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INTRODUCTION

Cutaneous lesions arising from infective agents are common reasons for seeing a dermatologist at the outpatient clinic or for inpatient admission. The diagnosis is often suspected clinically based on the localization of the lesions (e.g., over typical inoculation sites) and clinical appearance such as verrucous growth, inflammation, and pus formation. In immunocompetent patients, the intact epidermal skin barrier and innate immunity keep the skin infection-free. In immunosuppressed patients, cutaneous infections may take on exaggerated and atypical features. Presumptive diagnosis of cutaneous infections such as serology may assist in early diagnosis, but definitive diagnosis often requires the demonstration of the causative organism within the skin lesion by direct visualization, cultures, or specific antigenic detection. The sample is often from direct swabs or biopsy taken from the infected skin.

Cutaneous infections are classically categorized under the group of causative organisms: viral, bacterial, fungal, and parasitic.

VIRAL INFECTIONS

Viral infections commonly cause exanthems and enanthems, leading to maculopapular or morbiliform rashes (Figures 6–1, 6–2, 6–3, 6–4, 6–5, 6–6, 6–7, 6–8, 6–9, 6–10, 6–11, 6–12, 6–13, 6–14, 6–15, 6–16, 6–17, 6–18, 6–19, 6–20, 6–21, 6–22, 6–23, 6–24, 6–25, 6–26, 6–27). Vesicobullous eruptions may be caused by herpes simplex virus type 1 and 2, varicella zoster virus, enteroviruses, and coxsackievirus; clinical entities include herpes labialis, herpes genitalis, chicken pox, herpes zoster, and hand-foot-mouth disease. Human papilloma virus (HPV) infection is extremely common in children and adults and manifests as common and genital warts. Patients with underlying skin barrier defects such as atopic dermatitis, may develop secondary infections with HSV (eczema herpeticum) and molluscum contagiosum. In the immunocompromised, infection can be florid and atypical and malignant transformation may occur. HIV infection gives rise to a variety of skin manifestations occurring at different CD4 count levels, and is covered in Chapter 26.

Figure 6-1

Molluscum contagiosum

Multiple discrete pearly papules with central umbilication are seen on the popliteal fossa of this 3-year-old boy. This contagious infection, caused by a pox virus, is spread by direct contact and is most commonly seen in young school-going children. Spontaneous resolution occurs in most children although treatment with a variety of destructive methods (e.g., cryotherapy, electrocautery, application of trichloroacetic acid, cantharidin) may be used if the patients can tolerate them.

Figure 6-2

Molluscum contagiosum—histology

Several acanthotic epidermal invaginations are present in this biopsy of molluscum contagiosum. Within the crateriform lobules, there are intracytoplasmic eosinophilic inclusion bodies present in the keratinocytes, which are then extruded to the surface.

Figure 6-3

Viral wart—palm

Human papilloma viruses (HPVs) are a large group of DNA viruses with around ...

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