Chapter 12: Parasitic Skin Infestations

Scabies results from infestation by the itch mite Sarcoptes scabiei var. hominis, which burrows into the stratum corneum and triggers a delayed hypersensitivity reaction that produces intense nighttime itching weeks after initial exposure, though subsequent infections cause immediate symptoms. The condition presents with characteristic intraepidermal burrows appearing as thin linear marks, typically distributed across interdigital spaces, wrists, axillary folds, and the pelvic girdle. Clinical variants include crusted scabies, which occurs predominantly in immunocompromised or neurologically impaired individuals and involves massive mite populations forming protective warty crusts, and nodular scabies, characterized by firm pruritic dome-shaped lesions. Diagnosis relies on clinical presentation and can be confirmed through burrow ink testing or microscopic identification of mites and fecal material from skin scrapings. Treatment requires topical permethrin as first-line therapy, with systemic ivermectin reserved for refractory or crusted cases, alongside antihistamines and corticosteroids for pruritus management and potential antibiotics for secondary bacterial infections. Pediculosis encompasses three distinct louse species affecting different body regions and populations: head lice primarily infect school-age children through direct contact, body lice inhabit clothing fibers and associate with disease transmission in crowded settings, and crab lice affect sexually active individuals in pubic and perianal areas. Like scabies, louse-induced itching results from inflammatory reactions to parasitic saliva, with diagnosis confirmed by visualizing live lice or nits attached to hair shafts. Fine-toothed combing proves more effective than visual inspection alone for case identification and differentiation from pseudonits. Management emphasizes treating only active infestations using permethrin, pyrethrin, or prescription agents like malathion and spinosad, with manual delousing gaining acceptance due to chemical resistance. Both conditions require comprehensive patient education regarding environmental cleaning, contact notification, and the temporary persistence of symptoms following successful pharmacological treatment.