Scabies and Pediculosis

• Mite burrowing and sensitization in scabies: Sarcoptes scabiei females burrow into the stratum corneum at a rate of 0.5–5 mm per day, creating serpentine tunnels where they deposit eggs and fecal material. This mechanical disruption triggers a biphasic immune response: the initial infestation (primary scabies) causes minimal symptoms for 3–6 weeks while IgE-mediated sensitization develops; subsequent exposures trigger immediate hypersensitivity reactions within 24–48 hours. The immune response includes release of histamine and activation of type 2 T helper cells (Th2), driving intense pruritis that is often worse at night when mite activity peaks due to circadian temperature-dependent behavior.

• Parasitic antigen-driven inflammation and pruritus: Both scabies and pediculosis trigger production of IL-4, IL-5, and IL-13, promoting eosinophilic and IgE-mediated responses. The pruritis arises from both mechanical irritation and neurogenic inflammation: mite fecal pellets contain proteases that activate protease-activated receptors (PAR-2) on sensory neurons, directly stimulating C-fiber nociceptors. In scabies, the immune response becomes dysregulated in immunocompromised hosts, leading to Norwegian scabies (crusted scabies), where reduced cellular immunity permits uncontrolled mite proliferation (>1 million mites versus typical 10–15), resulting in minimal pruritus despite massive infestation due to impaired Th2 responses.

• Louse feeding mechanics and anticoagulant adaptation: Pediculus humanus and Phthirus pubis survive by feeding on host blood approximately every 4–6 hours, injecting saliva containing anticoagulants and vasoactive substances that prevent platelet aggregation. These substances include apyrase (nucleotidase) and lipocalins that inhibit platelet activation and promote vasodilation. The host response includes localized erythema, papules, and secondary excoriation from scratching; unlike scabies, lice remain on the skin surface and do not burrow, making them potentially visible to the naked eye (nits are particularly visible on hair shafts).

• Secondary bacterial infection mechanism: Intense scratching disrupts the epidermis, creating portals for bacterial entry, particularly Staphylococcus aureus and Streptococcus pyogenes. These organisms produce superantigens that amplify T cell activation and inflammation, potentially leading to poststreptococcal glomerulonephritis in untreated cases, particularly in resource-limited settings.

• **Scabies caused by Sarcoptes scabiei with human-specific variants**: The var. hominis is obligately parasitic on humans and cannot complete its life cycle on animals (though rare zoonotic transmission from animals is possible with different strains). Transmission occurs through 15–20 minutes of direct skin-to-skin contact with an infested individual; sexual contact poses particularly high transmission risk. Fomite transmission is possible but less frequent, as mites can survive only 2–3 days off the human host in cool, humid conditions.

• Risk factors for widespread infestation: Immunocompromised states (HIV/AIDS with CD4 <200 cells/μL, malignancy, immunosuppressive therapy) eliminate the normal inflammatory response that controls mite numbers, leading to Norwegian scabies. Other risk factors include crowded living conditions (shelters, prisons, military barracks, nursing homes), poor hygiene, very young age (<5 years), advanced age (>65 years with compromised cell-mediated immunity), and developmental disabilities. Scabies incidence peaks in 3-year cycles, suggesting population immunity cycling.

• Pediculosis transmission and risk factors: Head lice (Pediculus humanus capitis) spread through direct contact, shared combs, brushes, hats, and bedding, particularly affecting children aged 3–12 years; prevalence is 1–3% in developed countries and higher in resource-limited settings. Body lice (P. h. corporis) are associated with homelessness, poor hygiene, and crowded conditions and can transmit epidemic typhus (Rickettsia prowazekii), trench fever, and relapsing fever. Pubic lice (Phthirus pubis) spread through sexual contact and are now less common in developed countries due to grooming practices; infestation of children should raise suspicion for sexual abuse.

• Pruritus as cardinal symptom in scabies: The intense, often intolerable pruritus is typically worse at night and in warm environments (bed warmth), reflecting mite nocturnal activity. Pruritus often precedes visible lesions by 1–2 weeks, making history crucial; patients often describe "something crawling under the skin." Scratching creates secondary excoriations and potential for impetigo, so examination findings may not match the severity of symptoms.

• Characteristic burrows and distribution patterns: Classic serpentine burrows appear as wavy, threadlike lines 5–15 mm long, typically gray or flesh-colored, most commonly found in web spaces between fingers, flexural wrists, anterior forearms, axillae, belt line, genitalia, and periumbilical region. However, burrows are present in only 10–15% of patients and are easily obscured by excoriations. Lesions often appear as inflammatory papules, nodules (particularly in genital region), or vesicles rather than burrows; the inflammation reflects the host immune response rather than direct mite damage.

• Distribution differences in special populations: Infants and young children may develop lesions on palms, soles, face, and scalp (rare in adults). Immunocompromised patients with Norwegian scabies develop hyperkeratotic, crusted plaques on hands, feet, elbows, and knees that may resemble psoriasis; these crusts contain millions of mites and are highly contagious.

• Pediculosis presentation with species-specific findings: Head lice present with visible nits (ova) firmly cemented to hair shafts, particularly at the nape and behind ears, accompanied by pruritus and often secondary impetigo of the scalp; adult lice may be visible moving on the scalp. Body lice create pruritic papules and linear excoriations on the trunk and proximal extremities; lice and nits are found primarily in clothing seams rather than on skin. Pubic lice present as pruritic papules in the pubic and perineal region with possible involvement of axillae, trunk, and eyelashes (phthirus causes "blue spots" or maculae ceruleae from blood pigment deposition); nits are visible on pubic hair.

• Important clinical variants: Scabies incognito occurs in immunocompromised or regularly treated patients with subtle or absent burrows; clinical suspicion must remain high. Bullous scabies presents with fluid-filled lesions resembling bullous pemphigoid, with IgG autoantibodies against BP180 collagen. Crusted (Norwegian) scabies appears in severely immunocompromised hosts with minimal pruritus, making diagnosis frequently delayed.

• Clinical diagnosis with microscopic confirmation: Diagnosis of scabies is primarily clinical, based on typical distribution, nocturnal pruritus, and household contact history. Confirmation requires demonstration of mites, eggs, or fecal pellets using microscopy. The scabicide ink test (applying ink to burrow, wiping away, and observing dark material tracking into burrow) has limited sensitivity but may help identify burrows. Dermoscopy can reveal mite, eggs, or fecal matter as a "triangular sign" or jet stream pattern within the burrow.

• Microscopic examination techniques and interpretation: Mineral oil preparation (applying mineral oil to suspected area, scraping firmly with a blade, transferring to microscope slide) has sensitivity of 50–70% but is observer-dependent. The KOH mount (10% potassium hydroxide treatment) dissolves cellular material but can damage mite morphology. Tape stripping followed by microscopy may increase yield. Finding even one mite, egg, or fecal pellet confirms diagnosis; absence does not exclude scabies given the low parasite load.

• Dermoscopy and imaging adjuncts: Dermoscopy at 10× magnification may reveal the mite body with anterior appendages, eggs arranged in a linear pattern, or triangular fecal pellets; specificity is >95% when findings are present but sensitivity is variable (60–80%). High-power magnification (×40 objective) on standard microscopy with immersion oil facilitates visualization of mite morphology. In resource-limited settings lacking microscopy, empiric treatment based on clinical presentation is justified.

• Diagnosis of pediculosis and nit identification: Head lice are diagnosed by visualization of live lice or nits; nits appear as tan-white, oval, 0.8 mm structures firmly attached to hair shafts and are distinguished from lint by their location (typically within 0.5 cm of scalp), firm attachment, and inability to be brushed away. Dermoscopy clearly visualizes nit morphology and confirms diagnosis. Body lice are identified by examination of clothing seams; pubic lice may be visible to naked eye or require magnification.

• Differential diagnosis considerations: Scabies must be distinguished from atopic dermatitis (lichenification, involvement of eyelids, family history), contact dermatitis (asymmetric distribution, clear exposure history), lichen planus (violaceous polygonal papules, reticular pattern), and drug reactions. Pediculosis must be differentiated from seborrheic dermatitis or lint (which do not adhere firmly and are easily removed). Scabies-like eruptions from other mites (Demodex, Cheyletiella) or hypersensitivity reactions occasionally mimic scabies but lack the characteristic distribution and response to scabicides.

• First-line treatment of scabies: permethrin cream: Topical permethrin 5% is the gold standard first-line treatment, applied from neck down to all skin surfaces (including web spaces, genitalia, buttocks), left on for 8–14 hours, then washed off. The recommended dose is 30 g per application; application to the entire body ensures coverage of mites at all locations. Two applications 1–2 weeks apart are standard to eliminate any mites that emerge from eggs after the first treatment. Permethrin acts as a neurotoxin, blocking sodium and chloride channels in mite nerve and muscle cell membranes, causing paralysis and death; selectivity for arthropod nervous systems over mammals provides safety (though neurotoxicity theoretically possible with excessive percutaneous absorption).

• Second-line and alternative treatments: Oral ivermectin 200 μg/kg (typically 12 mg per dose) given as two doses 1–2 weeks apart is highly effective and preferred for Norwegian scabies, immunocompromised patients, and institutional outbreaks. Ivermectin binds to glutamate-gated chloride channels unique to invertebrates, paralyzing mites; it does not kill eggs, necessitating repeat dosing. Topical sulfur 5–10% is safe in pregnancy and infants but is messy and has lower efficacy; application for 3 consecutive nights followed by 1–2 week interval before repeating is standard. Benzyl benzoate 10–25% is commonly used internationally but is irritating and less effective than permethrin; it requires multiple applications. Crotamiton 10% applied twice daily for 5 days is less effective than permethrin.

• Treatment of pediculosis and special considerations: Permethrin 1% lotion (or shampoo) applied to dry hair, left on for 10 minutes, then rinsed is first-line for head lice in non-resistant populations. Oral ivermectin (0.2 mg/kg twice, 1 week apart) is recommended for resistant lice or when topical treatment fails. Malathion 0.5% lotion (organophosphate insecticide) is increasingly used given pyrethroid resistance. Benzyl alcohol 5% works by anoxia rather than neurotoxicity, reducing resistance development. Spinosad 0.9% (derived from Saccharomyces spinosus) is ovicidal (kills nits directly) and requires single application. Manual nit removal with fine-toothed combs is adjunctive therapy. Body lice treatment involves permethrin applied to entire body or improved hygiene and clothing cleaning.

• Non-pharmacological measures: Fomite management is critical: all clothing, bed linens, and towels should be washed in hot water (≥50°C) and dried on high heat, or placed in plastic bags for ≥2 weeks (mites die without a host). Carpets, furniture, and other items should be vacuumed; chemical treatment of the environment is generally unnecessary and not recommended. Simultaneous treatment of all household members and close contacts is essential to prevent reinfection, even asymptomatic individuals. For head lice, environmental cleaning is minimal since lice do not survive off the scalp; brushes and combs should be washed in hot water.

• Monitoring parameters and treatment failure: Pruritus may persist for 2–4 weeks despite successful mite elimination due to residual inflammation and IgE sensitization; antihistamines and topical corticosteroids provide symptomatic relief. Persistent pruritus beyond 4 weeks suggests treatment failure (inadequate application, resistant mites, or reinfection). Repeat microscopy is NOT necessary if clinical response is appropriate (though reassessment of burrows at 2 weeks can confirm treatment success). Patients should be counseled that pruritus does not indicate treatment failure; absence of new burrows or lesions after 4 weeks indicates cure. Resistance to pyrethroids is now common in head lice populations in developed countries, making alternatives necessary in treatment-refractory cases.

• Secondary bacterial infection: impetigo and cellulitis: Aggressive scratching creates epidermal breaks allowing colonization by Staphylococcus aureus and/or Streptococcus pyogenes, leading to impetigo (honey-crusted lesions, highly contagious). This is among the most common complication in children and can progress to cellulitis with systemic symptoms; in tropical settings, scabies-associated impetigo predisposes to poststreptococcal glomerulonephritis and acute rheumatic fever. Treatment requires both scabicide and appropriate antibiotics (e.g., amoxicillin-clavulanate, cephalosporin, or cloxacillin depending on local resistance patterns).

• Norwegian scabies: life-threatening complication in immunocompromised hosts: In severely immunocompromised patients (particularly HIV/AIDS with CD4 <200), the normal immune response fails to control mite proliferation, resulting in extensive hyperkeratotic crusts with mite counts exceeding 1 million. This creates a highly contagious state with rapid transmission to caregivers; the massive antigenic load can trigger systemic reactions. The crusts are relatively non-pruritic (paradoxically, due to impaired immune response), making diagnosis frequently missed until widespread environmental contamination occurs. Treatment requires oral ivermectin (typically 200 μg/kg twice, one week apart, often repeated) combined with topical scabicide and sometimes keratolytic agents; relapse is common if immune reconstitution does not occur.

• Long-term sequelae and scabies nodules: Scabies nodules (indurated nodules 5–10 mm, typically in genital region, buttocks, axillae) represent a hypersensitivity reaction to mite antigens

Buzzwords that give away the diagnosis

• Nocturnal pruritus plus interdigital burrows: intense itching worse at night with serpentine burrows in finger web spaces, wrists, axillae, umbilicus, and genitals — plus itchy household contacts — is scabies until proven otherwise.
• Genital nodules in a man / itchy scrotal papules: highly specific for scabies; these are hypersensitivity nodules, not active infestation.
• Maculae ceruleae (slate-blue macules) point to Phthirus pubis.

The single best next step

• Treat empirically and treat every contact: the CDC STI Treatment Guidelines list two recommended regimens for classic scabies — permethrin 5% cream applied neck-down and washed off after 8–14 hours, or oral ivermectin 200 µg/kg repeated in 14 days. Lindane 1% is a second-line/alternative agent because of neurotoxicity. Permethrin remains the usual practical first choice and the preferred agent in pregnancy and young children. A negative skin scraping does not rule out scabies — the parasite load is low.
• Crusted (Norwegian) scabies: the answer is combined oral ivermectin plus topical scabicide, contact isolation, and a workup for immunosuppression (HIV with low CD4, HTLV-1, chronic glucocorticoids). Minimal itch with thick hyperkeratotic crusts is the tip-off.

The association examiners love

• Body lice as a vector: Pediculus humanus corporis transmits Rickettsia prowazekii (epidemic typhus), Bartonella quintana (trench fever), and Borrelia recurrentis (relapsing fever). Head and pubic lice are not established vectors of human disease — the vector-borne triad is a body-louse association. Body lice are treated with hygiene and laundering or discarding clothing, not scalp shampoo.
• Pubic lice in a prepubertal child: mandates evaluation for sexual abuse; the CDC also recommends screening for other STIs in adults with pubic lice.

Vignette traps

• Itch at week 3 after correct treatment: the answer is an antihistamine or topical corticosteroid, not re-treatment; new burrows or lesions signal true failure.
• The stem treats only the index patient: reinfestation is the expected outcome — treat all household and sexual contacts simultaneously.
• Eyelash involvement: phthiriasis palpebrarum is treated with occlusive petrolatum and mechanical nit removal, never permethrin near the eye.
• "Can she return to school with nits?": yes — the AAP opposes "no-nit" exclusion policies.
• Infant scabies: scalp, face, palms, and soles are involved, so the application must include them.