Chapter 143. Scabies

A 2-year-old boy is seen with severe itching and crusting of his hands (Figures 143-1 and 143-2). He also has a pruritic rash over the rest of his body. The child has had this problem since 2 months of age and has had a number of treatments for scabies. Other adults and children in the house have itching and rash. Various attempts at treatment have only included topical preparations. A scraping was done and scabies mites and scybala (feces) were seen (Figures 143-3 and 143-4). The child and all the family members were put on ivermectin simultaneously and the Norwegian scabies cleared from the child. The family cleared as well and the child was given a repeat dose of ivermectin to avoid relapse.

Seven-year itch.

• Three hundred million cases per year are estimated worldwide.1 In some tropical countries, scabies is endemic.
• The incidence of scabies in a study performed in general practices in England and Wales was 351 per 100,000 person-years in men and 437 per 100,000 person-years in women.2
• Data from the Royal Infirmary in Edinburgh show that 5% of patients with skin disease between 1815 and 2000 had scabies; the prevalence during wartime reached over 30%.3

• Human scabies is caused by the mite Sarcoptes scabei, an obligate human parasite (Figure 143-3).1,4
• Adult mites spend their entire life cycle, around 30 days, within the epidermis. After copulation the male mite dies and the female mite burrows through the superficial layers of the skin excreting feces (Figure 143-4) and laying eggs (Figure 143-5).
• Mites move through the superficial layers of skin by secreting proteases that degrade the stratum corneum.
• Infected individuals usually have less than 100 mites. In contrast, immunocompromised hosts can have up to 1 million mites, and are susceptible to crusted scabies also called Norwegian scabies (Figures 143-1, 143-2, and 143-6, 143-7, 143-8, 143-9, 143-10).1
• Transmission usually occurs via direct skin contact. Scabies in adults is frequently sexually transmitted.5 Scabies mites can also be transmitted from animals to humans.1
• Mites can also survive for 3 days outside of the human epidermis allowing for infrequent transmission through bedding and clothing.
• The incubation period is on average 3 to 4 weeks for an initial infestation. Sensitized individuals can have symptoms within hours of reexposure.

• Scabies is more common in young children, healthcare workers, homeless and impoverished persons, and individuals who are immunocompromised or suffering from dementia.1
• Institutionalized individuals and those living in crowded conditions also have a higher incidence of the infestation.1

Clinical Features

• Pruritus is a hallmark of the disease.1
• Skin findings include papules (Figure 143-10), burrows (Figure 143-11 and 143-12) nodules (Figures 143-13), and vesiculopustules (Figure 143-14).
• Burrows are the classic morphologic finding in scabies and the best location to find the mite (Figures 143-11 and 143-12).
• Infants and young children can also exhibit irritability and poor feeding.
• Pruritic papules/nodules around the axillae (Figure 143-13), umbilicus, or on the penis and scrotum (Figure 143-15 and 143-16) are highly suggestive of scabies.

Typical Distribution

• Classic distribution in scabies includes the interdigital spaces (Figure 143-17), wrists, ankles (Figure 143-18), waist (Figures 143-19 and 143-20), groin, axillae (Figure 143-13), palms, and soles (Figures 143-1, 143-2, 143-6, and 143-7).
• Genital involvement can also occur (Figures 143-15 and 143-16).
• In children, the head can also be involved (Figure 143-21).

Laboratory Studies and Imaging

• Light microscopy of skin scrapings provides a definitive diagnosis when mites, eggs, or feces are identified (Figures 143-3, 143-4, 143-5). This can be challenging and time-consuming, even when mites, eggs, or feces are present. Packing tape stripping of skin has also been used instead of a scalpel to find mites for examination under the microscope.6 The inability to find these items should not be used to rule out scabies in a clinically suspicious case. In what is believed to be a recurrent case, it is helpful to find definitive evidence that your diagnosis is correct.
• Dermoscopy is a useful and rapid technique for identifying a scabies mite at the end of a burrow (see Appendix C: Dermoscopy).7 The mite has been described as an arrowhead or a jet plane in its appearance (Figure 143-22). The advantage of the dermoscope is that multiple burrows can be examined quickly without causing any pain to the patient. Children are more likely to stay still for this than scraping with a scalpel or skin stripping with tape.
• If a dermoscope is available, start with this noninvasive examination. If the findings are typical, then a microscopic examination is not needed. If the findings are not convincing, or a dermoscope is not available, perform a scraping. It is best to scrape the skin at the end of a burrow. Use a #15 scalpel that has been dipped into mineral oil or microscope immersion oil. Scrape holding the blade perpendicular to the skin until the burrow (or papule) is opened (some slight bleeding is usual). Transfer the material to a slide and add a cover slip.
• Tips for microscopic examination—Start by examining the slide with the lowest power available as mites may be seen under 4 power and the slide can be scanned most quickly with the lowest power. If no mites are seen switch to 10 power and scan the slide again looking for mites, eggs, and feces. Forty power may be used to confirm findings under 10 power.
• In one study comparing dermoscopic mite identification with microscopic examination of skin scrapings, found the former technique to be of comparable sensitivity (91% and 90%, respectively) with specificity of 86% (vs. 100% by definition), even in inexperienced hands.8 Another study reported sensitivity of dermoscopy at 83% (95% confidence interval, 0.70 to 0.94).9 In this study, the negative predictive value was identical for dermoscopy and the adhesive tape test (0.85), making the latter a good screening test in resource-poor areas.
• Videodermatoscopy can also be used to diagnose scabies.10 Videodermatoscopy allows for skin magnification with incidental lighting at high magnifications for viewing mites and eggs. The technique is noninvasive and does not cause pain.
• S. scabiei recombinant antigens have diagnostic potential and are under investigation for identifying antibodies in individuals with active scabies.11

Biopsy

Rarely necessary unless there are reasons to suspect another diagnosis.

• Atopic dermatitis—Itching is a prominent symptom in atopic dermatitis and scabies. The distribution of involved skin can help to differentiate the 2 diagnoses. Look for burrows in scabies and a history of involved family members. In children, atopic dermatitis is often confined to the flexural and extensor surfaces of the body. In adults, the hands are a primary site of involvement (see Chapter 145, Atopic Dermatitis).
• Contact dermatitis—Characterized by vesicles and papules on bright red skin, which are rare in scabies. Chronic contact dermatitis often leads to scaling and lichenification and may not be as pruritic as scabies (see Chapter 146, Contact Dermatitis).
• Seborrheic dermatitis—A papulosquamous eruption with scales and crusts that is limited to the sebum rich areas of the body; namely, the scalp, the face the postauricular areas, and the intertriginous areas. Pruritus is usually mild or absent (see Chapter 151, Seborrheic Dermatitis).
• Impetigo—Honey-crusted plaques are a hallmark of impetigo. Scabies can become secondarily infected, so consider that both diagnoses can occur concomitantly with papules and pustules present (Figure 143-23) (see Chapter 116, Impetigo).
• Arthropod bites—Bites may exhibit puncta that allow for differentiation from scabies.
• Acropustulosis of infancy (Figure 143-24)—A vesicopustular recurrent eruption limited to the hands, wrists, feet, and ankles. It is rare after 2 years of age (see Chapter 110, Pustular Diseases of Childhood).

Nonpharmacologic

• Environmental decontamination is a standard component of all therapies. SOR B Clothing, bed linens, and towels should be machine washed in hot water. Clothing or other items (e.g., stuffed animals) that cannot be washed may be dry cleaned or stored in sealed bags for at least 72 hours.12

Medications

Treatment includes administration of an antiscabicide and an antipruritic.1,13

• Permethrin 5% cream (Elimite, Acticin) is the most effective treatment based upon a systematic review in the Cochrane Database.13 SOR A The cream is applied from the neck down (include the head when it is involved) and rinsed off 8 to 14 hours later. Usually, this is done overnight. Repeating the treatment in 1 to 2 weeks may be more effective. SOR C In patients with crusted scabies, use of a keratolytic cream may facilitate the breakdown of skin crusts and improve penetration of the cream.14 Unfortunately, scabies resistance to permethrin is increasing.
• Ivermectin is an oral treatment for resistant or crusted scabies. Studies have demonstrated its safety and efficacy. Most studies used a single dose of ivermectin at 200 mcg/kg.13 SOR A Taking the drug with food may enhance drug penetration into the epidermis.14 Some experts advocate repeating a dose 1 week later. It is worth noting that the FDA has not labeled this drug for use in children weighing less than 15 kg. Ivermectin is currently available only in 3- and 6-mg tablets, so dosing often needs to be rounded up to accommodate the use of whichever tablets are available. As there is no oral suspension available, tablets may need to be cut and given with food for use in children.
• Diphenhydramine, hydroxyzine, and mid-potency steroid creams can be used for symptomatic relief of itching. SOR C. It is important to note that pruritus may persist for 1 to 2 weeks after successful treatment because the dead scabies mites and eggs still have antigenic qualities that may cause persistent inflammation.
• All household or family members living in the infested home and sexual contacts should be treated. SOR C Failure to treat all involved individuals often results in recurrences within the family. Use of insecticide sprays and fumigants is not recommended.
• Other less-effective medications include topical benzyl benzoate, crotamiton, lindane (no longer used in the United States because of concerns regarding neurotoxicity), and synergized natural pyrethrins.9 SOR A Topical agents used more commonly in other countries include 5% to 10% sulfur in paraffin (widely in Africa and South America), 10% to 25% benzyl benzoate (often used in Europe and Australia), and malathion.14 In infants younger than 2 months of age, crotamiton or a sulfur preparation is recommended by one author instead of permethrin because of theoretical concerns of systemic absorption of permethrin.14
• Antibiotics are needed if there is evidence of a bacterial superinfection (Figure 143-23). SOR C

Complementary and Alternative Therapy

• Tea tree oil contains oxygenetic terpenoids, found to have rapid scabicidal activity.15

• Avoid direct skin-to-skin contact with an infested person or with items such as clothing or bedding used by an infested person.
• Treat members of the same household and other potentially exposed persons at the same time as the infested person to prevent possible reexposure and reinfestation.

• The prognosis with proper diagnosis and treatment is excellent unless the patient is immunocompromised; reinfestation, however, often occurs if environmental risk factors continue.1
• Postinflammatory hyper- or hypopigmentation can occur.1

• Routine follow-up is indicated when symptoms do not resolve.
• Consider an immunologic work-up for individuals with crusted scabies.

• Patients should avoid direct contact including sleeping with others until they have completed the first application of the medicine.
• Patients may return to school and work 24 hours after first treatment.
• Patients should be warned that itching may persist for 1 to 2 weeks after successful treatment but that if symptoms are still present by the third week, the patient should return for further evaluation.

Patient Resources

• http://www.cdc.gov/parasites/scabies/.
• http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001833/.

Provider Resources

• http://emedicine.medscape.com/article/1109204.
• http://dermnetnz.org/arthropods/scabies.html.