Infestations (Scabies, Lice)

• Itch mite infestation
• "Seven-year itch"

• Human scabies is a pruritic condition caused by infestation with the host-specific mite Sarcoptes scabiei var. hominis, which lives its entire life within the epidermis.
• Although the scabies mite is not a known vector for any systemic disease, secondary bacterial infections with Streptococcus pyogenes or Staphylococcus aureus may develop.
• Transmission typically occurs via direct close contact with an infested person; fomite transmission is also possible, especially with the crusted variant.
• Permethrin 5% cream is generally the first-line treatment for classic scabies.

• Scabies mite plagues all countries of the world.
• Estimated 150–200 million individuals affected globally.
• Overall worldwide prevalence ~2.5%.
• In 2017, WHO designated scabies as a neglected tropical disease to highlight need for global control strategies.
• Pruritus is usually severe, especially at night, and treatment typically requires prescription scabicidal therapy.

• Scabies has been a common companion of the human species for over 2500 years.

• Worldwide problem; all ages, races, and socioeconomic groups susceptible.
• Environmental factors promoting spread: overcrowding, institutional settings, delayed treatment, lack of public awareness.
• Prevalence in low-income countries ranges from 4% to 70%.
• Higher incidences in tropical regions; outbreaks in natural disasters, wars, economic depression, refugee displacement.
• Transmitted directly by close personal contact (sexual or otherwise) or indirectly via fomites.
• Prevalence higher in children and sexually active people; spread common among household members/close contacts.
• Scabies mite is not a known vector for systemic disease.
• Crusted scabies (formerly Norwegian scabies) found in:
• Immunocompromised individuals (elderly, HIV, HTLV-1 infection, solid organ transplant recipients).
• Those with decreased sensory function and/or ability to scratch (e.g. leprosy, paraplegia).
• Patients may have minimal pruritus despite large mite burden; highly contagious.

• Species-specific, eight-legged mite Sarcoptes scabiei var. hominis causes human scabies.
• S. scabiei var. canis (dogs) not a source of human infestation but can cause bite reactions (Ch. 85).
• Mite size: 0.35 × 0.3 mm — too small to see with naked eye.
• Entire 30-day life cycle completed within the epidermis.
• Female mite lays 2–3 eggs/day, requiring ~10 days to mature.
• Number of mites on host usually <100, often 10–15.
• Crusted scabies: patients may have thousands of mites; live mites recoverable from sheets, floor, curtains, chairs in environment.
• Mites usually live ≤3 days off host; crusted scabies mites may live up to 7 days by feeding on sloughed skin.
• Incubation period: days to months.
• First-time infestation: 2–6 weeks before immune sensitization causes pruritus/lesions.
• Subsequent infestation: symptomatic within 24–48 hours.
• Asymptomatic scabies-infested individuals ("carriers") not uncommon.
• Life cycle stages: Egg → Larva (3 pairs of legs vs 4 in adult) → Protonymph (2–2.5 days) → Tritonymph (3–4 days) → Adult scabies mite.
• Female mite burrows and lays egg over 3 days.
• 1 day spent on skin then burrows back into skin (1–2 days).
• 15-minute copulation occurs once per female mite lifetime.

• Diagnosis based on epidemiologic history (pruritus in household/close contacts), distribution/type of lesions, and pruritus.
• Intense pruritus classically accentuated at night and by hot bath/shower.
• Pruritus may precede overt physical signs; thought to be mediated by nonhistaminergic mechanisms.
• Cutaneous lesions symmetrical, typically involving:
• Interdigital web spaces of hands, flexural wrists, axillae, posterior auricular area, waist (including umbilicus), ankles, feet, buttocks.
• Men: penile and scrotal lesions common.
• Women: areolae, nipples, vulvar area often affected.
• Infants, elderly, immunocompromised: all skin surfaces susceptible, including scalp and face.
• Small erythematous papules with variable excoriation.
• Vesicles, indurated nodules, eczematous dermatitis also common.
• Pathognomonic sign: the burrow — tunnel excavated by female mite while laying eggs.
• Wavy, thread-like, grayish-white, 1–10 mm in length.
• Many patients lack obvious burrows on inspection, especially in warm climates.
• Acral vesiculopustules — clue to scabies diagnosis in infants.
• Similar findings seen in acropustulosis of infancy, which can follow successful scabies treatment.
• Crusted scabies:
• Marked hyperkeratosis favoring acral sites, including subungual areas; involvement may be widespread.
• Affected areas referred to as pachyderma.
• "Scabies incognito" can present as asymptomatic hyperkeratotic "rash."
• Secondary bacterial infections (S. aureus, Str. pyogenes) sometimes complicate scabies.
• In resource-poor endemic countries, post-streptococcal glomerulonephritis is a significant issue.
• Peripheral eosinophilia can be the primary sign of scabies in patients with disorders of keratinization.

• Light microscopic examination of mineral oil preparations of skin scrapings for adult mites, eggs, and/or fecal pellets (scybala).
• Scalpel or curette used to obtain sample.
• Microscopic examination of transparent adhesive tape applied to infested skin — alternative technique.
• Dermoscopy and confocal microscopy useful for direct in vivo visualization of mites/eggs.
• Consensus criteria for confirmed diagnosis: visualization of mites, eggs, or feces by light microscopy of skin samples OR high-powered imaging device/dermoscopy.
• Skin biopsy may confirm diagnosis only if specimen contains mite or eggs.
• Often diagnosis is clinical, based on history and physical findings (burrows, genital lesions) and response to treatment.
• PCR-based analysis of skin scrapings/swabs and serologic IgE testing for recombinant S. scabiei antigen — under investigation.

• Patchy to diffuse infiltrate with prominent eosinophils, plus lymphocytes and histiocytes in reticular dermis.
• Transected scabies mite may occasionally be seen within epidermis.
• Pink "pigtail"-like structures attached to stratum corneum represent fragments of adult mite exoskeleton — clue to diagnosis when mites/scybala/eggs not identified.

• Unless burrows or (dermoscopy) mites/eggs noted, wide differential of pruritic skin diseases:
• Atopic, allergic contact, autosensitization ("id" reaction), and nummular dermatitis.
• Arthropod bites, pyoderma, dermatitis herpetiformis, bullous pemphigoid.
• Can mimic Langerhans cell histiocytosis clinically and histologically (dense Langerhans cell infiltrate).
• Infantile scabies clinical/histologic features may resemble inflammatory stage of incontinentia pigmenti.
• Acropustulosis of infancy can both mimic scabies and follow it as a hypersensitivity phenomenon.

• Two treatments 1 week apart with antiscabetic medication typically recommended.
• Permethrin 5% cream — standard first-line therapy.
• Topical preparations applied to entire body surface head-to-toe in infants and elderly; other age groups may exclude face/scalp.
• Special attention to interdigital spaces, intergluteal cleft, umbilicus, subungual areas.
• To reduce fomite reinfestation: at each treatment, wash clothing/linens/towels used within previous week in hot water and dry on high heat, or store in bag for 7–10 days.
• All family members/close contacts treated simultaneously, even if asymptomatic (due to common asymptomatic carriers).
• Pets cannot harbor human mites — do not need treatment.
• Secondary bacterial infections treated with appropriate antibiotics.
• Postscabetic pruritus/dermatitis:
• Pruritus/lesions can persist 2–4 weeks or longer after successful treatment, especially acral vesiculopustules in infants and nodules.
• Does NOT imply treatment failure — represents response to dead mites sloughed off (within 2 weeks) with normal epidermal exfoliation.
• Many patients experience relief from pruritus within 3 days.
• Second treatment reduces reinfestation risk from fomites and kills nymphs that survived within egg's semi-protective environment.

Permethrin

• Synthetic pyrethroid, 5% cream — current standard topical scabicide.
• Inhibits sodium transport in arthropod neurons → paralysis.
• Adverse reactions rare, usually brief stinging on application.
• Prolonged in vitro survival noted after exposure, but clinical resistance not documented.

Spinosad

• 2021: 0.9% spinosad suspension FDA-approved for scabies in patients ≥4 years.
• RCT (n=206): single application to entire body (neck to toes; left on ≥6 hours) → complete cure on day 28 in 78% vs 40% placebo (p<0.001).

Lindane

• Gamma-hexachlorocyclohexane, organochlorine agent; 1% lotion/cream.
• CNS side effects may occur from increased percutaneous absorption (damaged skin, misuse, overuse, accidental ingestion).
• Contraindicated in: premature infants, crusted scabies or pre-existing skin conditions (e.g. extensive atopic dermatitis) increasing absorption, uncontrolled seizure disorder.
• Potential toxicity, poor efficacy, widespread resistance, environmental contamination issues → inferior to permethrin or oral ivermectin.
• 2003: FDA "black box" warning — use with caution in infants, children, individuals <110 lb (50 kg), and elderly (increased neurotoxicity risk).

Crotamiton

• 10% lotion/cream.
• Has antipruritic effect but can cause irritation of denuded skin.
• Less effective than other prescription options.

Sulfur ointment

• 5%–10% sulfur in petrolatum base, three consecutive overnight applications.
• Must be compounded by pharmacist; toxicity studies not performed.
• Messy, malodorous, irritating, can stain clothing.
• Efficacy reported as high as 60%–96% with three consecutive 24-hour applications.

Benzyl benzoate

• Ester of benzoic acid and benzyl alcohol, neurotoxic to mites.
• OTC 10%–25% lotion/emulsion (not sold directly in US pharmacies; lower concentrations for children); left on 24 hours.
• Regimens: 2–3 applications over 24 hours; applications on 3 consecutive days; or applications on day 1 and day 8.
• Cure rates ~90%; main side effect skin irritation.

Ivermectin

• Macrocyclic lactone produced by Streptomyces avermitilis.
• Not FDA-approved for scabies but effective for this and other ectoparasitic infestations.
• Blocks transmission across nerve synapses utilizing glutamate or GABA → paralysis of peripheral motor function in insects/acarines.
• Blood–brain barrier prevents CNS penetration after early infancy in humans.
• Neurotoxicity risk in rare individuals with biallelic nonsense mutations in ABCB1 (ATP-binding cassette subfamily B member 1) gene encoding P-glycoprotein.
• Not recommended for pregnant/breastfeeding women (lack of safety data).
• Use in infants/children <33 lb (15 kg) — well-tolerated in recent large series, though traditionally not recommended in this group.
• Extremely safe in clinical studies; >18 million people >5 years treated annually in WHO Onchocerciasis Eradication Program.
• Increased death rate in elderly in one retrospective study determined causally unrelated to medication by WHO, medical examiners, scientific community.
• Moxidectin (related, longer half-life) recently FDA-approved for onchocerciasis; under investigation for scabies.
• Standard dose: oral 200 mcg/kg, administered twice, 1–2 weeks apart.
• Only one report of clinical resistance to date; increased in vitro survival times described in high-prevalence communities.
• Mass administration (oral ivermectin or topical permethrin, ± azithromycin) in endemic areas can control scabies and reduce impetigo prevalence.
• Topical ivermectin 1% (FDA-approved for rosacea) appears effective for scabies but needs further study.
• Subungual disease: add topical scabicide since systemic drug won't penetrate thickened keratotic debris.
• Combination of oral ivermectin + topical permethrin often required for crusted scabies.
• Topical keratolytic (salicylic or lactic acid) helps decrease hyperkeratosis.

Topical and Oral Treatments for Scabies

• Permethrin cream (5%): Topically overnight (8–12 h) days 1 and 8*. Concerns: allergic contact dermatitis (formaldehyde sensitivity). Efficacy: excellent, some tolerance developing. Infants: FDA-approved ≥2 months. Pregnancy: considered safe.
• Spinosad suspension (0.9%): Topically ≥6 h day 1. Concerns: skin irritation, dry skin. Efficacy: good. Infants: FDA-approved ≥4 years. Pregnancy: considered safe.
• Lindane lotion/cream (1%): Topically overnight days 1 and 8. Concerns: potential CNS toxicity, especially <110 lb (50 kg), elderly, crusted scabies, other skin conditions, seizure history. Efficacy: poor, resistance very common. Infants: not recommended for infants/children/breastfeeding mothers; contraindicated in premature infants. Pregnancy: not recommended.
• Crotamiton lotion/cream (10%): Topically overnight 2–5 consecutive days. Concerns: irritant contact dermatitis especially denuded skin. Efficacy: very poor; antipruritic, may be used for postscabetic pruritus. Infants: considered safe. Pregnancy: not established.
• Sulfur ointment (5%–10%): Topically overnight, 3 successive days. Concerns: toxicity studies not performed. Efficacy: good. Infants: considered safe. Pregnancy: considered safe.
• Benzyl benzoate lotion/emulsion (10%–25%): Topically (24 h) days 1 and 8. Concerns: irritant contact dermatitis. Efficacy: good. Infants: not well established. Pregnancy: not established.
• Ivermectin (3 mg tablets): Oral 200 mcg/kg day 1, repeated day 8 or 14*†. Concerns: potential CNS toxicity in infants/young children. Efficacy: excellent. Infants: not well-established, though recent studies suggest safety age ≥1–2 months weighing <33 lb (15 kg). Pregnancy: generally not recommended.
• 0.5%–1% ivermectin lotion/cream topically overnight days 1 ± 8: Concerns potential skin/eye irritation. Efficacy: not established. Not established for infants/pregnancy.
• *For crusted scabies, CDC recommends:

1. Topical permethrin 5% every 2–3 days for 1–2 weeks.
2. Oral ivermectin (200 mcg/kg/dose) as three doses (days 1, 2, 8), five doses (days 1, 2, 8, 9, 15), or seven doses (days 1, 2, 8, 9, 15, 22, 29) depending on severity.
3. Topical keratolytic (salicylic or lactic acid) in areas of hyperkeratosis.

• †Second dose at 1 week recommended by CDC.

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• Pediculosis capitis
• "Cooties"

• Worldwide infestation caused by bloodsucking, wingless, six-legged insects living only on scalp hairs.
• Diagnosis usually confirmed by presence of 0.8 mm eggs ("nits") firmly attached to scalp hairs.
• Spread by head-to-head contact as well as fomite transmission.
• Resistance to traditional OTC preparations (pyrethrins, permethrin) is growing.
• Newer treatments, including topical ivermectin, now available.

• Lice are bloodsucking, wingless insects of suborder Anoplura.
• 12 million cases per year in the US alone; resistance to traditional treatments increasing.
• Prescription products (topical malathion, topical/oral ivermectin) often needed for resistant cases.

• Head lice have infested mankind for thousands of years; nits found on hair of Egyptian and Peruvian mummies.

• Found worldwide, no strict age/sex/race/socioeconomic limitations.
• Children 3–11 years highest incidence.
• Worldwide prevalence in school-aged children estimated 19% in recent meta-analysis, up to 60% in some countries.
• More frequent in girls (longer hair, sharing brushes/barrettes/hair accessories).
• Distinctly uncommon in African-Americans — US lice unable to properly position themselves to lay eggs on coarse curly hair.

• Pediculus capitis — highly host-specific insect, size of sesame seed (2–3 mm).
• Obligate human parasites; feed on host blood approximately every 4–6 hours.
• Female louse lives 30 days, lays 5–10 eggs/day on hair shafts.
• Nits (oval egg capsules) measure 0.8 mm in length; laid close to scalp for warmth.
• Eggs >1 cm from scalp generally unhatched.
• In warm climates, viable nits found 15 cm or more from scalp, especially above nape of neck.
• Nits cemented to hairs by proteinaceous matrix resembling amino acid constituents of human hair shaft.
• Head lice rarely live >36 hours away from host without blood meal.
• Given appropriate temp (82–90°F/28–32°C) and humidity (70%–90%), nits can survive and hatch after 10 days away from host.
• Transmission: direct head-to-head contact, or fomites (combs, brushes, blow-dryers, hair accessories, bedding, helmets, headgear).
• Life cycle: Egg (0.8 mm, hours to 1 day) → First instar/nymph → Second instar/nymph (3–4 day molt) → Third instar (3–4 day molt) → Adult louse (2–4 mm); louse must take blood meal before copulating; copulation; total cycle ~10 ± 2 days.

• Skin findings limited to scalp, behind ears, and nape of neck.
• Intense pruritus varies among patients.
• Initial infestation: pruritus may take 2–6 weeks to develop (delayed immunologic response to saliva/excreta components).
• Repeat infestation: pruritus develops within 24–48 hours.
• Some individuals asymptomatic "carriers."
• Common findings: excoriations, erythema, pyoderma, scaliness of scalp/posterior neck.
• Definitive diagnosis: identification of nits and/or adult lice on scalp hair.
• Viable eggs: tan to brown.
• Hatched eggs: clear to white.
• Occasionally low-grade fever and lymphadenopathy from secondary bacterial infection.
• Head lice can carry S. aureus and Str. pyogenes on surfaces — common cause of scalp pyoderma.
• Increasing evidence head lice may harbor Bartonella recurrentis, B. quintana, Coxiella burnetii, Acinetobacter spp. — unclear if associated with disease transmission.

• Epidermis and dermis may show nonspecific inflammation.

• Seborrheic dermatitis, psoriasis can cause scalp pruritus, but finding nits/adult lice is diagnostic.
• Nits more firmly adherent to hair shaft than dandruff, dried hair products, or hair casts.
• Must differentiate from other hair shaft nodule causes: piedra (white or black), trichorrhexis nodosa.

• Choice based on efficacy, potential toxicity, insecticide resistance patterns in geographic area, ease of prescription access.
• Pediculicides mainstay of therapy.
• Two applications, 1 week apart, advisable with all topical preparations to: (1) kill survived nits; (2) defend against growing resistance; (3) reduce fomite reinfestation.
• Conditioner should NOT be applied prior to topical medications; hair should not be rewashed for 1–2 days.
• Non-pesticidal products (petroleum jelly, hair pomade, olive oil, mayonnaise, vegetable oil, mineral oil) — unsubstantiated claims; may slow lice movement/ease combing but not lethal.
• Essential oils (tea tree, lavender, eucalyptus combinations) reported effective; need more clinical study.
• Oral trimethoprim–sulfamethoxazole may potentially improve efficacy of topical agents but routine use not recommended.
• Many school authorities enforce "no-nit" policy — no return to school regardless of nit viability; requires physical nit removal via closely-spaced metal-tooth combs; no products currently ease nit removal effectively.

Pyrethrins

• Pyrethrins, 1% permethrin, and 0.5% ivermectin — FDA-approved, OTC topical pediculicides.
• Derived from natural extract of Chrysanthemum cinerariaefolium flower heads.
• Patients allergic to chrysanthemums/ragweed/related plants — wheezing, dyspnea with crude extract.
• Formulated as lotions, shampoos, foam mousse, cream rinses.
• Piperonyl butoxide added — slows biotransformation by partially inhibiting insect cytochrome P450 enzymes, initially improving effectiveness.
• Applied for 10 minutes, then rinsed.
• Growing resistance → treatment failures now commonplace.

Permethrin

• Only synthetic pyrethroid used worldwide for head lice.
• OTC 1% cream rinses/lotions applied for 10 minutes; alternatively 8–12-hour application of 5% cream (used for scabies).
• Resistance developed even to higher-concentration products.
• Point mutations in gene encoding α-subunit of insect voltage-sensitive sodium channels commonly reduce permethrin/pyrethroid binding affinity.
• Some studies found >85%–90% success with permethrin in children whose lice had these mutations — other factors play role in resistance.

Malathion

• Prescription organophosphate, cholinesterase inhibitor.
• Recommended application: 8–12 hours; repeat in 7 days if live lice present.
• High efficacy reported also with duration reduced to 20 minutes.
• US product (Ovide® lotion) contains 78% isopropyl alcohol — enhances effectiveness but flammable.
• Some resistance reported, especially Europe and Australia.

Ivermectin

• 0.5% topical ivermectin (Sklice® lotion) FDA-approved 2012 for head lice ≥6 months of age, now OTC.
• Kills permethrin-resistant head lice; viability of lice hatched from treated eggs severely compromised.
• Two RCTs (total n=765): single 10-minute application, 0.5% lotion to dry hair → 74% lice-free at 15 days vs 18% vehicle control (p<0.001).
• Oral ivermectin — another option for resistant infestations.
• Large multicenter trial: 400 mcg/kg on days 1 and 8 → 95% lice-free day 15 in patients who failed prior topical therapy (pyrethrin/malathion), vs 85% with two topical malathion applications.
• Other controlled studies: >90% of children treated with 200 mcg/kg on days 1 and 8 lice-free at 2-week follow-up.
• Because lice only exposed while feeding, oral ivermectin has no ovicidal activity — unlike topical, two treatments required.
• Clinical resistance associated with mutation in ivermectin target site documented in two children from rural Senegal after second oral treatment.

Dimethicone

• Silicone oil, used as emollient.
• May suffocate lice or block ability to excrete water, causing death by osmotic stress.
• Several RCTs: 10-minute to overnight application of 4%–100% liquid gel/lotion/spray (LiceMD®, Hedrin®, NYDA®) on day 1 ± day 8 → 70%–97% lice-free at day 14, higher efficacy than 1% permethrin cream rinse or 0.5% malathion in some reports.

Spinosad

• 2011: 0.9% spinosad topical suspension (Natroba™) FDA-approved, indicated ≥6 months of age.
• Fermentation product of Saccharopolyspora spinosa; induces muscle spasms/paralysis.
• Trial (n=1038): 10-minute application without nit combing, day 1 ± day 7 → superior efficacy to 1% permethrin cream with nit combing (~86% vs ~44%; p<0.001); no significant adverse effects.

Abametapir

• 2020: 0.74% abametapir lotion (Xeglyze™) FDA-approved for head lice ≥6 months of age.
• Metalloproteinase inhibitor targeting proteinases critical to lice/egg development.
• Two trials (n=704): single 10-minute application → 86% lice-free through day 14 vs 63% vehicle (p<0.001).

Benzyl alcohol

• 2009: 5% benzyl alcohol lotion (Ulesfia®) FDA-approved, prescription, ≥6 months of age.
• Acts via asphyxiation — prevents lice closing respiratory spiracles; not ovicidal.
• Two RCTs (total n=250): 75% lice-free 14 days after last of two 10-minute applications (1 week apart) vs 5%–25% vehicle.
• Ulesfia discontinued; generic benzyl alcohol may be available.

Lindane

• Chlorinated hydrocarbon; 1% prescription shampoo, applied 4 minutes.
• FDA "black box" warning — reserved for treatment failures of other approved therapies due to CNS side effect potential.
• Resistance commonly observed.

Carbaryl

• Cholinesterase inhibitor like malathion; 0.5% lotion/shampoo available in UK and other countries; not available in US.
• More potentially toxic to patients, less lethal to lice, compared to malathion.

Treatments for Head Lice (days 1 and 8 unless noted)

OTC products:

• Ivermectin lotion (0.5%)‡ (avermectin): topical 10 min to dry hair. Concerns: skin/eye irritation. Efficacy: good; single report of resistance to date.
• Dimethicone liquid gel/lotion/spray (4%–100%) (silicone oil): topical 10–15 min or overnight. Concerns: none. Efficacy: good (varies by formulation).
• Pyrethrins (0.33%) + piperonyl butoxide (4%) (natural botanicals): topical 10 min to dry hair. Concerns: allergic reactions (chrysanthemum/ragweed sensitivity). Efficacy: poor–fair; resistance common.
• Permethrin cream rinse/lotion (1%) (synthetic pyrethroid): topical 10 min to clean dry hair. Concerns: none. Efficacy: poor–fair; resistance common.

Prescription products (ranked by efficacy; "Good" often first-line):

• Malathion lotion/gel (0.5%)† (organophosphate cholinesterase inhibitor): topical 8–12 h to dry hair (Ovide® with isopropyl alcohol effective at 20 min). Concerns: flammable isopropyl alcohol base; burning/stinging on eroded skin. Efficacy: excellent (US); resistance noted Europe/Australia, not yet US.
• Oral ivermectin (3 mg tablets) (avermectin): oral 200–400 mcg/kg. Concerns: potential CNS toxicity; not recommended breastfeeding/pregnant (category C); recent studies suggest safety ≥1–2 months weighing <33 lb (15 kg), use not well-established in this group. Efficacy: excellent; single report of resistance to date.
• Spinosad suspension (0.9%)‡ (bacterial fermentation product): topical 10 min to dry hair. Concerns: skin/eye irritation. Efficacy: good; no resistance noted to date.
• Abametapir lotion (0.74%)‡ (metalloproteinase inhibitor): topical 10 min to dry hair. Concerns: skin irritation. Efficacy: good; no resistance noted to date.
• Benzyl alcohol lotion (5%)‡ (alcohol): topical 10 min to dry hair. Concerns: skin irritation. Efficacy: good; no resistance noted to date.
• Permethrin cream (5%)\* (synthetic pyrethroid): topical overnight to clean, dry hair. Concerns: allergic contact dermatitis (formaldehyde sensitivity). Efficacy: poor–fair; resistance common.
• Lindane shampoo (1%): topical 4 min to clean dry hair, then lather/rinse. Concerns: potential CNS toxicity; not recommended for infants, children, breastfeeding/pregnant (category C). Efficacy: poor; resistance common.
• Carbaryl shampoo (0.5%) (carbamate cholinesterase inhibitor): topical 8–12 h. Concerns: possible carcinogen. Efficacy: poor–fair; resistance common (not approved in US).
• *Approved for individuals ≥2 months of age; considered safe during pregnancy.
• †Approved for individuals ≥6 years of age; considered safe during pregnancy.
• ‡Approved for individuals ≥6 months of age; spinosad cream rinse, abametapir lotion, benzyl alcohol lotion considered safe in pregnancy.
• FDA-approved regimen for 0.5% ivermectin solution and 0.74% abametapir lotion: single application only.
• Airallé® — FDA-cleared medical device using hot air to treat head lice by dehydrating eggs (and to lesser degree hatched lice).
• Isopropyl myristate 50% in cyclomethicone solution (Resultz®) — FDA-approved OTC treatment in 2017; available in Canada/Europe, not yet marketed in US.
• Benzyl benzoate 10%–25% lotion/emulsion (OTC, not directly sold in US pharmacies) may be used for head lice, few investigational studies; applied to hair 12–24 hours on days 1 and 8.

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• Pthirus pubis or Phthirus pubis
• Pubic lice
• Pediculosis pubis
• "Crabs"

• More properly designated "crab lice" (rather than "pubic lice") because infestations may involve other hair-bearing sites — beard, eyelashes, axillae, perianal region.
• Transmitted by sexual or close contact and, to lesser extent, via contaminated clothing, towels, bedding.

• Infestation with Pthirus pubis, the crab louse, causes discomfort, pruritus, embarrassment; may coexist with other STIs.

• Parasitic relationship between humans and crab lice dates to prehistoric times.

• Incidence may be slightly higher in men (greater coarse body hair).
• Found in all socioeconomic/ethnic groups; those of Asian descent or with minimal pubic hair rarely affected.
• Incidence has decreased with current pubic hair removal practices.
• Most frequent in ages 15–40 years, correlating with increased promiscuous sexual activity.
• Highest prevalence: men who have sex with men.
• Often considered STI, but non-sexually-exposed individuals occasionally infested via fomite transmission (contaminated clothing, towels, bedding).

• Crab lice, Pthirus pubis, ~1 mm in length, resemble tiny crabs, wider/shorter body than head lice.
• Eggs viable for up to 10 days; adult louse can live at least 36 hours away from host.
• Serrated edges on first claw enable ambulation on entire body surface.
• Infests: pubic hair, scalp, eyebrows, eyelashes, moustache, beard, axillae, perianal area.
• 60% of patients with pubic lice are infested in at least two hair-bearing sites.
• When pubic area shaved/treated, surviving lice can travel to other hairy areas including scalp.

• Typically presents with pruritus in pubic region.
• Crab lice cling to base of hairs; can be brown to skin-colored or mimic hemorrhagic crusts.
• Other findings: nits at base of hair shafts, erythema around hair follicles, excoriations, secondary bacterial infection, lymphadenopathy.
• Eyelash involvement: feces accumulate resembling flecks of mascara.
• Macula caerulea: asymptomatic, slate-gray to bluish, irregularly shaped macules, 0.5–1 cm diameter, favor trunk and thighs.
• Develop in chronic crab lice infestations.
• Thought to result from breakdown of bilirubin to biliverdin by enzymes in louse saliva.
• Consider possibility of additional STIs and seek original infestation source to reduce recurrence risk.

• Nonspecific inflammatory changes in epidermis and dermis.
• Lice live on skin surface — not evident histologically.

• Identification of crab lice and/or nits is diagnostic.
• Consider other infestations (e.g. scabies), arthropod bites.
• Nits on pubic/axillary hairs must be distinguished from white piedra and trichomycosis pubis or axillaris.

• Topical insecticides are standard therapy; sexual partners treated simultaneously.
• Most commonly used: permethrin (1% or 5%) and synergized pyrethrin products.
• Applied on two occasions, 1 week apart.
• All hairy body areas must be inspected and treated if possible infestation.
• Single application limited to pubic hair only may limit success rates to as low as 55%.
• Safest/most effective: 5% permethrin cream, applied generously overnight to all potentially infested hairy areas, repeated 1 week later.
• Lindane: poor efficacy, higher toxicity; shampoo not approved for extensive body application.
• Oral ivermectin days 1 and 8: for perianal or eyelash involvement, or when topical therapy unsuccessful.

Treatments for Crab Lice (days 1 and 8)

• Permethrin (1%) cream rinse or synergized pyrethrin shampoo\* (OTC): topical 10 min to clean dry hair. Concerns: none. Efficacy: fair.
• Permethrin cream (5%): topical 8–12 h. Concerns: allergic contact dermatitis (formaldehyde sensitivity). Efficacy: good.
• Lindane shampoo (1%): topical 4 min to dry hair, then lather/rinse. Concerns: potential CNS toxicity; not recommended infants, children, breastfeeding/pregnant (category C). Efficacy: poor.
• Ivermectin 0.5%–1% lotion/cream: topically overnight. Concerns: potential skin/eye irritation. Efficacy: not established.
• Oral ivermectin (3 mg tablets): oral dose 250 mcg/kg. Concerns: potential CNS toxicity; not recommended breastfeeding/pregnant (category C)†. Efficacy: excellent.
• *OTC products.
• †Recent studies suggest safety for children ≥1–2 months weighing <33 lb (15 kg), not well-established in this group.
• Benzyl benzoate 10%–25% lotion/emulsion (OTC, not directly sold in US pharmacies) may be used for crab lice, few investigational studies; applied three times over 24 hours without intervening bath.

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• Pediculosis corporis
• Clothing lice

• Infestation of humans and their clothing; most commonly affects homeless, refugees, victims of war and natural disasters.
• Body lice do NOT live or lay eggs on people but in their clothing.
• Infections transmitted by body lice: epidemic typhus, trench fever, relapsing fever.

• Associated with overcrowding, poor hygiene, poverty, war, natural disasters.
• Primary vectors for diseases caused by Rickettsia, Borrelia, and Bartonella spp. (see Chs. 74 & 76).
• Resemble head lice but larger in size.

• Found worldwide; increased prevalence tied to poor hygiene, poverty, homelessness.
• No racial, age, or sex restrictions.

• Pediculosis corporis caused by Pediculus humanus var. corporis infesting humans and clothing.
• Requires inability to wash and change clothes.
• Diseases transmitted by body louse:
• Epidemic typhus (Rickettsia prowazekii).
• Relapsing fever (Borrelia recurrentis).
• Diseases caused by Bartonella quintana: trench fever, bacillary angiomatosis, endocarditis.
• Unclear role in transmission of Acinetobacter baumannii or Yersinia pestis to humans.
• Transmission of microorganisms occurs NOT from louse bites but from:
• Inoculation of louse feces into skin via scratching, or
• Inhalation of dry, powdery louse feces from infested bedding/clothing.

• P. humanus var. corporis measures 2.5–4.0 mm in length.
• Unlike head/crab lice, nits and lice rarely found on skin (except while feeding) — reside primarily on clothing.
• Causes severe pruritus.
• Back, neck, shoulders, waist areas commonly involved.
• Findings: pinpoint red macules, small erythematous papules, crusts, excoriations; occasionally impetigo and lymphadenopathy.
• Blood and fecal pellets stain clothing/bedding.
• Nits and adult lice found along clothing seams in contact with neck, axillae, waistline.

• Epidermis: minimal spongiosis; dermis: nonspecific infiltrates of eosinophils and neutrophils.

• Systemic diseases (hepatic or renal impairment).
• Drug reactions.
• Atopic dermatitis, contact dermatitis (irritant or allergic).
• Other infestations (e.g. scabies).

• Preferably discard clothing/bedding in tightly sealed plastic biohazard bags and incinerate.
• If not possible, disinfect: fumigate or launder in hot water (≥130°F/55°C), then machine dry with high heat.
• Hot ironing of seams of upholstered furniture also helpful.
• Avoid exposure to untreated infested items for 2 weeks.
• Topical insecticide regimens similar to scabies treatment can eradicate concurrent scabies/crab lice and kill any body lice adherent to hairs.
• Mass delousing in crowded populations (natural disaster, war, famine): dusting powders containing DDT, malathion, dieldrin, lindane, or permethrin.

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• Tunga penetrans: chigoe flea, jigger flea, burrowing flea, sand flea
• Pique
• Nigua
• Pio and bicho de pie

• Caused by the burrowing flea, Tunga penetrans.
• Endemic in Caribbean islands and parts of Central and South America, Africa, Pakistan, and India.
• Female flea burrows into skin, then enlarges to create a nodule with a central punctum through which eggs are ejected.

• Female flea burrows into upper dermis, enlarges to approximately 1 cm diameter.
• Creates nodule with central punctum through which fertilized or unfertilized eggs eventually discharged.

• First reported in crew members on Christopher Columbus' original trip to the New World in 1492.

• Occurs regardless of sex, race, or age.
• High-incidence areas: Caribbean islands, parts of Central and South America, sub-Saharan Africa.
• Risk factors in endemic areas: walking barefoot, open-toed shoes, homes with dirt/unclean floors.

• Tunga penetrans: wingless flea, 1 mm in size, lives in warm dry soil.
• Both sexes require occasional blood meal from warm-blooded animal for maturation.
• Limited jumping ability → most common bite location is the foot.
• Female fleas burrow headfirst into mammalian skin.
• Undergo marked abdominal hypertrophy, growing to 1 cm diameter.
• Flea protrudes rear cone through a central punctum that is always maintained.
• Over 100 eggs discharged within 3 weeks (whether or not fertilization occurred via mating with free-roaming male).
• After expelling eggs, female dies; remains eventually sloughed from skin.

• Bites of immature fleas cause no symptoms.
• Burrowing of female flea causes significant clinical morbidity.
• Initially asymptomatic, then varying degrees of pain/pruritus develop.
• Evolution: small black dot → pearl-like whitish papule → larger nodule resembling a "watch glass," with clearly demarcated white halo surrounding black central punctum.
• Peripheral zone of erythema frequently evident.
• When flea dies, a black crust covers the involuting lesion.
• Fortaleza classification scheme divides tungiasis into stages based on symptoms, lesion size, and clinical appearance.
• Most common site: periungual area of the toes, followed by soles and toe webs.
• Complications: ulceration, secondary infection, lymphangitis; less frequent: tetanus, gangrene, amputation of a digit.
• Over half of periungual infections lead to deformation or loss of the nail.
• Heavy infestations produce substantial morbidity in resource-poor endemic communities.

• Nonspecific inflammatory infiltrate surrounds the flea within the dermis.

• Myiasis, tick bite reactions, cercarial dermatitis, pyoderma, plantar warts, verruga peruana, squamous cell carcinoma.

• Spontaneous recovery does occur; most prefer removal of female flea.
• Early infestation: sterile needle to pry insect from skin.
• Covering punctum with dimethicone, liquid paraffin, or a mixture of neem and coconut oils may prevent flea growth and expedite removal.
• If treatment delayed: curettage, electrodesiccation, or surgical excision may be required.
• Careful cleaning of cystic cavity speeds resolution; antibiotics for secondary infection.
• Tetanus prophylaxis should be strongly considered.
• Topical ivermectin, metrifonate, or thiabendazole may hasten demise of embedded fleas but has limited clinical benefit.
• RCT: oral ivermectin not found to have significant efficacy compared to placebo.
• Prevention: closed shoes/boots, avoid ground contact in endemic areas.
• Plant-based insect repellent containing coconut and jojoba oils shown to prevent sand flea infestation.

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• Various fly names: human botflies, screwworms, blowflies, fleshflies, tumbu flies.

• Infestation of skin by fly larvae (maggots).
• Larvae inhabit cutaneous wounds (wound myiasis) or burrow into dermis causing boil-like lesions (furuncular myiasis).
• Usually minimal morbidity, but larvae can cause problems when involving nasal cavity and sinuses.

• Infestation of skin by developing larvae (maggots) of fly species within arthropod order Diptera.
• Two main clinical types: wound myiasis and furuncular myiasis.

• Maggots central to Francesco Redi's experiment rejecting theory of spontaneous generation.
• Wound debridement by maggots used for centuries, including American Civil War and Napoleon's troops.

• Worldwide with seasonal variation; prevalence related to latitude and fly life cycle.
• Higher incidence in tropics/subtropics of Africa and the Americas.
• Flies prefer warm/humid environment; restricted to summer months in temperate zones, year-round in tropics.

• Infestation of living humans (or other vertebrates) with dipterous larvae feeding on host tissues, liquid body substances, or ingested food.
• Classified by: (1) taxonomic family; (2) differences in pathogenic behavior of fly species.
• Most common causative flies worldwide:
• Dermatobia hominis (human botfly).
• Cordylobia anthropophaga (tumbu fly).
• Transmission routes differ:
• D. hominis lays eggs on mosquitoes, which deposit them on warm-blooded mammal.
• C. anthropophaga deposits eggs on moist clothing, soiled blankets, and sand.
• Larva can live 15 days without feeding; once contacting host, penetrates skin, initiating maturation.
• In endemic areas, people iron clothes after hanging to dry to kill fly eggs.
• Wound myiasis: open wound or orifice attracts flies to deposit eggs.
• Any body area can be infested; most serious sequelae when nasal cavity, sinuses, or scalp involved.
• Incubation for larvae to mature into adulthood: 1 to 12 weeks, depending on fly.
• Life cycle of Dermatobia hominis:
• Adult botfly emerges from soil after 4–11 weeks.
• Phoresis: gravid female seizes blood-sucking insect and oviposits 10–50 eggs.
• Larvae hatch in response to elevated temperature when carrier insect feeds on mammalian host, then rapidly penetrate skin.
• First stage larva → Second stage larva (difficult to dislodge due to shape and posterior spines) → Third stage larva emerges from aperture in 5–10 weeks and falls to ground to pupate in soil.

• Furuncular myiasis (human botfly, tumbu fly) — causes boil-like lesions.
• Tumbu fly: typically trunk, thighs, buttocks.
• Botfly: usually exposed areas — scalp, face, forearms, legs.
• Pruritic papule develops within 24 hours of penetration, enlarging to a 1–3 cm nodule with a 2–3 mm central punctum.
• Can be painful/tender; may become crusted and purulent; sensation of movement possible.
• Wound myiasis: larvae deposited in suppurating wound or on decomposing flesh.
• Cochliomyia hominivorax (screwworm) — most common cause in the Americas.
• Chrysomya bezziana — typically seen in Africa, Australia, and Asia.
• Diagnosis obvious when larvae visible on wound surface; more difficult when burrowed beneath surface.
• Creeping (migratory) cutaneous myiasis:
• Caused by Hypoderma bovis (exposure to infested cattle) or Gasterophilus intestinalis (those working with horses).
• Resembles cutaneous larva migrans, but fly larvae migrate more slowly, persist longer (often months), and are larger than helminth larvae.
• Myiasis is self-limiting with minimal morbidity in vast majority of cases.
• Major reasons for treatment: reduction of pain, cosmesis, psychological relief.
• Larvae such as C. hominivorax can infest around orifices of the head and may burrow into brain tissue.

• Inflammatory response occurs in stages, involving lymphocytes, giant cells, neutrophils, eosinophils, mast cells, plasma cells.
• Larvae can be seen in cross-section.

• Furuncular cutaneous myiasis should be differentiated from:
• Ruptured epidermoid cyst, abscess, furunculosis, foreign body reaction, onchocerciasis, tungiasis, exaggerated arthropod bite reaction, lymphadenopathy.

• Larva in furuncular myiasis should NOT be forcibly removed through central punctum — tapered shape with rows of spines/hooks prevents simple extrusion.
• Surgical debridement under local anesthesia is curative; foreign body reaction can occur if parts of larva remain.
• Occlusion/suffocation approaches: petroleum jelly, liquid paraffin, beeswax, nail polish, heavy oil, lard, or strips of bacon over central punctum.
• Blockage forces aerobic larva to surface for air over several hours; forceps then aid capture.
• Alternative: ethyl chloride sprays, liquid nitrogen, chloroform in vegetable oil, or insecticides — alone or in combination.
• Larva can be forced to surface by injecting lidocaine into base of tissue cavity or using a suction-based venom extractor.
• After removal: antiseptic dressings; antibiotics if secondary infection present.
• Wound myiasis: requires debridement with irrigation to eliminate larvae, or surgical removal.
• If burrowing occurred, extraction method identical to furuncular myiasis.
• Alternative treatment for all types of myiasis: oral ivermectin, especially helpful with oral and orbital involvement.
• Surgical removal not required unless requested by patient, as larvae naturally sloughed within 2 weeks.
• Myiasis can be a portal of entry for Clostridium tetani — vaccination of affected individuals should be considered.

• Avoid activities increasing risk in endemic areas — wearing damp clothing, resting in sandy areas.
• Use of insect repellents (including on scalp) useful in preventing mosquitoes harboring human botfly larvae from depositing their cargo.

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• Major phyla causing bites/stings/parasitic infection in humans: Arthropoda, Chordata, Cnidaria, Nemathelminthes, Platyhelminthes, Annelida, Protozoa

• Ticks, mites, spiders, scorpions; adult/nymph have 4 leg pairs, larvae have 6; body = cephalothorax + abdomen (vs. 3-segment insects)

Order Acarina — Tick Bite

• Ixodidae (hard ticks, remain attached) and Argasidae (soft ticks) attack humans
• Ornithodoros hermsi, O. turicata, O. parkeri: tick-borne relapsing fever (US)
• Wood tick (Dermacentor andersoni): western US — Rocky Mountain spotted fever (RMSF), tularemia, ehrlichiosis, Colorado tick fever
• Dog tick (Dermacentor variabilis): eastern US — most common RMSF vector, also tularemia
• Dermacentor marginatus: tick-borne lymphadenopathy (Spain)
• Haemaphysalis longicornis: severe fever with thrombocytopenia syndrome (high mortality — China, S. Korea, Japan; now established in US)
• Brown dog tick (Rhipicephalus sanguineus): RMSF, tularemia, boutonneuse fever
• Lone star tick (Amblyomma americanum): RMSF, tularemia, human monocytic ehrlichiosis
• Ixodes ricinus (Europe), I. scapularis and I. pacificus (US): Borrelia burgdorferi (Lyme disease) vectors; Ixodes also transmit human granulocytic ehrlichiosis, babesiosis
• Risk of transmission increases with attachment duration; ticks often attach unnoticed
• Attachment mechanism: proboscis insertion generally unnoticed; may be mistaken for new mole; engorges over 7–12 days then falls off; tick bite pyrexia (fever, chills, headache, abdominal pain, vomiting) during attachment, resolves 12–36h after removal
• Sequelae: pruritic fibrous nodules (tick bite granulomas) persisting months; pruritic circinate/arciform erythemas persisting months; tick bite-induced alopecia; A. americanum and Ixodes bites associated with IgE to galactose-α-1,3-galactose (alpha-gal) → delayed urticaria/anaphylaxis after red meat consumption
• Histology: wedge-shaped necrosis, neutrophilic infiltrate, vascular thrombosis/hemorrhage; chronic reactions may show atypical CD30+ lymphocytes/eosinophils; pseudolymphomas/immunocytomas possible

Tick Paralysis

• Most often affects children; ~10% mortality; ascending flaccid paralysis (legs → arms → neck), resembles Guillain-Barré; bulbar paralysis, dysarthria, dysphagia, death from respiratory failure possible
• Prompt recovery if tick found/removed before terminal stage
• Dermacentor ticks (North America) and Ixodes ticks (Australia) most important causes; Dermacentor typically attach to scalp — may go unnoticed

Mites

Scabies

• Sarcoptes scabiei: oval, ventrally flattened, dorsal spines; fertilized female burrows into stratum corneum, deposits eggs
• Predilection sites: finger webs, wrists, axillae, areolae, umbilicus, lower abdomen, genitals, buttocks — "circle of Hebra" (imaginary circle: axillae, elbow flexures, wrists/hands, crotch)
• Adults: face/scalp usually spared; infants: entire cutaneous surface may be involved
• Burrows: slightly elevated grayish tortuous lines, ± terminal vesicle/pustule (especially infants/children); India ink or gentian violet applied then removed with alcohol highlights burrows (retained in threadlike tracks)
• Variable eruption depending on infestation duration, prior sensitization/treatment, climate, host immune status; lichenification, impetigo, furunculosis; bullous lesions (eosinophil-rich, may mimic bullous pemphigoid, can show positive immunofluorescence); can trigger EB pruriginosa (dystrophic EB variant); may mimic Langerhans cell histiocytosis clinically/histologically — misdiagnosis has led to inappropriate toxic systemic treatment
• Nodular scabies: dull-red 3–5mm nodules on scrotum/penis/vulva, may persist, may not itch; histology may suggest lymphoma; treatment: intralesional steroids, tar, excision
• Crusted (Norwegian/hyperkeratotic) scabies: immunocompromised/debilitated (neurologic disorders, Down syndrome, transplant, GVHD, ATL, Hansen disease, AIDS); heavily scaling/crusted, face/scalp involved, itching may be slight; psoriasis-like periungual scaling, swollen/crusted/distorted nail-area fingertips; severe genital/buttock fissuring/scaling; pressure-bearing areas favored, teeming with mites
• Transmission: close personal contact, contaminated linens/clothing; STD screening appropriate in adults
• Sensitization begins ~2–4 weeks after onset (asymptomatic burrowing before this); reinfection: itching within days, more intense; itching worst at night; adults spare face/scalp
• Women: nipple itching + generalized pruritic papular eruption characteristic; men: scrotal/penile itchy papules typical; multiple affected family members suggests scabies
• Diagnosis: microscopic mite demonstration via burrow scraping (mineral/immersion oil); mites found most on hands/wrists, then elbows, genitalia, buttocks, axillae (decreasing order); children — subungual material via blunt curette; noninvasive: dermoscopy, digital photography
• Treatment: permethrin 5% cream, oral ivermectin, 6–10% precipitated sulfur in petrolatum, benzyl benzoate most widely used; spinosad also available in some areas
• Permethrin: low toxicity, some concern re: topical insecticide/hematopoietic malignancy association
• Lindane: rarely first-line, banned many locations (neurotoxicity)
• Oral ivermectin 200 µg/kg, typically repeated in 7–10 days (or single dose for mass infestations in resource-limited settings)
• European guidelines: permethrin, oral ivermectin, benzyl benzoate first-line; topical malathion/ivermectin/sulfur alternatives
• Japanese guidelines: topical phenothrin or oral ivermectin first-line; topical sulfur/crotamiton/benzyl benzoate alternatives
• German guidelines: crotamiton second-line
• Moxidectin and Tinospora cordifolia lotion promising
• Application: neck to feet thoroughly (creases, perianal, umbilicus, nail folds/edges), wash off 8–10h later; launder clothing/linens; crotamiton (Eurax) lower cure rate — applied 5 successive nights, washed off 24h after last use
• Crusted scabies: ivermectin + topical agent combined, repeated 2–3× at 1–2 week intervals; close contacts treated; long-term care facility scabies is increasing problem, delays in contact treatment cause outbreaks
• Pregnancy: ivermectin no selective fetal toxicity in animal studies; permethrin often used, but some prefer sulfur given pyrethroid-leukemia association concern

Animal Scabies

• Zoonotic scabies/scab mites: self-limited course in humans, burrows usually absent

Demodex Mites

• Demodex folliculorum: vermiform mite, pilosebaceous units of nose/forehead/chin/scalp; flattened head, 4 pairs short peglike legs, elongated abdomen
• Demodex brevis: shorter, more often trunk
• Human associations: demodectic blepharitis, folliculitis, abscess, alopecia — respond to mite eradication; some rosacea-like lesions may be Demodex-caused
• Treatment: permethrin, sulfur, ivermectin, benzyl benzoate, benzoyl peroxide topically; oral ivermectin, metronidazole also used

Cheyletiella Dermatitis

• Cheyletiella yasguri, C. blakei, C. parasitovorax: nonburrowing mites parasitic on dogs, cats, rabbits respectively ("walking dandruff" on the animal)
• Bite humans in close animal contact, itchy dermatitis resembling scabies or immunobullous disease
• Similar diameter to Sarcoptes but elongated, prominent anterior hooked palps; found by brushing animal hair over dark paper, floating mites in alcohol (scales/hair sink); veterinary treatment of pet

Chigger Bite

• Trombiculid mites ("chiggers," "mower's mites," "red bugs"); North America — Trombicula (Eutrombicula) alfreddugesi; Europe — harvest mite Neotrombicula autumnalis
• Summer/fall, contact with infested grass/bushes; legs, belt line, clothing-constriction sites; penile lesions common in males
• Severely pruritic hemorrhagic puncta with red swelling; grouped excoriated papules on ankles
• East Asia/South Pacific trombiculid species vector scrub typhus (tsutsugamushi fever)

Gamasoidosis

• Ornithonyssus and Dermanyssus genera; contact with canaries, pigeons, poultry; common among urban dwellers (pigeon association); small mite size + tendency to leave host after biting makes diagnosis easy to miss
• Pet stores: bird mites → rodents → human disease from gerbil/hamster contact
• Active at night, hide during day; hands/arms itchy macules/papules/vesicles, but any area (groin, areolae, umbilicus, face, scalp) possible; mites wander from nests as young birds fledge, infest terrace cushions/patio furniture
• Tropical fowl mite (Ornithonyssus bursa) and red chicken mite (Dermanyssus gallinae) major culprits; Dermanyssus may carry Erysipelothrix rhusiopathiae

Grocer's Itch

• Pruritic forearm dermatitis ± trunk inflammatory/urticarial papules; from handling figs/dates/prunes (Carpoglyphus passularum) or storage mite (Glycyphagus domesticus) exposure; distinct from grocer's eczema (flour/sugar/cinnamon/chocolate sensitization)

Grain Itch (Straw/Barley/Mattress/Prairie Itch)

• Mites: Pyemotes tritici, P. ventricosus, Cheyletus malaccensis, Tyrophagus putrescentiae (copra itch mite)
• Affects wheat/hay/barley/oat harvesters, straw handlers; typical lesion: urticarial papule with small vesicle, intense pruritus, mainly trunk; initial hemorrhagic punctum → ecchymosis with hemosiderin pigmentation

Other Mite-Related Dermatitides

• Dermatophagoides pteronyssinus and D. farinae: dust mites, atopic disease implication
• Lepidoglyphus destructor: hay mite
• Pyemotes boylei: outbreaks in homes fumigated for termites (mites thrive on termite carcasses despite not surviving alongside live termites)
• Vanillism: Acarus siro, vanilla pod handlers
• Copra itch: Tyrophagus longior, copra handlers
• Coolie itch: Rhizoglyphus parasiticus, tea plantation workers (India), sore feet
• Rat mite itch: Ornithonyssus bacoti (tropical rat mite), intensely pruritic papulovesicular urticarial eruption in store/factory/warehouse/stockyard workers; vectors endemic typhus, rickettsialpox, equine encephalitis, tularemia, plague, relapsing fever
• Feather pillow dermatitis: Dermatophagoides scheremetewskyi (psoroptid carpet mite)
• House mouse mite (Allodermanyssus/Liponyssoides sanguineus): vector of Rickettsia akari (rickettsialpox)

Order Scorpionidae — Scorpion Sting

• Elongated abdomen ending in stinger; cephalothorax, 4 leg pairs, pincers, mouth pincers; 2 poison glands empty into stinger
• Worldwide, especially tropics; nocturnal, hide under tabletops/closets/shoes/blankets; ground scorpions burrow into gravel/sandboxes
• Buthid scorpions: most venomous medically important species — Tityus serrulatus (Brazil), Buthotus tamulus (India), Leiurus quinquestriatus and Androctonus crassicauda (North Africa/SW Asia), Centruroides suffusus (Mexico); C. exilicauda and C. sculpturatus (most toxic in US); Vaejovis (SE US) — "brown recluse-like" dermonecrotic reactions reported
• Sting only accidentally/defensively; venom causes pain, paresthesia, variable local swelling
• Egyptian scorpion (L. quinquestriatus): 50% mortality in children; neurotoxic venom → numbness, laryngeal edema, profuse sweating/salivation, cyanosis, nausea, tongue paresthesia; minimal/no visible site change, histologically confirmed absence of inflammation typically; death from cardiac/respiratory failure (especially children); renal/hepatic toxicity possible
• Treatment: species/symptom-dependent — antiarrhythmics, antiadrenergics, vasodilators, calcium channel blockers; antivenin available for many species, generally most effective intervention

Order Arachnidae — Arachnidism (Spiders)

• Most spiders beneficial (insect control); few dangerous; most spider venoms poorly characterized, responsible spider rarely identified
• Brazilian armed spider (Phoneutria nigriventer): well-characterized neurotoxins, can be fatal in children
• Reactions: dermonecrotic, systemic toxicity, AGEP

Latrodectism

• Latrodectus species share similar toxins/reactions
• Black widow (L. mactans): chief US concern, also Caribbean; web-building, found in woodpiles/under outhouse seats; venom less potent than brown widow but injects more
• Other regional species: L. curacaviensis (South America), red-back spider L. mactans hasselti (Australia/NZ), L. indistinctus (Africa), brown widow L. geometricus (southern Africa/Madagascar)
• Female L. mactans: 13mm, shiny black, red hourglass marking, legs spread to 4cm; not aggressive, bites only when disturbed
• Severe pain within minutes, spreads to extremities/trunk; hours later: chills, vomiting, violent cramps, delirium/partial paralysis, spasms, abdominal rigidity (may mimic appendicitis/colic/food poisoning); toxic morbilliform erythema, myocarditis reported
• Treatment: antivenin for severe envenomation; benzodiazepines for tetany

Loxoscelism

• Brown recluse spider (Loxosceles reclusa): major US cause of necrotic arachnidism, lower Midwest/Southwest; dark violin-shaped cephalothorax marking, three sets of eyes (vs. usual 4); light brown, ~1cm, small body/long delicate legs; storage closets, basements, cupboards, clothing; outdoors: woodpiles, grass, rocky bluffs, barns; defensive stinger, not aggressive; incidence of bites grossly overestimated
• Related species (lesser necrosis): L. rufescens, L. deserta, L. arizonica; Latin America (similar to L. reclusa): L. laeta, L. intermedia, L. gaucho, L. similis
• Venom: phospholipase enzyme sphingomyelinase D (major toxin); hyaluronidase → gravity-dependent lesion spread
• Necrotic cutaneous loxoscelism: painful severe edematous reaction within 8h, bulla with erythema/ischemia zones, central portion dark/demarcated/gangrenous by ~1 week
• Systemic loxoscelism: rare, may occur despite minor-appearing bite; associated with DIC
• Treatment: rest, ice, elevation; tetanus toxoid if not current; some data favor intralesional triamcinolone (anecdotal injection-site sparing with surrounding necrosis); antibiotics/conservative debridement for necrotic wounds; dapsone used but no better than placebo in some studies, toxic especially with venom-induced hemolysis; colchicine disappointing in animal models; tetracyclines promising, need further study; hyperbaric oxygen for slow-healing wounds

Funnel Web Spiders

• Tegenaria agrestis (hobo/aggressive house spider, Pacific NW), Atrax robustus (Sydney funnel web, Australia — dangerous but antivenin available)

Tarantulas (Lycosidae: Theraphosidae)

• Large hairy hunting spiders; American species have urticating hairs causing wheal/flare reactions, can embed in cornea causing ophthalmia nodosa

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• Pathophysiology: Mite burrows into stratum corneum; Type IV hypersensitivity reaction to mite/eggs/feces causes intense nocturnal itch after 4 to 6 weeks (1st exposure) or 24–48 hours (re-exposure).
• Clinical: Nocturnal intractable pruritus; Burrows (pathognomonic 2–15 mm linear serpiginous ridges); papules, excoriations.
• Anatomic Sites: Web spaces of fingers, flexor wrists, axillae, umbilicus, waistline, buttocks, genitalia (penile nodular scabies). SPARES HEAD AND NECK IN ADULTS (involves head/neck in infants!).
• Crusted (Norwegian) Scabies: Hyperkeratotic crusted plaques harboring millions of mites in immunocompromised hosts; highly contagious; non-pruritic.
• Treatment Protocols:
• Permethrin 5% Cream: Apply head-to-toe, leave on for 8 to 14 hours, wash off; repeat in 7 days. Safe ≥ 2 months of age and in pregnancy.
• Oral Ivermectin: 200 mug/kg single oral dose, repeated in 7 to 14 days. Essential for crusted scabies.
• Wash clothing/bedding in hot water (>60°C) and dry on high heat; treat all household contacts simultaneously.

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• Pediculosis Capitis (Head Lice) (Pediculus humanus capitis): Direct head contact; nits cemented to hair shaft. Treatment: Permethrin 1%, Ivermectin 0.5% lotion, Spinosad 0.9%, Malathion 0.5%.
• Pediculosis Corporis (Body Lice) (Pediculus humanus humanus): Lice live in clothing seams. Vector for Epidemic Typhus (Rickettsia prowazekii), Trench Fever (Bartonella quintana), and Relapsing Fever (Borrelia recurrentis).
• Pediculosis Pubis (Crab Lice) (Pthirus pubis): Sexually transmitted. Broad crab-like claws attached to pubic hair and eyelashes (Phthiriasis Palpebrarum). Maculae Caeruleae (slate-blue macules on thighs/abdomen from louse saliva). Treatment: Permethrin 1%.

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