What causes scabies and what is the mite's life cycle?
Scabies is a highly contagious skin infestation caused by the mite Sarcoptes scabiei var. hominis, an obligate ectoparasite (class Arachnida). The mite burrows into the skin and, weeks after the first infestation, triggers intense nocturnal pruritus. Transmission is mainly by prolonged skin-to-skin contact. The WHO recognised scabies as a neglected tropical disease in 2017.
| Feature | Fact |
|---|---|
| Size | Female up to about 0.45 × 0.35 mm, roughly twice the size of the male; 8 legs |
| Mating and burrowing | Adults mate on the skin; the fertilised female penetrates the epidermis, dissolving the stratum corneum with her saliva |
| Egg laying | 2 to 4 eggs per day along the burrow; faecal pellets (scybala) are left in the tunnel; burrows extend about 1–10 mm |
| Life span | Adult mites survive about 1 to 2 months |
| Larvae | Hatch in 2–4 days as 6-legged larvae, make new burrows and moult to nymphs and then adults |
| Transmission | Usually needs about 10–15 minutes of direct skin contact; fomites matter little in classic scabies but more in crusted scabies |
| Mite burden (classic) | Only about 10–15 mites per person |
What are the clinical features and typical sites of scabies?
The hallmark is intense pruritus, worse at night, often affecting several family members or close contacts. The classic lesion is the burrow: a short, wavy white line with a vesicle or erosion at the distal end where the mite sits. Burrows are pathognomonic but uncommon and hard to find; most patients show a polymorphous eruption of excoriated papules, eczematous or lichenified plaques and nodules.
| Group | Typical sites |
|---|---|
| Adults | Finger webs (interdigital spaces), flexor surface of wrists, axillae, umbilicus, beltline, nipples and areolae, buttocks, scrotum and penile shaft |
| Infants and children < 2 years | Vesicles; finger web spaces, wrists and trunk; scalp, face, palms and soles can also be involved (unlike adults) |
| Nodular scabies | Erythematous, itchy nodules in moist, thin-skinned sites such as the axillae and groin; a hypersensitivity reaction to the female mite |
| Crusted scabies | Hands and feet (palms, soles), head, earlobes, subungual areas |
Chronic scratching leads to excoriation, crusting, lichenification, prurigo nodularis and secondary bacterial infection (impetiginisation). Impetigo in scabies is caused by Staphylococcus aureus or Streptococcus pyogenes.
What is crusted (Norwegian) scabies?
Crusted scabies occurs mainly in the immunocompromised — for example diabetes mellitus, HIV, old age or immunosuppressive therapy. Instead of 10–15 mites, the patient may carry millions, so the condition is extremely contagious: even brief contact with the patient or contaminated material can transmit it. It shows hyperkeratotic plaques, diffuse or localised to palms, soles and subungual areas, and can mimic erythroderma, psoriasis, eczema, seborrhoeic dermatitis or pityriasis rubra pilaris.
| Feature | Classic | Crusted |
|---|---|---|
| Host | Usually immunocompetent | Immunocompromised |
| Mite number | About 10–15 | Thousands to millions |
| Itch | Intense, nocturnal | May be minimal |
| Lesions | Papules, burrows, excoriations | Thick hyperkeratotic crusts and plaques |
| Spread by fomites | Uncommon | More frequent; outbreaks in institutions |
| Treatment | Permethrin or ivermectin | Combination therapy: topical agent plus oral ivermectin on multiple days, plus keratolytics |
How is scabies diagnosed?
Scabies is mainly a clinical diagnosis: typical lesions in a typical distribution with a supportive history (itch in contacts). In 2020 the International Alliance for the Control of Scabies published consensus criteria for classic scabies with three levels of certainty.
| Level | Requirement |
|---|---|
| Suspected scabies | Typical lesions in a typical distribution plus one history feature |
| Clinical scabies | The above plus visible burrows |
| Confirmed scabies | The above plus visualisation of mites, eggs or faeces |
- Skin scraping of the stratum corneum with microscopy can show mites, eggs or scybala, but is time-consuming and low in sensitivity because of sampling error.
- Dermoscopy is non-invasive and quick; burrows resemble a jet-with-contrail or delta-wing jet sign. It is limited on dark or hairy skin and in the genital region.
- Videodermatoscopy (×10 to ×1000) can show burrows, mites, eggs, larvae and faeces but is costly and uncommon.
- Skin biopsy may show mites and eggs in the reticular dermis with an inflammatory infiltrate, but mites are often missed, but is not recommended routinely.
What is the treatment of scabies?
First-line options are topical permethrin 5% cream and oral ivermectin, which have comparable efficacy. Permethrin is generally preferred first because of its safety, including in children older than 2 months and in pregnancy. Whatever the drug, the whole household must be treated at the same time, because people in the early stage of infestation are asymptomatic.
| Drug | Regimen and points |
|---|---|
| Permethrin 5% cream | Applied from the neck down (including under the nails) to the whole body, once weekly for 2 weeks; in infants also scalp, hairline, face and ears, avoiding eyes and mouth; licensed from 2 months of age |
| Oral ivermectin | 200 µg/kg, repeated after 14 days (it is scabistatic and not ovicidal, so the second dose kills newly hatched mites). Not established in pregnancy or in children under 15 kg |
| Sulfur ointment 5–10% | Safe alternative for infants under 2 months: applied for 3 consecutive days and repeated after 2 weeks |
| Benzyl benzoate 25% | Alternative; safe in pregnancy and children; used in crusted scabies with oral ivermectin |
| Spinosad 0.9%, crotamiton 10% | Other approved topical options (crotamiton on 2 consecutive days, repeated after 2 weeks) |
| Topical ivermectin 1% lotion | Less effective alternative; washed off after 8–14 hours, repeated after 1 week if symptoms persist |
| Lindane 1% | Reserved for failure or intolerance of other options; neurotoxicity and haematological adverse effects; contraindicated under 10 years, in pregnancy, lactation and extensive dermatitis |
- Crusted scabies: combination of permethrin 5% (whole body daily for 1 week, then twice weekly until cure) or benzyl benzoate 25% plus oral ivermectin 200 µg/kg on days 1, 2, 8, 9 and 15, with keratolytics such as urea or salicylic acid to improve penetration.
- Itch may worsen for 1–2 weeks after effective treatment; patients should be told this.
- Treatment failure usually reflects untreated contacts, inadequate decontamination of bedding and clothing, or poor adherence; ivermectin-resistant mites may occur in crusted scabies.
- Repeat treatment after 7–14 days according to the drug chosen.
What are the complications of scabies?
The main complications are persistent pruritus, insomnia and secondary bacterial infection. According to WHO, scabies is frequently complicated by impetigo, which can lead to abscesses, sepsis and invasive infection with S. aureus and S. pyogenes; the latter can result in kidney disease and rheumatic heart disease. Scabies mites release complement-inhibiting proteins that impair opsonisation, which favours bacterial growth in the skin. The itch and visible rash also cause stigmatisation, poor sleep and missed school or work.
- Differential diagnosis: atopic or contact dermatitis, irritant dermatitis, fleas, bedbugs and chiggers; in crusted scabies psoriasis, hyperkeratotic eczema and Darier disease; in bullous scabies bullous impetigo and pemphigus vulgaris.
- Prevalence is highest in crowded, resource-poor settings: poverty, homelessness, poor water and sanitation.
What is pediculosis and which lice infest humans?
Pediculosis is infestation by lice, obligate blood-feeding insects with no free-living stage. Lice are about 1 to 3 mm long, cannot jump or fly, and so need close contact or fomites to spread. Itch arises from an immune-mediated hypersensitivity: after a first exposure symptoms may take 2 to 6 weeks, but only 1 to 2 days on re-exposure.
| Species | Site and spread | Notable points |
|---|---|---|
| Pediculus humanus capitis (head louse) | Scalp hair; head-to-head contact, shared hats and hair accessories | Most common; children 3–12 years, girls more often than boys; nits stick to hair shafts |
| Pediculus humanus corporis (body louse) | Lives and lays eggs in clothing and bedding, moves to skin only to feed | Crowded, homeless or displaced populations; colder months; vector of trench fever, relapsing fever and epidemic typhus |
| Pthirus pubis (pubic or crab louse) | Pubic hair; sexual contact, bedding or clothing | Smaller than the others; screen for other sexually transmitted infections |
How is pediculosis diagnosed and treated?
Diagnosis is made by finding at least one live louse, helped by bright light, a lens or a fine-toothed lice comb. Nits alone do not prove active infestation, because nits can stay attached to hair for months after successful treatment; dandruff, hairspray residue and debris can mimic nits. Nits are oval and firmly attached to the hair shaft, and under a Wood lamp they may show greenish-yellow fluorescence. When lice are found, examine all household members.
| Agent | Mechanism and notes |
|---|---|
| Permethrin 1% | Synthetic pyrethroid; disrupts neuronal sodium transport and paralyses lice; residual activity; resistance increasingly reported; retreat at 7 days |
| Malathion 0.5% | Organophosphate cholinesterase inhibitor; respiratory paralysis; foul odour; leave on 8–12 hours |
| Lindane 1% | Organochloride; neurotoxic if absorbed; avoid repeat use; contraindicated in children, older adults and people under 50 kg |
| Benzyl alcohol 5% | Suffocates lice by preventing the spiracles from closing |
| Spinosad 0.9% | Neuronal hyperexcitation and paralysis; often needs only one application |
| Ivermectin 0.5% (topical) | Increases chloride permeability in muscle cells, causing hyperpolarisation and paralysis; single application |
| Oral ivermectin | Only systemic option; for resistant cases; neurotoxicity risk |
- Most pediculicides do not reliably kill eggs, so a second application after 7–10 days is usually needed.
- Wet combing from root to tip with a fine-toothed comb is a physical method; cure rates vary widely.
- Body lice: improved hygiene, changing clothes at least weekly and laundering; insecticide treatment of clothing if necessary.
- Pubic lice: same agents as head lice (permethrin preferred); treat sexual partners and look for other STIs.
- Wash infested clothing, bedding and towels in hot water: at least 52 °C for 30 minutes kills lice and nits.
- Treatment failure is most often non-adherence (especially skipping the repeat treatment), untreated contacts, resistance and unremoved nits.
How are scabies and pediculosis asked in NEET PG and INI-CET?
- Causative organism of scabies — Sarcoptes scabiei var. hominis.
- Sites — finger webs, wrists, axillae, areolae, genitals; face and scalp spared in adults, involved in infants.
- Type of hypersensitivity — delayed (type IV).
- Crusted scabies — immunocompromised host, millions of mites, very contagious.
- Drug of choice — permethrin 5% (including pregnancy); oral ivermectin 200 µg/kg repeated at 14 days.
- Treat contacts simultaneously, and why treatment fails.
- Lice — body louse transmits epidemic typhus, trench fever and relapsing fever; pubic lice prompt STI screening; nits alone do not prove active infestation.
- Dermoscopy sign — jet with contrail (delta-wing jet).