Applied Evidence

Scabies: Refine your exam, avoid these diagnostic pitfalls

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Nearly half of all infections are missed when first examined. Attentiveness to specific details, particularly in 3 common scenarios, can help ensure an accurate Dx.

VIDEO shows scabies mite in motion


› Consider scabies with any severe pruritic eruption. Conduct a thorough physical exam, preferably with a dermatoscope, for burrows in the webs and sides of fingers, proximal palm, and wrists. A

› Consider scabies in all patients—especially the immunocompromised—who have distal white or yellow thick, scaly, or crusted plaques. C

› Include scabies in the differential when patients present with smooth nodules of the genitals or pruritic smooth papules and plaques in other locations. A

Strength of recommendation (SOR)

A Good-quality patient-oriented evidence
B Inconsistent or limited-quality patient-oriented evidence
C Consensus, usual practice, opinion, disease-oriented evidence, case series



It is estimated that up to 45% of cases of scabies are misdiagnosed as another condition.1 This can occur when common clinical features are overlooked, a skin exam is rushed (and the rash is chalked up to dermatitis), or the wrong part of the pruritic lesion is scraped (the papule, rather than the burrow). There are also atypical presentations of scabies, which can confound even the most astute clinician.1 Misdiagnosis can increase health care costs due to repeat office visits or multiple referrals. In this article, we review the typical and atypical presentations of scabies and provide recommendations to aid physicians in its early recognition and correct diagnosis.

The scope of scabies infection, and its clinical stages

The prevalence of scabies, a common skin infection caused by the mite Sarcoptes scabiei, is estimated at 300 million cases worldwide annually, with the greatest incidence occurring in children and adolescents.1 In the developing world, its clinical burden is highest among the homeless, those of lower socioeconomic status, and those with poor hygiene. In the developed world, the clinical burden is highest among hospitalized patients and residents of long-term living facilities.

The S scabiei mite is an obligate parasite that elicits an adaptive immune response in susceptible hosts. The female mite lays 60 to 90 eggs that mature into adult mites after completing the mite life cycle in human hosts. In immunocompetent patients, roughly 10 to 15 surviving mites can be found at any given point in the disease process.2 In crusted or disseminated scabies, which often occur in immunocompromised patients, thousands of mites may be found at any given point in the disease process. 2

Scabies infection has 2 stages: the latent primary infection and the symptomatic secondary infection.

The primary infection starts with the initial mite invasion, typically with the transfer of impregnated females during person-to-person contact. Females deposit eggs as they burrow into the epidermis at the level of the stratum corneum with the use of proteolytic enzymes (creating the mite burrow). Surviving eggs hatch into larvae that then mature into nymphs and adult mites. After these adult mites mate, the impregnated females create new burrows and lay additional eggs.3 Patients may be asymptomatic during this initial stage and the infection may be transmitted from person to person through direct skin contact.

In children with scabies, vesicles, pustules, and nodular pruritic lesions may coexist with eczema and impetigo.

The second stage of infection is when patients experience severe pruritus with inflammatory papules seen on exam. The pruritus associated with scabies results from a delayed type IV hypersensitivity reaction to mite infestation. This requires host sensitization to the scabies mite. Clinically, there is a delayed onset (weeks) of numerous erythematous papules and, later, excoriated papules.

Conditions that scabies can mimic

The differential of typical scabies includes diagnoses manifesting with moderate to severe pruritus. In the immunocompetent adult, conditions to consider are atopic dermatitis, tinea corporis, papular urticaria, seborrheic dermatitis, poison ivy and other causes of contact dermatitis, drug eruptions, and irritant dermatitis. In immunocompetent infants, think of seborrheic dermatitis, atopic dermatitis, acropustulosis, and viral exanthems.

Continue to: Nodular scabies variants...


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