10.3 Skin Infestations & Cutaneous Malignancies

Scabies: Presentation, Diagnosis and Eradication

Scabies is a highly contagious parasitic dermatosis caused by the microscopic mite Sarcoptes scabiei var. hominis.

• Transmission: Occurs via direct, prolonged skin-to-skin contact. Transmission via fomites (clothing, bedding) is less common but possible.
• Clinical Presentation: The hallmark symptom is intense, generalized pruritus that is characteristically worse at night. Pathognomonic lesions are burrows—thin, short (2-15 mm), wavy, thread-like gray or skin-colored lines representing the tunnels created by the female mite in the stratum corneum.
  • Distribution: Web spaces of the fingers, flexor surface of the wrists, elbows, axillae, periumbilical area, buttocks, female nipples, and male genitalia (erythematous nodules on the scrotum or penis are highly suggestive). The neck and head are spared in adults, though they can be involved in infants and the elderly.
  • Crusted (Norwegian) Scabies: A severe, highly contagious variant occurring in immunocompromised, senile, or disabled patients. It presents with thick, hyperkeratotic, scaling plaques containing millions of mites. Itch may be absent or minimal.
• Management:
  • Topical Permethrin 5% cream: First-line therapy. It must be applied thoroughly from the neck down to the toes (including web spaces, under fingernails, and genital folds). The cream is left on the skin for 8-14 hours (usually overnight) and then washed off. Because permethrin is not ovicidal, a second application 7-14 days later is mandatory to kill newly hatched mites.
  • Oral Ivermectin: A systemic alternative (200 mcg/kg single dose, repeated in 7-14 days). Indicated for institutional outbreaks, non-compliance with topicals, or crusted scabies (used in combination with topical permethrin).
  • Household Contacts: All household and close physical contacts must be treated simultaneously, even if they are asymptomatic, to prevent re-infestation.
  • Environmental Decontamination: All clothing, bedding, and towels used within the preceding 3 days must be washed in hot water (>50°C) and dried on a high-heat cycle. Non-washable items must be sealed in a plastic bag for at least 72 hours, as the mites cannot survive off human skin for more than 2-3 days.
  • Post-Scabetic Pruritus: Itching may persist for 2-4 weeks after successful mite eradication due to a persistent hypersensitivity reaction to mite antigens. This should be managed with topical steroids or oral antihistamines, not repeated scabicide applications.

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Pediculosis (Lice Infestation)

Pediculosis is caused by wingless, host-specific insects that feed on human blood. There are three clinically significant types:

1. Pediculosis Capitis (Head Lice): Caused by Pediculus humanus capitis. Most common in school-aged children. Transmitted by direct head-to-head contact. Presents with scalp pruritus, especially in the postauricular and occipital regions. Diagnosis is confirmed by finding live lice or nits (oval eggs) firmly cemented to the hair shaft within 1 cm of the scalp. Unlike dandruff, nits cannot be easily slid down the hair shaft.
2. Pediculosis Corporis (Body Lice): Caused by Pediculus humanus corporis. Typically affects individuals living in crowded, unhygienic conditions (e.g., homelessness). The lice reside and lay eggs in the seams of clothing, rather than on the skin, visiting the skin only to feed. Treatment primarily involves body hygiene and thorough laundering of clothing.
3. Pediculosis Pubis (Pubic Lice / "Crabs"): Caused by Pthirus pubis. Typically sexually transmitted. Presents with pubic pruritus and occasionally blue-gray macules (maculae caeruleae) on the lower abdomen or thighs due to salivary toxins. Patients should be screened for other sexually transmitted infections (STIs).

• Treatment of Head and Pubic Lice: First-line treatment is topical Permethrin 1% cream rinse or pyrethrins with piperonyl butoxide applied to washed, towel-dried hair for 10 minutes and rinsed. A repeat treatment is required 7-10 days later to target newly hatched nymphs. A fine-toothed nit comb should be used to remove dead lice and nits.

Epithelial Skin Cancers: Basal Cell and Squamous Cell Carcinoma

Non-melanoma skin cancers (NMSCs) represent the most common malignancies worldwide, arising from epidermal cells and closely linked to ultraviolet (UV) radiation.

Basal Cell Carcinoma (BCC)

BCC is the most common human cancer, arising from the pluripotential cells in the basal layer of the epidermis.

• Risk Factors: Intermittent, intense sun exposure, fair skin types, radiation therapy, and immunosuppression.
• Clinical Subtypes:
  • Nodular (80%): A pearly, translucent papule or nodule with prominent, dilated branching vessels (telangiectasias) and a rolled border. Central ulceration ("rodent ulcer") occurs as the lesion enlarges. Most common on the head and neck.
  • Superficial: An erythematous, scaly patch with a fine thread-like border, commonly on the trunk.
  • Morpheaform/Sclerosing: An indurated, flat, scar-like white or yellow plaque. Highly aggressive and infiltrative.
• Clinical Behavior: BCC is locally invasive and destructive, but it has extremely low metastatic potential (<0.1%).
• Management: Surgical excision with 4 mm margins is standard. Mohs micrographic surgery is indicated for lesions on the face (especially the central face, ears, and eyelids) or recurrent/aggressive subtypes to spare tissue and ensure clear margins.

Squamous Cell Carcinoma (SCC)

SCC is the second most common skin cancer, arising from epidermal keratinocytes.

• Risk Factors: Cumulative lifetime UV exposure, chronic non-healing wounds, thermal burns, exposure to arsenic, and immunosuppression (particularly in organ transplant recipients, who have an extremely high risk).
• Precursors: Actinic Keratosis (AK)—rough, sand-paper-like erythematous papules on sun-damaged skin. Bowen's disease represents SCC in situ.
• Clinical Presentation: A firm, erythematous, hyperkeratotic nodule or plaque with adherent scaling or crusting. It frequently ulcerates and bleeds with minimal trauma. Commonly located on sun-exposed areas such as the face, lower lip, ears, and hands.
• Clinical Behavior: Unlike BCC, SCC has a significant metastatic potential (2-5%), particularly for lesions on the lip or ear, large lesions (>2 cm), deep invasion, or in immunocompromised hosts.
• Management: Surgical excision with 4-6 mm margins or Mohs micrographic surgery.

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Malignant Melanoma: ABCDE Assessment and Staging

Malignant melanoma is a potentially lethal neoplasm arising from melanocytes, responsible for the vast majority of skin cancer-related deaths.

• Risk Factors: Fair skin, family history of melanoma, multiple atypical (dysplastic) nevi, a history of blistering sunburns during youth, and indoor tanning bed use.
• Clinical Screening (The ABCDE Criteria):
  • A - Asymmetry: One half of the lesion does not match the other half.
  • B - Border irregularity: The edges are notched, ragged, blurred, or poorly defined.
  • C - Color variegation: The color is not uniform; it includes shades of brown, black, red, pink, blue, or white.
  • D - Diameter: The lesion is larger than 6 mm (about the size of a pencil eraser), though melanomas can sometimes present at smaller sizes.
  • E - Evolving: The lesion is changing in size, shape, color, or symptoms (e.g., new onset of itching, bleeding, or tenderness).
• Subtypes: Superficial spreading (most common, 70%), nodular (vertical growth phase, aggressive), lentigo maligna (chronic sun damage in elderly), and acral lentiginous (palms, soles, subungual; most common in dark-skinned populations).
• Prognostic Factors & Staging:
  • Breslow Thickness: The most important prognostic indicator. It measures the vertical depth of the tumor in millimeters from the granular layer of the epidermis to the deepest point of invasion.
  • Sentinel Lymph Node Biopsy (SLNB): Used to detect subclinical micrometastasis in the draining lymph node basin. Generally recommended for melanomas with a Breslow thickness > 1.0 mm, or > 0.8 mm with ulceration or high mitotic rate.
• Management: Surgical management requires wide local excision with margins determined by tumor thickness:
  • Melanoma in situ: 0.5-1.0 cm margins.
  • Thickness < 1.0 mm: 1.0 cm margins.
  • Thickness 1.0 - 2.0 mm: 1.0-2.0 cm margins.
  • Thickness > 2.0 mm: 2.0 cm margins.
  • Systemic Therapy: Checkpoint inhibitor immunotherapy (e.g., Pembrolizumab, Nivolumab) or BRAF/MEK inhibitors (e.g., Dabrafenib/Trametinib) for BRAF-mutated advanced metastatic melanoma.