6th July 2023

Other Skin Infections

Scabies 

A skin infestation caused by the mite, Sarcoptes scabiei. The fertilised female mite burrows into the stratum corneum laying up to 3 eggs daily. Eggs develop into adult mites within 2 weeks. Symptoms may take 4-8 weeks to develop.

Scabies can affect any age and presents mostly in the winter months. It is spread via direct contact and is more frequent in crowded conditions, institutions and in immunosuppressed patients.

Scabies classically present with intense itching that is worse at night and can persist for several weeks despite treatment. Sites of infestation include the fingers, wrist, ankles, nipples and genitalia. Irregular burrows around 1cm long may be evident, with a mite visible at one end. A hypersensitivity reaction produces a rash that can consist of erythematous papules, vesicles or nodules.

Scabies

Image Source: Wikimedia Commons (2005). Acarodermatitis Hand, Licensed under a creative commons licence. Available for free at: https://commons.wikimedia.org/wiki/File:Acarodermatitis_Hand.jpg (Accessed on: 10 March 2022).

Crusted scabies is due to an impaired host immune response and is more frequent in elderly and immunodeficient patients. It is highly contagious and represents hyper infestation with thousands/millions of mites. It is characterised by a scaly rash with minimal/absent itch.

Crusted scabies in a patient with AIDS

Image Source: Wikimedia Commons (2008). Norwegian Scabies in Homeless AIDS Patient, Licensed under a creative commons licence. Available for free at: https://commons.wikimedia.org/wiki/File:Norwegian_Scabies_in_Homeless_AIDS_Patient.jpg (Accessed on: 10 March 2022).

Scabies is diagnosed clinically. Uncertain cases can be confirmed by dermatoscopy and skin scraping microscopy, which may demonstrate burrows, mites, ova and mite faeces. If crusted scabies is suspected, underlying immunodeficiency should be investigated. Management involves treating the individual, household members, close contacts and sexual contacts with permethrin 5% cream. Alternative: malathion 0.5% liquid. Crusted scabies requires oral ivermectin. Clothing/bedding/towels should be washed at least 60 degrees and dried in a hot dryer. Pruritus can be managed with crotamiton 10% cream, antihistamines and sometimes mild-topical corticosteroids.

Erythema Multiforme (EM)

EM is a self-limiting hypersensitivity reaction caused predominantly by infection, for which HSV causes the majority of cases. Other causes include certain medications or conditions (table 7). It is now considered a distinct entity from Stevens-Johnson syndrome and Toxic Epidermal Necrolysis.

Table 7: Triggers for erythema multiforme

Mycoplasma pneumoniae is often listed as a cause of EM, however this has recently been classified as its own distinct entity, mycoplasma pneumoniae-induced rash and mucositis (MIRM).

EM acutely manifests on the distal extremities as symmetrical erythematous papules that transform into target-like lesions. These have a dusky centre, a pale and oedematous intermediate ring and an erythematous outer ring. Lesions may be painful or itchy. EM minor has cutaneous manifestations only, whereas EM major has the addition of mucosal involvement. Diagnosis is clinical but investigations to review the cause may be required. Treatment is supportive. Precipitating factors should be managed and offending medication avoided. Ocular involvement requires ophthalmology input. Recurrent cases may benefit from antiviral therapy.

Cutaneous erythema multiforme

Image Source: Wikimedia Commons (2011). Erythema Multiforme EM 01 ajustement niveaux auto, Licensed under a creative commons licence. Available for free at: https://commons.wikimedia.org/wiki/File:Erythema_Multiforme_EM_01_ajustement_niveaux_auto.jpg (Accessed on: 10 March 2022).

Mucosal erythema multiforme

Image Source: Wikimedia Commons (2021). Erythema multiforme tongue, Licensed under a creative commons licence. Available for free at: https://commons.wikimedia.org/wiki/File:Erythema_multiforme_tongue.jpg (Accessed on: 10 March 2022).