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

Related posts:

Skin Disease in Acute and Chronic Immunosuppression Dermatitis Herpetiformis Body ART Benign Neoplasias and Hyperplasias of Melanocytes Cutaneous Manifestations of Internal Malignant Disease: Cutaneous Paraneoplastic Syndromes Leprosy

Stay updated, free articles. Join our Telegram channel

Join