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Ectoparasite Infestations and Arthropod Injuries

Chapter 472 | Part 14: Poisoning, Drug Overdose, and Envenomation · Part 14 – Poisoning, Overdose & Environmental · Chapter 472

Key Clinical Points

  1. Scabies is caused by Sarcoptes scabiei var. hominis; diagnosis requires microscopic examination of burrows showing mites, eggs, or fecal pellets.
  2. Tick paralysis results from neurotoxins in tick saliva blocking neuromuscular transmission; removal of the tick leads to rapid improvement.
  3. Alpha-gal syndrome (red meat allergy) is caused by IgE antibodies cross-reacting with galactose-α-1,3-galactose in mammalian meat after tick bites.
  4. Furuncular myiasis by Dermatobia hominis presents with a central breathing pore emitting bubbles when submerged in water.
  5. Recluse spider bites (Loxosceles spp.) often result in MRSA infections or minor injuries; Widow spider bites (Latrodectus spp.) cause latrodectism (cramps, hypertension, tachycardia) due to α-latrotoxin.
  6. Head lice are acquired by direct head-to-head contact; body lice are vectors for epidemic typhus, relapsing fever, and trench fever.
  7. Crusted scabies (Norwegian scabies) is associated with immunodeficiency and requires repeated courses of oral ivermectin and topical permethrin.
  8. Tick-borne disease prophylaxis with doxycycline is recommended within 72 hours of Ixodes scapularis bite in endemic areas.
  9. Chigger lesions are small (<1 cm) but intensely itchy, often appearing as minute (0.5 mm) red dots.
  10. Pentastomiasis (halzoun or marrara) presents rapidly (<12 h) with throat/ear pain and potential airway obstruction.

DEFINITION & OVERVIEW

• Ectoparasites: Arthropods or other creatures infesting skin/hair for sustenance and shelter. They may remain on surfaces or penetrate the skin to reside in the epidermis, dermis, or subcutaneous tissues. • Mechanism of Injury: ◦ Direct mechanical injury ◦ Consumption of blood or nutrients ◦ Induction of hypersensitivity reactions ◦ Inoculation of toxins ◦ Transmission of pathogens ◦ Creation of openings in the skin for secondary bacterial infection ◦ Incitement of fear or disgust • Host Status: ◦ Obligate: Scabies mites (hominis) and human-infesting lice. ◦ Facultative, Dead-end, or Paratenic: Most other arthropods. • Classification: ◦ Insects: Lice, fleas, bed bugs, wasps, ants, bees, flies. ◦ Arachnids: Spiders, scorpions, mites, ticks. ◦ Myriapods: Millipedes, centipedes. ◦ Other Phyla: Pentastomes (tongue worms), leeches.

EPIDEMIOLOGY

• Scabies: ◦ Prevalence: ~250 million persons worldwide. ◦ Settings: Preschools, hospitals, nursing homes, prisons, and other congregate settings. ◦ Transmission: Facilitated by crowding, poor hygiene, and close physical contact. Mites die within a day or so without a host. • Head Lice: ◦ Prevalence: Low in North America; episodic high-prevalence focal outbreaks among school-aged children. ◦ Transmission: Acquired mainly by direct head-to-head contact (not via fomites). • Body Lice: ◦ Occurrence: Uncommon in developed countries; found on indigent persons or during societal upheaval/disasters (e.g., World War I). ◦ Risk: Proliferate when access to clean clothing/laundry is limited.

ETIOLOGY & PATHOPHYSIOLOGY

• Scabies: ◦ Cause: Sensitization reaction to mites and their secretions/excretions. ◦ Timeline: Initial infestation may be asymptomatic for up to 6 weeks; reinfestation causes immediate hypersensitivity. ◦ Histopathology: Burrows surrounded by inflammatory infiltrates (eosinophils, lymphocytes, histiocytes). • Tick Saliva: ◦ Effects: Transmission of pathogens; 'sterile reactions' (local inflammation, fevers). ◦ Tick Paralysis: Caused by toxins in saliva blocking neuromuscular transmission and decreasing nerve conduction. • Spider Venom: ◦ α-Latrotoxin (Widow spiders): Binds irreversibly to presynaptic nerve terminals → release/depletion of acetylcholine and norepinephrine. ◦ Symptoms: Cramps, hypertension, tachycardia, etc. • Leech Anticoagulation: ◦ Hirudin: Powerful anticoagulant causing continued bleeding after the leech detaches.

CLINICAL FEATURES

• Scabies: ◦ Distribution: Volar wrists, digital web spaces, axillae, intertriginous areas, navel, belt line, buttocks, upper thighs. ◦ Male Genitalia: Penile glans, shaft, and scrotum nearly always involved. ◦ Morphology: Small papules, vesicles, eczematous plaques, pustules, nodules (symmetrical). ◦ Sparing: Face, scalp, neck, palms, soles (except in infants). ◦ Crusted Scabies: Thick yellowish-white keratotic crusts, scaly plaques, dystrophic nails; no characteristic burrows; often not itchy. • Chigger: ◦ Morphology: Small (<1 cm) papular, urticarial, or pustulovesicular lesions. ◦ Appearance: Minute (~0.5 mm) red dots with a hemorrhagic base. ◦ Location: Ankles, belt line, areas where clothing constricts movement. • Tick Bite: ◦ Hard Tick: Small induration (often purpuric) with erythema; may form a 'tâche noire' (necrotic eschar). ◦ Granulomas: Persistent tick-bite granulomas (1–3 cm) can last months. ◦ Fever: Often accompanied by headache, nausea, malaise; resolves ≤36 h after removal. ◦ Soft Tick: Red macules at bite site. • Lice: ◦ Head Lice: Edematous pink wheals along posterior hairline; blepharitis common. ◦ Puber/Pubic Lice: 'Maculae ceruleae' (intensely pruritic, bluish macules <5 mm). • Myiasis: ◦ Furuncular: Lesions with a central breathing pore emitting bubbles in water; causes significant emotional distress. ◦ Non-furuncular: Pruritic, serpiginous eruption. ◦ Ophthalmomyiasis: Nodules in eyelid, retinal detachment, globe destruction. • Pentastomiasis: ◦ Syndromes: Halzoun (Lebanon) or Marrara (Sudan). ◦ Presentation: Rapid onset (<12 h) of pain/itching of throat and ears, coughing, hoarseness, dysphagia, dyspnea. ◦ Complication: Severe edema may require tracheostomy. • Spider Bite: ◦ Recluse: Painful; most 'noduloulcerative' lesions are MRSA or minor injury. ◦ Widow: Neurotoxic symptoms (cramps, hypertension, tachycardia). ◦ General: Most bites do not cause local tissue damage.

DIFFERENTIAL DIAGNOSIS

• Scabies Mimics: ◦ Unrelated skin diseases. ◦ 'Outbreak' situations (misinterpreted cases). ◦ Eczematous plaques, pustules, or nodules. • Spider Bite Mimics: ◦ Unrelated minor injury. ◦ Acute bacterial infection (MRSA). ◦ Vasculitis. • Louse Vector Mimics: ◦ Rickettsia prowazekii (louse-borne typhus).  Bartonella quintana (trench fever).

DIAGNOSTIC APPROACH

  1. Scabies Confirmation: • Gold Standard: Microscopic examination of material scraped from burrows. → Step 1: Unroof burrows with sterile needle/scalpel blade. → Step 2: Examine for mites, eggs, and fecal pellets. → Alternative methods: Biopsy, tape lift (cellophane or cyanoacrylate), or dermatoscopic imaging. → Clinical Diagnosis: Triad of pruritus, physical exam findings, and epidemiologic link.
  2. Mite Identification: • Direct Sampling: Obtain from patient (ideally by medical personnel). • Indirect Sampling: Traps or monitoring devices in home/workplace.
  3. Tick-Borne Disease Prophylaxis: • Indication: Ixodes scapularis bite in endemic areas. → Action: Administer one-time oral dose of Doxycycline within 72 h of tick removal. → Note: Prevents Lyme disease only; does not prevent other tick-borne infections.

MANAGEMENT & TREATMENT

  1. Scabies Treatment: • Topical Scabicides: Permethrin, crotamiton, spinosad, or lindane. • Application Protocol: → Apply thinly but thoroughly from jawline to toes after bathing. → Ensure coverage of interdigital spaces, navel, and under nails. → Wash off with soap and water 6–14 h later. • Follow-up: Second round of treatment in 1 week (due to egg resistance). • Crusted Scabies Management: → Pre-treatment: Keratolytic agent (e.g., 6% salicylic acid) to debulk crusts. → Topical: Repeated applications of permethrin. → Oral: Ivermectin for multiple doses over several days. • Supportive Care: Antihistamines, topical steroids, or emollients for pruritus; topical glucocorticoids for lingering itch.
  2. Note: Patients are noncommunicable within 1 day of proper treatment.
  3. Lice Treatment: • Initial Treatment: Topical application of permethrin lotion or pyrethrin-based shampoo. • Follow-up: Second application roughly 10 days later to clear newly hatched nits.
  4. Myiasis Treatment: • Identification: Use water to check for breathing pores.
  5. Note: Removal of larvae is required; identification of the specific parasite is important.
  6. Tick Management: • Removal: Prompt removal of tick.
  7. Note: Fever usually resolves ≤36 h after removal.
  8. Prophylaxis (Ixodes scapularis): • Medication: Doxycycline oral dose.
  9. Timing: Within 72 h of bite in endemic areas.

COMPLICATIONS & PROGNOSIS

• Scabies Complications: → Secondary bacterial infections (S. aureus, S. pyogenes). → Consequences: Impetigo, cellulitis, poststreptococcal glomerulonephritis, acute rheumatic fever. • Tick Paralysis Prognosis: → Rapid improvement upon removal of the tick.

SPECIAL CONSIDERATIONS

• Immunocompromised/Neurological Patients: → High risk for Crusted Scabies (Norwegian scabies). → Requires intensive treatment: Keratolytic agents, repeated permethrin, and multiple doses of ivermectin. • Children: → Higher prevalence of head lice; requires 10-day follow-up for nits.

KEY PEARLS & CLINICAL TRAPS

• Scabies Timing: Initial infection is often asymptomatic for 6 weeks; reinfestation causes immediate pruritus. * Note: This helps distinguish primary infestation from reinfestation or secondary reactions. • Crusted Scabies: Characterized by thick crusts and lack of burrows; highly contagious. • Myiasis Identification: A 'breathing pore' that bubbles in water is pathognomonic for furuncular myiasis. • Tick Prophylaxis: Doxycycline within 72h is specific to Ixodes scapularis (Lyme disease). • Spider Bites: Most 'noduloulcerative' lesions are MRSA or minor trauma, not venom-related. • Lice as Vectors: Head lice are NOT known vectors; body lice ARE potential vectors for typhus and trench fever.