Describe the distribution of cutaneous findings in scabies.

Distribution of Cutaneous Findings in Scabies

The distribution of scabies lesions varies significantly based on age, immune status, and whether it’s a primary or secondary infection. The classic presentation differs in infants, children, and adults.

1. Infants and Young Children (under 2 years)

• Generalized Distribution: Unlike older children and adults, infants often present with a more widespread eruption. This is due to their less developed immune response and inability to communicate the itch effectively, leading to prolonged infestation.
• Common Sites: Face, scalp, palms, and soles are frequently involved. The face is often spared in older individuals due to thicker skin and less frequent exposure.
• Vesiculobullous Lesions: Infants are more likely to exhibit vesiculobullous lesions, sometimes resembling eczema or impetigo.
• Palmoplantar Involvement: The palms and soles are commonly affected in infants, a feature less common in adults.

2. Children (2 years to puberty)

• Interdigital Spaces: Scabies commonly affects the web spaces between the fingers and toes. This is a classic finding.
• Wrist and Elbow Flexures: These areas are frequently involved due to skin folds and increased friction.
• Axillae: The armpits are another common site of infestation.
• Belt Line: A characteristic feature is the involvement of the skin around the waistline, where clothing is tight.
• Genitalia: In boys, the scrotum and penis are often affected. In girls, the areolae and nipples may be involved.

3. Adults

• Similar to Children: Adults often exhibit the same distribution as children – interdigital spaces, wrist and elbow flexures, axillae, and belt line.
• Scrotum and Penis (Males): The scrotum, penis, and perineum are frequently involved in men.
• Areolae and Nipples (Females): The areolae and nipples are commonly affected in women.
• Wrist and Forearm: The extensor surfaces of the wrists and forearms are often involved.
• Buttocks: The buttocks can also be affected.
• Palmoplantar involvement: Less common than in infants, but can occur.

4. Immunocompromised Individuals (e.g., HIV/AIDS, organ transplant recipients, elderly)

• Crusted Scabies (Norwegian Scabies): This is a severe form of scabies characterized by thick, crusted lesions containing a massive number of mites.
• Generalized Distribution: Crusted scabies often presents with a widespread, non-pruritic eruption.
• Nails: Subungual hyperkeratosis (thickening under the nails) is common.
• Face and Scalp: The face and scalp are frequently involved in crusted scabies, unlike typical scabies.
• Minimal Inflammation: The inflammatory response is often diminished due to the compromised immune system.

5. Specific Lesion Morphology & Distribution

• Burrows: These are the hallmark of scabies – slightly elevated, serpiginous (snake-like) tracks in the skin, representing the mite’s tunnels. They are often seen on the sides of the fingers, wrists, and waistline.
• Papules: Small, red bumps are common, often clustered together.
• Vesicles: Small blisters can occur, especially in infants and immunocompromised individuals.
• Excoriations: Scratch marks are common due to intense pruritus.
• Secondary Infections: Bacterial superinfection (often with *Staphylococcus aureus* or *Streptococcus pyogenes*) is common due to excoriations, leading to impetigo or cellulitis.