Scabies
BY JENNA BOROK AND LAWRENCE EICHENFIELD, MD
Frontline Medical News
The scabies mite or the “itch mite” was discovered in 1687 as the first identifiable microorganism that caused human disease.1 Scabies is an infection of the epidermis with the mite, Sarcoptes scabiei variety hominis (S. scabiei) affecting approximately 300 million people worldwide.2 It is a common disease, especially among school-aged children and is particularly rampant in areas of poor sanitation and overcrowding.2,3
S. scabiei live in and on human skin where the impregnated female mite burrows into the stratum corneum and lays two to three eggs daily for as long as 30 days.3,4 The egg becomes a larva, which leaves the burrow and molts into a nymph, and it continues to molt into a mature mite.3 Mating then occurs during the mite stage. After mating, the male mite dies and the female completes the life cycle by burrowing back into the stratum corneum; this process takes about 2 weeks.3,4
Diagnosis
The diagnosis is usually based on strong clinical suspicion. The chief complaint is often a generalized itching rash that is often worse at nighttime.3,6 Small inflammatory papules are the main physical exam finding, and they usually are widely distributed, the favored locations including the finger webs, wrists, elbows, axillae, girdle area, and feet.3 In addition, male genitalia oftentimes are involved, and small itching papules on the penis should be considered scabies until proven otherwise.3 The head almost always is spared except in infants, who may present with pustules on the palms and soles of the feet and vesicles or lesions on the neck and face.7
The pathognomonic exam finding is the mite’s burrow, which appears as a 2-5 mm white, superficial, threadlike line. Upon close inspection, a tiny black speck often can be seen at the end of the burrow, which represents the adult mite.3 The presence of mites, eggs, or feces also is diagnostic and is accomplished by a skin scraping with a No. 15 blade.3 A positive scraping is diagnostic, although a negative scraping does not rule out the condition. (We explain this to our patients by saying “If you call someone’s home phone and they answer the phone, you know they are home. If you call and there is no answer, it could be that they aren’t home, or they are home and just not answering.”) A biopsy usually is not required, but may provide a diagnosis when scabies is unsuspected.3
The differential for red papules that itch in children includes atopic dermatitis, impetigo, papular urticaria, contact dermatitis, and other infestations, including bites from mosquitoes, fleas, and bed bugs. Atopic dermatitis (AD) can present as eczematous, erythematous papular lesions with oozing in flexural areas similar to areas affected by scabies. However, there are no burrows seen in AD, and the lesions are not in the interdigital web spaces. Impetigo has erythematous vesiculopustular lesions that form honey-colored crusts. Papular urticaria may be a hypersensitivity reaction to another insect, and the urticarial lesions are usually on the exposed parts of extremities. Unlike scabies, there are no burrows and usually no symptomatic family members. Contact dermatitis can present with vesicular, bullous, and sometimes papular erythematous lesions. It occurs after exposure to an allergen and often has well-demarcated borders with geometric shapes.
Treatment
Few randomized control or head-to-head trials exist on scabies treatment.2 First-line treatment is permethrin 5% cream, which is applied to the entire body surface from the neck down in children, but includes the face and scalp in infants.8,9 It is applied at bedtime and is washed off in the morning, and a second application is recommended after 7 days.3,8 It can be used in infants 2 months of age and older, and in pregnant females.3 Household contacts should be treated too, and those who are asymptomatic only require one application of permethrin.3 Permethrin is effective and more cost effective than ivermectin. Oral ivermectin has been used to treat scabies with a dose of 0.2 mg/kg and then repeated 2 weeks later, but the safety in children under 15 kg has not been determined.9 Lindane is an alternative option and is not recommended as first-line therapy because of its toxicity. It only should be used if the patient cannot tolerate or failed the previously mentioned therapies, with particular concern in children less than 50 kg.8,9 Infants and young children should be treated with permethrin and not with lindane.8 Clothes and bed linens can be decontaminated by machine-washing at a hot temperature.3,8 Topical corticosteroids and oral antihistamines can be helpful post-treatment to minimize pruritus and secondary eczematous changes.9
References
1. Int J Dermatol. 1998 Aug;37(8):625-30.
3. Lookingbill and Marks’ Principles of Dermatology, 5th edition (Philadelphia: Elsevier, 2013).
4. BMJ. 2005 Sep 17;331(7517):619-22.
5. Br Med J. 1941 Sep 20;2(4211):405-6.
6. Lancet. 2006 May 27;367(9524):1767-74.
7. Am J Clin Dermatol. 2002;3(1):9-18.
8. MMWR June 5, 2015 / 64(RR3);1-137.
Dr. Eichenfield is chief of pediatric and adolescent dermatology at Rady Children’s Hospital–San Diego and professor of dermatology and pediatrics at the University of California, San Diego. Ms. Borok is a medical student at the University of California, Los Angeles. Dr. Eichenfield and Ms. Borok said they had no relevant financial disclosures. Email them at pdnews@frontlinemedcom.com.