Intro: Scabies & Pediculosis - Itchy Critter Chaos
- Scabies: Caused by mite Sarcoptes scabiei var. hominis. Intense itching, especially at night. Burrows visible.
- Pediculosis: Infestation by lice (Head: Pediculus humanus capitis; Body: P. humanus corporis; Pubic: Pthirus pubis). Itching, visible nits/lice.
- Transmission: Close personal contact, fomites.
- Goal of Pharmacotherapy: Eradicate parasites, manage symptoms (pruritus, secondary infections).
⭐ Scabies burrows are pathognomonic and often found in interdigital spaces, wrists, axillae, and genital areas. Secondary bacterial infections are common due to scratching.
Key Drugs: Permethrin & Ivermectin - The Dynamic Duo
| Feature | Permethrin | Ivermectin (Oral) |
|---|---|---|
| MOA | Disrupts Na+ channels in parasite neurons → paralysis & death. | Activates glutamate-gated Cl- channels in parasite nerve/muscle → hyperpolarization, paralysis & death. |
| Spectrum | Sarcoptes scabiei (scabies), Pediculus humanus (lice). | Scabies (incl. crusted), Pediculosis (esp. resistant), some Helminths. |
| Key SE | Transient burning, stinging, pruritus at application site. | Generally well-tolerated. Headache, dizziness. Mazzotti reaction (if treating filariasis). |
| Dosing Highlights | Scabies: 5% cream, neck-down, wash off 8-14h. Lice: 1% lotion, wash off 10min. | Scabies/Lice: 200 mcg/kg single oral dose. May repeat in 1-2 weeks. |
| Pregnancy Cat. | B | C |
Other Anti-Parasitic Agents - Supporting Squad Scourge
- Lindane (1%): Organochloride.
- MOA: Blocks GABA-A chloride channels → CNS hyperexcitability.
- Use: Scabies, pediculosis (2nd line). 📌 L for Lingering neurotoxicity.
- ⚠️ Avoid: Infants, children <2yrs, pregnancy, seizure Hx. Apply 8-12h (scabies), 4min (lice).
- Benzyl Benzoate (10-25%): Ester.
- MOA: Neurotoxic to parasite nervous system.
- Use: Scabies (adults 25%, children 10-12.5%), pediculosis.
- Apply neck down for 24h; may repeat. Irritant.
- Crotamiton (10%):
- MOA: Scabicidal mechanism unclear; antipruritic.
- Use: Scabies (less effective), symptomatic pruritus relief.
- Apply 2-5 days for scabies; wash off 24-48h after last dose.
- Malathion (0.5-1%): Organophosphate.
- MOA: Irreversible cholinesterase inhibitor.
- Use: Pediculosis (head/pubic lice), ovicidal. Apply 8-12h. Flammable.
⭐ Malathion shows high efficacy against permethrin-resistant head lice.
- Sulfur (5-10%):
- MOA: Converted to pentathionic acid ($H_2S_5O_6$), a scabicide.
- Use: Scabies (safe in pregnancy/infants), acne, seborrhea.
- Apply nightly for 3-7 days. Odor, staining of clothes/bedding.
Treatment: Special Considerations - Smart Strategy Central
- Agent Selection: Tailor to patient (see flowchart).
- Crusted (Norwegian) Scabies: Highly contagious! Oral Ivermectin + repeated topical (Permethrin/BB) + keratolytic. Isolate.
- Contacts & Environment: Treat ALL contacts simultaneously. Hot wash (≥50°C) linens. Seal items >72h.
- Post-Scabies Itch: Common post-cure. Emollients, antihistamines, topical steroids. Not active infestation.
- Resistance: Live mites post-therapy? Switch drug class or combine.
⭐ Permethrin 5% cream is the drug of choice for scabies in pregnancy (FDA Category B) and in children older than 2 months.
High‑Yield Points - ⚡ Biggest Takeaways
- Permethrin 5% cream is DOC for scabies; 1% for pediculosis.
- Oral Ivermectin is key for crusted scabies and resistant lice/scabies.
- Lindane is second-line due to neurotoxicity risks, especially in children.
- Benzyl Benzoate is an older, irritant alternative for scabies.
- Crotamiton offers both scabicidal and antipruritic effects.
- Malathion for lice is an organophosphate; note its flammability and odor.
- Spinosad is a newer, safe topical option for head lice.
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