nails), or from infestation of unusual areas (e.g. the face or the scalp).

PRACTICE POINTS

Crusted scabies

Etiology and pathogenesis

In crusted scabies, also known as Norwegian scabies, thousands of mites have populated the skin, in contrast to conventional scabies, in which the number is thought to be less than 20. Patients usually have neurologic disease (e.g. Down syndrome or mental retardation) or immunosuppression (e.g. AIDS, hematologic malignancy, or advanced age).

Clinical

Scaling, crusting, and itching begin on the hands, feet, and groin, as with typical scabies. As the number of mites increases, thick, white crusting may develop virtually anywhere (Figs 27.16 and 27.17). The appearance of this crust has been likened to fine white sand. Unlike classic scabies, the face is commonly involved (Fig.27.18). Secondary infection with Staphylococcus aureus is common, and septicemia has been reported. A scabies epidemic affecting other patients and healthcare providers in hospitals and institutions is a common complication.

Figure 27.16 Crusted scabies. Thousands of mites populate the skin, in contrast to conventional scabies, in which the number is thought to be less than 20.

Figure 27.17Crusted scabies. Heavy crusting in the web spaces is shown.

Figure 27.18Crusted scabies of the face. In typical scabies, the face is not affected. This is not true in crusted scabies, which may be found on any part of the body, including the face.

Differential diagnosis

The white, crusted hyperkeratosis is quite characteristic. However, this pattern is not common, and most patients who have not presented to a dermatologist initially have been treated for eczema or for the secondary infection. In unusual cases, psoriasis might appear similar but does not have the same degree of itch.

Treatment

Either permethrin or ivermectin, as with classic scabies, should be prescribed, but repeated courses should be given. Daily bathing to soak off the crust is appropriate. The area under the nails should be given special attention, as it is a source of residual mites. The face, scalp, and ears should be treated.

Pediculosis capitis (head louse infestation)

Etiology and pathogenesis

Pediculus humanus capitis is a flat, wingless, clawed insect whose only natural host is humans (Fig.27.19). They live on the hair but feed from the skin of the scalp. Lice die after failing to find a blood meal for 24–72h. Nits (the eggs of the louse) can survive for up to 10 days. Direct contact is the most common form of transmission, but fomite transmission also occurs. Common transferring agents include hats, brushes, combs, earphones, bedding, furniture, headgear, and rugs.

Figure 27.19 Pediculosis capitis, louse. (a) Note the long, slender body and the claws that grip the hair. (b) A lighter-colored louse is shown here.

Clinical

The patient complains of itching in the scalp (Fig.27.20). Close inspection reveals the small, white, ovoid eggs or nits attached firmly to the base of the hairs. The nits are usually much easier to see than the lice, which can move rapidly along the hair shaft (Figs 27.21 and 27.22); the hair above the ears is often the best place to find nits. Bite reactions, pruritus, excoriations, lymphadenopathy, and conjunctivitis may occur.

Figure 27.20IPediculosis capitis, adult. If a patient complains of intense itching of the scalp, a close inspection for nits should always be performed. Nits are much more easily found than the lice that lay them.

Figure 27.21Pediculosis capitis, louse. Close inspection of the hair near the scalp reveals the wingless louse. Note that head and body lice have longer, more slender bodies than the pubic louse, which is more crab-like.

Figure 27.22 Pediculosis capitis, microscopic view of the nit. The egg case (nit) is shown firmly cemented to the hair shaft.

Differential diagnosis

The most relevant are as follows.

Treatment

Permethrin cream 1% rinse or gamma benzene hexachloride shampoo applied for 5–10min, followed by rinsing, is curative. Most (but not all) of the nits will be killed but still be attached. Successful treatment requires their removal. They may be combed out with a fine-tooth nit comb after pretreating the hair with various over-the-counter preparations containing formic acid. Appropriate cleansing of combs, brushes, hats, etc. is also important. The furniture, car seats, and bedding should be vacuumed. It is important to apply any medication to dry hair, as water may cause the louse to close down its respiratory airways for half an hour.

    Unfortunately, there is an alarming increase in lice resistant to permethrin cream and lindane. For these, permethrin applied overnight to the scalp may work. Malathion is an effective alternative, although the medicine is flammable due to its alcohol base.

    Alternative treatments include Vaseline (or mayonnaise) applied in sufficient quantity to suffocate the louse (e.g. 30–40g) and left on overnight. Diligent shampooing in the morning is necessary to remove the Vaseline. Other recommendations have included co-trimoxazole in otitis media doses; the blood-parasitic lice ingest this antibiotic, which either has a toxic effect on the louse or destroys the intestinal bacteria on which it depends. A single oral dose of ivermectin (200mg/kg repeated in 10 days) has been shown to be effective.

    A significant problem is that of knowing whether nits are viable and represent reinfection if they are found some time after treatment. Their position can help with this. Lice lay their eggs close to the scalp; any nits more than 1cm or so from the scalp are just old, hatched-out egg cases that have remained attached to the growing hair.

Pediculosis corporis (body louse infestation)

A variant of Pediculus humanus causes pediculosis corporis. Excoriations, crusting, and urticarial papules may be seen in pediculosis corporis (Fig.27.23), but the lice do not actually live on the skin; rather, they live in clothing and move on to the skin to feed (Fig. 27.24). Any part of the skin covered by clothes may be affected. Usually, it is the homeless or other indigent patients, who do not routinely wash their clothes, who are affected.

    Disposing of the clothes and bathing the patient is usually curative. Any bedding or clothing that cannot be thrown away can be laundered with hot water. Topical steroids may be given to control the itching until the rash subsides.

Figure 27.23 Pediculosis corporis. Any indigent or homeless patient (or any patient who does not routinely wash his or her clothes) with chronic pruritus and excoriations must be evaluated for pediculosis corporis.

Figure 24.24 Pediculosis corporis. Inspection of the clothing allows for diagnosis. Shown here are multiple eggs and a louse. Note that the shape of the louse is very similar to that of the parasite that causes pediculosis capitis.

Pediculosis pubis (pubic louse infestation)

Phthirus pubis is the technical name for the pubic louse or ‘crab louse' (Fig. 27.25). Its short, wide body is better suited for grasping pubic hairs, which are thicker and spaced further apart than scalp hairs. This disease is spread by close, usually sexual, physical contact.

    Pruritus of the groin is often the only symptom noted by the patient. Close inspection of the area shows multiple nits and crab-like organisms hanging to the base of adjacent hairs. The organisms may spread upward to the abdomen and chest. Hairy men may find the louse attached not only to the pubic hair (Fig.27.26) but also to the hairs on the abdomen, chest (Fig.27.27), axilla, thighs, beard, and eyelashes (Fig.27.28). Application of lindane lotion or permethrin cream from trunk to knees for 10 min with combing is effective treatment.

    For eyelashes, apply physostigmine drops, which paralyze the lice so they can be brushed off, and use aqueous permethrin application to kill the nits.

Figure 27.25 Pediculosis pubis. The pubic louse is shown here. Note the shorter, wider body compared with the louse that causes pediculosis capitis. This adaptation is better suited for grasping the wider-spaced pubic hairs.

Figure 27.26 Pediculosis pubis. Pruritus of the groin is often the only symptom of pediculosis pubis. Inspection of the area shows small, ovoid nits (eggs) attached firmly to the hair and pointing away from the skin.

Figure 27.27 Pediculosis pubis. Closer inspection will show crab-like organisms hanging to the base of adjacent hairs. The organisms may spread upward to the abdomen and chest, as shown here. This disease is spread by close physical (e.g. sexual) contact.

Figure 27.28 Pediculosis of the eyelashes. Pediculosis may also affect the eyelashes, as shown here. Multiple nits are seen. Physostigmine will paralyze the adult lice, allowing them to be easily brushed off, with aqueous malathion applied as a single treatment to coat the nits and consequently kill them.

White/Cox: Diseases of the Skin, 2ed.(c) 2006, Elsevier Inc. All rights reserved.