Chapter 208. Scabies, Other Mites, and Pediculosis

Epidemiology

Scabies is worldwide and affects all ages, races, and socioeconomic levels. Prevalence varies considerably with some underdeveloped countries having rates from 4% to 100% of the general population.1 An infested host usually harbors between 3 and 50 oviparous female mites,2 but the number may vary considerably among individuals. For example, patients with crusted formerly “Norwegian” scabies (Fig. 208-1) who have a defective immunologic or sensory response (i.e., leprosy, paraplegic, or HIV-infected patients) harbor millions of mites on their skin surface, with minimal pruritus.

It has been well established that close personal contact is a prime route of transmission. Although sometimes considered a sexually transmitted disease, the equally high prevalence in children attests that casual contact or sharing of objects among children and other family members is also sufficient to transmit the disease. Transmission via inanimate objects has been best demonstrated with crusted scabies. This condition is notoriously contagious, and anyone roaming within the general vicinity of these patients risks acquiring the infestation. Indeed, 6,000 mites/g of debris from sheets, floor, screening curtains, and nearby chairs have been detected.3 Mites are also prevalent in the personal environment of normal scabies patients.4,5 In one study, live mites were recovered from dust samples taken from bedroom floors, overstuffed chairs, and couches in every patient's dwelling.5

Etiology and Pathogenesis

Scabies is an infestation by the highly host-specific mite, Sarcoptes scabiei var. homini, family Sarcoptidae, class Arachnida. The mite is pearl-like, translucent, white, eyeless, and oval in shape with four pairs of short stubby legs. The adult female mite is 0.4 × 0.3 mm with the male being slightly smaller—just slightly too small to be seen by the naked eye. The scabies mite is able to live for 3 days away from the host in a sterile test tube, and for 7 days if placed in mineral oil mounts.4,6 Mites cannot fly or jump.

The life cycle of mites is completed entirely on human skin. The female mite by a combination of chewing and body motions is able to excavate a sloping burrow in the stratum corneum to the boundary of the stratum granulosum.7,8 Along this path, which can be 1 cm long, she lays two to three eggs a day during her life span of 30 days. Eggs hatch in 10 days and larvae leave the burrow to mature on the skin surface. The male mite lives on the surface of the skin and enters burrows to procreate.

Clinical Findings

The diagnosis of scabies is suspected by pruritus associated with a characteristic distribution of lesions and epidemiologic history. Pruritus typically appears 4–6 weeks after initial infestation; with subsequent reinfestations, symptoms develop within 2 days.9 Similar to the human response to other insects such as fleas, yellow jackets, and mosquitoes, there is a wide range of clinical responses to an infestation with scabies and some individuals remain asymptomatic despite being infested. These individuals are considered “carriers.”

On physical examination, patients display excoriations and eczematous dermatitis that favors the interdigital webs (Fig. 208-1), sides of fingers, volar aspects of the wrists and lateral palms (Fig. 208-2), elbows, axillae, scrotum, penis (Fig. 208-3), labia, and areolae in women. The head and neck are usually spared in healthy adults, but in infants, elderly, and immunocompromised, all skin surfaces are susceptible. Indurated, crusted nodules can be seen in children on intertriginous areas. In crusted scabies, (Fig. 208-1) hyperkeratotic plaques develop diffusely on the palmar and plantar regions, with thickening and dystrophy of the toenails and fingernails.

The pathognomonic lesion is a burrow, which is a thin, thread-like, linear structure (Fig. 208-2) 1–10 mm in length. It is a tunnel caused by the movement of the mite in the stratum corneum. When present, the burrow is best seen in the interdigital webs and wrists; however, it can be difficult to find in early stages of the condition, or after the patient has extensively excoriated the lesions. Identification of a burrow can be facilitated by rubbing a black felt-tip marker across an affected area. After the excess ink is wiped away with an alcohol pad, the burrow appears darker than the surrounding skin because of ink accumulation in the burrow.

A definitive diagnosis is made by microscopic identification of the scabies mites, eggs, or fecal pellets (scybala). This is accomplished by placing a drop of mineral oil over a burrow and then scraping longitudinally with a number 15 scalpel blade along the length of the burrow or a suspicious skin area, being careful not to cause bleeding. The scrapings are then applied to a glass slide and examined under low power (Fig. 208-3). Confocal microscopy and dermoscopy can also be used to examine the mite in vivo10,11 and a skin biopsy can be diagnostic, if the mite happens to be transected in the stratum corneum (see eFig. 208-3.1). An enzyme-linked immunosorbent assay has been developed for serologic testing of other mite infestations in animals; however, no serologic tests for scabies exist for humans.12 Despite the possibility of confirming the presence of mites via multiple methods of testing, the diagnosis usually is based on clinical impression, and solidified by response to treatment.

Differential Diagnosis

(See Box 208-1)

Complications

Secondary impetiginization may occur and poststreptococcal glomerulonephritis has resulted from scabies-induced pyodermas caused by Streptococcus pyogenes. Lymphangitis and septicemia have also been reported in crusted scabies (see eFig. 208-3.2). Finally, scabies infestation can also trigger bullous pemphigoid.13,14

Treatment

Scabies is treated by a combination of a scabicide and fomite control. With all insecticidal therapies, a second application, usually a week after the initial treatment, is required to reduce the potential for reinfestation from fomites as well as to kill any nymphs that may have hatched after treatment as a result of a semiprotective environment within the egg. All household and close contacts must be simultaneously treated to prevent reinfestation from asymptomatic carriers.

Topical scabicides are applied overnight to the entire skin surface with special attention to finger and toe creases, cleft of the buttocks, belly button, and beneath the fingernails and toenails. In adults, one can exclude treating the scalp and face. Most treated individuals experience relief from symptoms within 3 days, but patients must be informed that even after adequate scabicidal therapy, the rash and pruritus may persist for up to 4 weeks. The itching experienced during this time period is commonly referred to as “postscabetic itch.” Patients should be educated that excessive washing of the skin with harsh soaps will aggravate their skin irritation. Instead, oral antihistamines and emollients can be beneficial. The treatments for scabies are summarized in Box 208-2 but a few comments are warranted:

• Lindane has received a “black box” warning as well as restrictive labeling changes by the US Food and Drug Administration (FDA) to greatly restrict its usage.15,16 Moreover, it is banned in California.17 A physician should write a prescription for lindane only when cognizant of all the caveats are noted by the FDA.18 (See Box 208-2 footnote.)
• There are no documented cases of scabies resistance to permethrin, but tolerance is beginning to develop.19 Pregnant females, breast-feeding mothers, and children under 2 years should limit their two applications (1 week apart) to 2 hours only when using permethrin.
• Crotamiton is considerably less effective than all other options offered.
• Five to ten percent sulfur is messy, malodorous, tends to stain, and can produce irritant dermatitis, but is inexpensive and may be the only choice in areas of the world in which a lack of funds dictates therapy.20 The efficacy and toxicity of sulfur has not been critically evaluated in recent years, but many feel that it is the safest choice for neonates and pregnant females.21

Ivermectin is an anthelmintic agent derived from a class of compounds known as avermectins. It has been used in veterinary medicine since 1981, and has excellent antiparasitic properties.22,23 Ivermectin has been approved since 1996 by the FDA for treatment of two diseases, namely onchocerciasis and strongyloides. Clinical efficacy for scabies has been impressive at a dosage of 200 μg/kg given twice 1 week apart.24,25 Given that millions of people have been treated for onchocerciasis worldwide without significant side effects, it appears to be extremely safe. Nevertheless, because the drug acts on nerve synapses utilizing glutamate or γ- aminobutyric acid, and because the blood–brain barrier is not fully developed in young children, it is not recommended for use in children less than 15 kg (33 lbs) or in pregnant or lactating women. Success rates approach 100% in studies where entire households and close contacts of infested individuals are treated while maintaining strict fomite controls.23,26

In crusted scabies, the combination of oral ivermectin and a topical scabicide are recommended as the oral medication will not penetrate into the thickness of the keratinous debris under the nails.

Prevention

Several measures should be considered to reduce the potential of reinfestation by fomite transmission. Because of the common occurrence of asymptomatic mite carriers in the household, all family members and close contacts should be treated simultaneously. After treatment, treated individuals should wear clean clothing, and all clothing, pillow cases, towels and bedding used during the previous week should be washed in hot water and dried at high heat. Nonwashables should be dry-cleaned, ironed, put in the clothes dryer without washing, or stored in a sealed plastic bag in a warm area for 2 weeks. Floors, carpets, upholstery (in both home and car) play areas, and furniture should be carefully vacuumed. Fumigation of living spaces is not recommended. Pets also do not need to be treated because they do not harbor the human scabies mite.

There are 45,000 described species of mites that belong to the subclass Acarina and the class Arachnida. Some of the mites that can affect humans are listed in Table 208-1. Human infestation by these mites occurs only accidently (save for Demodex species).

Demodex folliculorum hominis and Demodex brevis are the only mites that routinely live on humans. These organisms reside in the hair follicle and the infundibulum of the sebaceous gland, respectively. Their presence has been linked with rosacea, perioral dermatitis, and suppurative folliculitis, although a causal role for mites in these diseases has not been established.

Although animal and fowl mites are not primary parasites of humans, Pyemotes sp. can cause straw itch, oak leaf eruption, or itch mite eruptions. These mites can cause epidemics of dermatitis with outbreaks in the last decade occurring in several Midwestern States. Pyemotes ventricosus and Pyemotes tritici occur in animal handlers, farmers participating in harvesting of grain, and those exposed to decorative grain.27,28 Pyemotes herfsi's normal host is the leaf galls on oak trees, and therefore this eruption characteristically occurs in people who spend time outdoors in or near wooded areas. Typical bites appear on exposed skin as red macules with a small blister center 10–16 hours after contact.

Harvest mites (also called berry bugs, red bugs, scrub-itch mites, and chiggers) are in the family Trombiculidae and are distributed worldwide.29 In the United States they inhabit mostly the southeast, south, and midwest, in areas of grasslands, forest, and damp areas along lakes and streams. Humans are susceptible to the larvae from April until the first frost. The minute, reddish larvae of Trombicula are less than 0.5 mm long and feed on skin cells of animals, including humans. Rather than sucking blood, these mites inject digestive enzymes into the skin breaking down cells, which can subsequently cause severe reactions and swelling. Each bite has a characteristic red papule with a white, hard central area. After feeding, they drop off their hosts and mature into adults, which are harmless to humans. Rarely does a victim realize when the bite is occurring, as itching from a chigger bite does not develop until 1–2 days after the bite. Chiggers prefer warm covered areas of the body, and thus the bites are often clustered behind the knees, or beneath tight undergarments such as socks, underwear, or brassieres.

The house dust mite is a cosmopolitan guest in human habitation and feeds off flakes of shed human skin. The mites are harmless, but their bodies and excreta are believed to play a role in human disease. They are a common precipitant of asthma, hay fever, and allergic respiratory symptoms worldwide. In addition, atopic dermatitis may be exacerbated in some patients by dust mite allergens.30,31

Pediculosis, the infestation of man by lice, has been a human affliction since antiquity. Three species of lice infest humans: (1) Pediculus humanus capitis, the head louse, (2) Pediculus humanus humanus, the body or clothing louse, and (3) Phthirus pubis, the pubic, or crab, louse.

Pediculosis Capitis (Head Lice)

Epidemiology

Head lice infestations occur worldwide and are most common in children between the ages of 3 and 12.32 Based on pediculicide sales in the United States, an estimated 10–12 million children are infected each year. Head lice affect all levels of society, and all ethnic groups; however, the incidence is low among African-Americans in the United States, possibly due to an anatomic inability of female lice in America to deposit eggs on coarse curly hair.33

Transmission is by means of direct head-to-head contact or by indirect (fomite) transmission through combs, brushes, blow-dryers, hair accessories, upholstery, pillows, bedding, helmets, or other headgear.34,36–39 Lice can be dislodged by air movement, blow-dryers, combs, and towels, and passively transferred to fabric, facilitating new infestations.29–33

Etiology and Pathogenesis

Head lice are blood-sucking, wingless, highly host-specific insects belonging to the order Anoplura. They are almost 2 mm long with three pairs of claw-like legs that are well adapted for grasping hair. Their entire life cycle is on the scalp. More than 95% of infested individuals have fewer than 100 adult lice in their scalps. The female louse lays 5–10 eggs per day during her 30-day life span. After 10 days, the eggs hatch producing larvae, which are referred to as nymphs or “instars.” Instars look like miniature adult louse and go through three stages of development that take 14 days for full maturation. The eggs are laid about 1 cm from the scalp surface, firmly attached to individual hairs with a proteinaceous glue that closely resembles the amino acid composition of the human hair shaft itself.40,41

Lice typically survive less than 2 days away from the scalp, although under favorable conditions of heat and humidity, survival has been reported at 4 days. Nits can survive for 10 days away from the scalp.

Clinical Findings

Pediculosis capitis is confined to the scalp with nits (Fig. 208-4) found most readily in the occipital and retroauricular regions. Most patients experience pruritus. The average incubation before symptoms is 4–6 weeks. Some individuals remain asymptomatic despite infestation, and can be considered “carriers.”

Bites of the mites may produce 2 mm erythematous macules or papules, but usually an examiner only finds excoriations, erythema, and scaling. Other findings may include a low-grade fever, regional lymphadenopathy, and irritability.

Infestations are diagnosed by demonstrating egg capsules (nits) and live lice. Nits are readily seen by the naked eye and are an efficient marker of past or present infestation. They can be differentiated from dandruff, hair casts and the like, as nits are not easily removed from the hair shaft.42 The color of newly laid or viable eggs is tan to brown; the remains of eggs that have hatched are clear, white, or light in color. The presence of adult lice confirms active infestation. However, lice are fast, avoid light, and blend in with the hair, making them difficult to find. Finding live adult lice or immature nymphs is best achieved with fine combing the hair with a nit comb. Wet combing, in which water and conditioner are applied to the hair prior to using the nit comb, increases the yield by prying the adult lice from the hair follicles.41,42

Differential Diagnosis

See Box 208-3.

Complications

Secondary bacterial infections can occur with pediculosis capitis. In fact, head lice are thought to be the most common cause of pyodermas of the scalp in the developed world.43

Head lice and body lice are closely related, so it is not surprising that head lice can serve as host for rickettsiae and have the potential of transmitting diseases.44,45 Head lice in laboratory experiments have been readily infected with Rickettsia prowazekii.46 Bartonella quintana, which causes trench fever, has also been isolated in head lice.47,48,71,72 Transmission of these infections to humans by pediculosus capitis, however, has never been described and it is highly unlikely to occur outside of experimental conditions.

Treatment

Standard treatment recommendations for pediculosis capitis utilize a two-step process of confirming active infestation with live lice and then treating with an over-the-counter or prescription pediculocidal therapy. Pediculocide choice is typically based on local resistance patterns and access of patients to a physician for prescription medications.49 However, with increasing resistance to pediculocides, a multimodal approach, similar to Staphylococcus aureus therapy, is warranted to prevent widespread resistance to currently available products (see Fig. 208-5).50 This is especially important when treating with prescription pediculocides.

Physical methods to treat infestations, including shaving one's head to avoid infestation, dates back to the sixth century bc, when priests and wealthy Egyptians removed scalp hair and wore wigs. The routine of head shaving for military services today was founded on the same principle. While a buzz haircut may be a solution for boys, such an approach would be traumatic psychologically for girls. Combing out nits is difficult, tedious, time-consuming, and somewhat painful. While wet combing can be an adjuvant to topical insecticidal therapy, is not by itself sufficient in most situations.44

Pediculicides remain the most effective treatment for head lice.51–58 Given (1) variable ovicidal activity, (2) possible lack of patient compliance, (3) growing resistance to pediculocides, and (4) the potential of fomite reinfestation, it is recommended to repeat treatment with all insecticidal treatments in 1 week.

The array of treatments for head lice is summarized in Box 208-4. One of the leading factors for the increasing number of infestations is resistance of lice to topical therapies.59–65 Since the introduction of insecticides years ago, the louse has adapted by several genetic alterations. The agents with the highest success rates, namely malathion and ivermectin, are prescription products. One trial has shown oral ivermectin given twice at a 7-day interval to be more effective than topical malathion lotion.66 However, both pediculocides are highly effective and treatment decisions should be based on local resistance patterns and individual patient characteristics.

There are a number of anecdotal and market-driven reports with occlusive and suffocation methods (such as with application of petrolatum, mayonnaise, dimethicone, vegetable oil, mineral oil, hair pomade, and olive oil).67,68 To accurately evaluate pediculicidal activity of any compound, one must appreciate that head lice have the ability to “resurrect” from a state of seeming death, in which respiratory and motor function appear to have ceased.69,70 These insects are less dependent than mammals for continuous nervous control of respiration and circulation, and the exact point of death is not readily defined. Indeed, the World Health Organization recommends pediculicidal testing to be read 24 hours after application of insecticide since otherwise mortality rates are overestimated.71 Not following these guidelines has lead to overestimates of the efficacy of several alternative treatments with occlusive agents and essential oils from health food stores. Such products slow the movements of adult lice and may allow them to be more easily combed out of their scalps, but these substances are usually not lethal to lice.

Patients should be counseled in at least some effective measures to prevent reinfestation by fomite transmission. After treatment, treated individuals should wear clean clothing, and all clothing, hats, pillow cases, towels and bedding used during the previous week should be washed in hot water and dried at high heat. Nonwashables should be dry-cleaned, ironed, put in the clothes dryer without washing, or stored in a sealed plastic bag in a warm area for 2 weeks. Combs and brushes may be washed in very hot water (65°C) or may be coated with the pediculicide for 15 minutes. Floors, carpets, upholstery (in both home and car) play areas, and furniture should be carefully vacuumed to remove any hairs with viable eggs attached. Fumigation of living spaces is not recommended and pets do not need to be treated because they do not harbor the human head louse. There is no reliable evidence that strict “no nit” policies at schools are necessary to prevent spread of the infestation, and they should be abandoned in favor of family education regarding effective treatment.

Pediculosis Corporis (Body Lice)

Etiology and Pathogenesis

Pediculus humanus humanus, body lice, have a very similar morphology to head lice, except they are 30% larger. The body louse's lifespan is 20 days during which the female may lay up to 300 eggs. The lice lay their eggs in the seams of clothing, while obtaining their blood meals from the host. The body louse can survive without a blood meal for up to 3 days.

Epidemiology

Pediculosis corporis requires exposure to the louse and favors an inability to wash and change clothing. Thus, it is most commonly found on homeless individuals, refugees, victims of war and natural disasters, or those forced into crowded living condition with poor hygiene. The infestation is usually transmitted by contaminated clothing or bedding. After exposure, the inability to wash or change clothes allows the infestation to persist.

Clinical Findings

Symptomatically, patients complain of pruritus. Most commonly, the only sign of body lice is excoriations, often linear and primarily on the back, neck, shoulders, and waist. Postinflammatory pigmentation is seen in chronic cases. Adult lice are not easily seen except in heavy infestations. Diagnosis is made by closely examining the lining of the clothing, particularly at the seams, for the presence of nits. The clothing may also be shaken over a sheet of white paper, at which time the lice may be seen moving about on the paper.

Differential Diagnosis

Box 208-5.

Complications

Several important human diseases are transmitted by the body louse. The major diseases include epidemic typhus (caused by a rickettsiae, R. prowazekii), murine typhus (caused by Rickettsia typhi), trench fever (caused by B. quintana), and relapsing fever (caused by a spirochete, Borrelia recurrentis).73–77 Lice obtain organisms, such as rickettsiae and spirochetes, from ingestion of blood meals from infested hosts. Transmission of microorganisms from body lice is not from the louse bite, but rather by (1) contaminated fecal material being scratched into excoriated skin of bite sites, (2) inhalation of dry, powdery louse feces from handling typhus-contamined bedding or clothing or (3) infected lice having its gut ruptured, allowing an infective blood meal to enter excoriations on the skin. In addition, excoriation can lead to secondary infection with S. aureus, S. pyogenes, and other bacteria.

Treatment

The most important treatment of body lice is disinfestation of all clothing and bedding. Beds should be burned or sprayed with lice sprays, because the body louse may lay eggs on the seams of the mattress or couch. Clothing is best treated like biohazardous waste, bagged, and tightly sealed in specially marked, plastic, biohazard bags. The waste is handled separately from other trash until it can be incinerated, maintaining a temperature of 65°C for 30 minutes. If this is not possible, clothing and bedding should be fumigated, machine washed in hot water, and dried on high heat or dry-cleaned. Hot ironing of the seams of upholstered furniture should also be performed and exposure to infested items should be strictly avoided for 2 weeks. The patient should be treated from head to toe with a topical insecticide or given oral ivermectin.

Pediculosis Pubis (Crab Lice)

Etiology and Pathogenesis

Pediculosis pubis is caused by infestation of the body with Phthirus pubis. Crab lice range from 0.8 to 1.2 mm in length and have wide, short bodies resembling tiny crabs. They have a serrated edge on their first claw, which gives them traction on flat, hairless, surfaces; thus, they can navigate across the entire body surface. They most commonly are found in the pubic and perianal region, but occasionally they also reside in mustache, beard, axillae, eyelashes, eyebrows, and scalp hair. In hirsute individuals, they are also found on the short hairs of the thighs and trunk. The louse has a lifespan of less than 3 weeks during which time the female will lay about 25 eggs on human hairs. The adult crab louse can survive for 36 hours off the human host, while the eggs are viable for up to 10 days.

Epidemiology

Crab lice can be found in all levels of society and all ethnic groups. Patients with crab lice often have another concurrent sexually transmitted disease. Although pediculosis pubis is considered a sexually transmitted disease, transmission has been documented to occur from contaminated clothing, towels, and bedding.

Clinical Findings

In the case of crab lice, all hairy parts of the body should be examined, especially the eyelashes, eyebrows, and perianal area. Many individuals have two different hair-bearing sites infested.78 These lice can be mistaken for scabs or moles, or can blend in with skin color making them difficult to detect. Infested patients have an average of 10–25 adult organisms on their body. Nits can also be identified near the base of hairs (Fig. 208-6). The diagnosis can be confirmed by microscopic examination of the plucked hair to identify the nits and/or adult lice. Although rare, skin lesions named maculae caerulea (eFig. 208-6.1), representing hemorrhage, can be seen with pubic lice with slate gray to bluish, irregular-shaped macules about 1 cm in diameter. Pediculosis palpebrarum, or phthiriasis palpebrarum, is the infestation of the eyelashes with crab lice.

Differential Diagnosis

Box 208-6.

Treatment

Shaving is not curative as the louse will seek another hairy area of the body to reside. Crab lice are treated with the same topical therapy as that for pediculosis capitis (Box 208-4).79 In vitro and in vivo resistance to pyrethrins have been shown.80 There is a lack of appreciation for their tendency to inhabit rectal hair.78 Unless the physician is certain that only one body area is involved, all hairy areas of the body should be treated because (1) it is not uncommon to have other areas infested, and (2) lice can migrate away from a treated areas to other hair-bearing locations. For this reason, oral ivermectin is recommended for this entity.71 However, as ivermectin treatment relies on the insect obtaining a blood meal, so the nits are not affected and the patient requires repeat oral ivermectin on day 8 and day 15.

Phthiriasis palpebrarum (Fig. 208-7) has traditionally been treated with petrolatum (Vaseline), but this treatment is slow and needs to be applied at least five times a day for weeks. Ivermectin is the first-line therapy for this condition.81

Fomite precautions mirror those discussed previously for pediculosis capitis. Treatment failure is usually a result of failure to treat all hairy areas (especially perirectally) or reinfestation from neglecting to treat sexual contacts. Other household members are also infested occasionally and should be carefully questioned for symptoms and/or examined.