Dermatology: Skin infections and infestations

• Bacterial: Cellulitis and Erysipelas | Impetigo | Boils, Abscesses and PVL | Necrotising Fasciitis | Other Bacterial Skin Infections | Animal and Human Bites | Lyme Disease
• Fungal: Tinea (Dermatophyte) Infections | Fungal Nail and Scalp Infections | Candida | Pityriasis Versicolor
• Viral: Herpes Simplex | Shingles and Chickenpox | Warts and Molluscum | Other Viral Rashes
• Infestations: Scabies | Head Lice and Other Infestations
• Revision: Drug Reference Table | Spot the Diagnosis | Exam-style Scenarios | Key Points | Guidelines and Further Reading

🦠 Skin infections and infestations — very common in every setting. The key skills are recognising cellulitis (and its mimics — most "bilateral cellulitis" isn't), spotting necrotising fasciitis early, confirming fungal infection before treating, and treating scabies in the whole household at once. Use local antimicrobial guidelines.
Your four questions for every patient: Is the person systemically unwell (sepsis, necrotising infection)? What's the likely organism? Are they immunosuppressed or diabetic? Who else needs treatment or protection (contacts, household, pregnant or immunosuppressed people)?

Cellulitis and Erysipelas

• Cellulitis — infection of the deeper dermis and subcutaneous tissue, usually Streptococcus pyogenes or Staphylococcus aureus.
• Erysipelas — a superficial form with a sharply defined, raised red edge (often the face or leg); usually streptococcal.
• Features — unilateral red (or darker in skin of colour), hot, swollen, tender skin, spreading; may have blisters, lymphangitis, fever, malaise; entry point (cuts, tinea pedis between the toes, ulcers, insect bites, eczema).
• Risk factors — lymphoedema, obesity, diabetes, venous insufficiency, previous cellulitis, IV drug use, immunosuppression.
• Mimics — "red legs" are often not cellulitis, especially if bilateral: venous (stasis) eczema, lipodermatosclerosis, contact dermatitis, DVT, gout, haematoma, ruptured Baker's cyst, necrotising fasciitis (pain out of proportion).
• Severity (Eron classification) — I (no systemic toxicity), II (systemically unwell or comorbidity), III (significant systemic upset — confusion, tachycardia, hypotension, or limb-threatening), IV (sepsis or necrotising infection). Admit classes III and IV (and consider for II).
• Management (NICE NG141):
  • Mark the edge of the redness; elevate the limb; analgesia; treat the entry point (tinea pedis).
  • Oral flucloxacillin (500 mg to 1 g four times a day) for 5–7 days.
  • Penicillin allergy — clarithromycin, erythromycin (in pregnancy), or doxycycline.
  • Near the eyes or nose — co-amoxiclav (anaerobes and other organisms); consider periorbital and orbital cellulitis (see the Red Eye chapter).
  • Severe or systemically unwell — IV flucloxacillin (or co-amoxiclav, cefuroxime, ceftriaxone, clindamycin per local policy); MRSA — vancomycin, teicoplanin or linezolid.
  • Review if worsening after 2–3 days or not better after 5–7 days (abscess, wrong diagnosis, resistance).
• Recurrent cellulitis — treat lymphoedema (compression), skin care, tinea pedis; antibiotic prophylaxis (e.g. phenoxymethylpenicillin) for frequent episodes.

💡 Clinical Pearl: Cellulitis is almost always one-sided. Two red, itchy, scaly lower legs in an older person with varicose veins is usually venous eczema — antibiotics won't help, but emollients, topical steroids and compression will.

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Impetigo

• What it is: A highly contagious superficial skin infection — Staph aureus (and Strep pyogenes); most common in young children; spreads in nurseries and families.
• Types:
  • Non-bullous (most) — red sores that weep and form golden-yellow ("honey-coloured") crusts, around the mouth and nose; may complicate eczema, scabies or cold sores.
  • Bullous — fragile, fluid-filled blisters that burst to leave raw areas with crust; usually Staph aureus (exfoliative toxin); more common in babies.
• Treatment (NICE NG153):
  • Localised non-bullous (not unwell) — hydrogen peroxide 1% cream for 5 days; if unsuitable — topical antibiotic (fusidic acid; mupirocin if fusidic acid resistance).
  • Widespread non-bullous — topical or oral flucloxacillin for 5 days.
  • Bullous impetigo, systemically unwell, or high risk of complications — oral flucloxacillin for 5 days; clarithromycin (erythromycin in pregnancy) if penicillin allergic.
  • Swab if recurrent or not responding (MRSA, PVL).
• Hygiene — wash hands, don't share towels, avoid scratching.
• School or nursery exclusion — until lesions are crusted and healed, or 48 hours after starting antibiotics.
• Complications (rare) — cellulitis, staphylococcal scalded skin syndrome, post-streptococcal glomerulonephritis, scarlet fever.

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Boils, Abscesses and PVL

• Folliculitis — small pustules around hair follicles (Staph aureus; "hot tub folliculitis" — Pseudomonas; shaving-related pseudofolliculitis); antiseptic washes, topical antibiotics; oral flucloxacillin if extensive.
• Boil (furuncle) — a deep, painful, red nodule around a hair follicle that comes to a head; carbuncle — several joined boils (back of neck; diabetes). Warm compresses; incision and drainage if fluctuant; antibiotics if spreading cellulitis, systemically unwell, or on the face.
• Skin abscess — drainage is the main treatment; send pus for culture; antibiotics if surrounding cellulitis or systemic features.
• Panton-Valentine leukocidin (PVL)-producing Staph aureus:
  • Suspect with recurrent boils or abscesses, clusters in families or close contacts (sports teams, prisons, military), more pain than expected.
  • Can cause necrotising pneumonia (often after flu — haemoptysis, rapid deterioration) and necrotising soft tissue infection.
  • Send swabs for PVL testing; drainage; antibiotics per microbiology; decolonisation (chlorhexidine body wash and nasal mupirocin for 5 days) for the patient and close contacts once lesions heal.
• Recurrent boils — check for diabetes, Staph aureus carriage (nasal swab), PVL, hidradenitis suppurativa (if in skin folds).

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Necrotising Fasciitis

• What it is: A rapidly spreading, life-threatening infection of the fascia and subcutaneous tissue — a surgical emergency (mortality about 20–30%).
• Types:
  • Type 1 (polymicrobial) — mixed aerobes and anaerobes; diabetes, older people, after surgery; includes Fournier's gangrene (perineum and genitals).
  • Type 2 (monomicrobial) — group A Streptococcus (sometimes with toxic shock), or Staph aureus (including PVL); can affect healthy young people after minor injuries.
  • Others — Vibrio (seawater), Clostridium (gas gangrene — IV drug users, trauma).
• Features:
  • Pain out of proportion to the visible signs (the key early clue) — then numbness as nerves die.
  • Rapid spread (hours), tense swelling beyond the redness, dusky or purple skin, blisters (haemorrhagic), skin necrosis.
  • Crepitus (gas).
  • Systemic toxicity — fever, tachycardia, hypotension, confusion, AKI.
• Tests — FBC, CRP, U&E, CK, lactate, glucose, blood cultures (LRINEC score can support but can't rule out); imaging only if it doesn't delay surgery (gas on X-ray or CT).
• Management:
  • Urgent surgical exploration and debridement — diagnosis is confirmed in theatre; repeat debridement often needed.
  • IV broad-spectrum antibiotics immediately (per local policy — e.g. a broad beta-lactam plus clindamycin, which reduces toxin production).
  • Sepsis care — fluids, ICU.
  • IV immunoglobulin in streptococcal toxic shock (specialist).
  • Invasive group A strep is notifiable — close contacts may need antibiotic prophylaxis.

⚠️ Safety Pearl: Severe pain with little to see, or "cellulitis" spreading quickly despite antibiotics, is necrotising fasciitis until proven otherwise. Call the surgeons straight away — a normal CRP or LRINEC score doesn't exclude it.

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Other Bacterial Skin Infections

• Staphylococcal scalded skin syndrome (SSSS) — babies and young children: fever, irritability, tender red skin, then superficial blistering and peeling (positive Nikolsky sign) — exfoliative toxin from a localised Staph infection; mucous membranes are spared (unlike TEN); admit, IV flucloxacillin (plus clindamycin), fluids, pain relief; heals without scarring.
• Toxic shock syndrome — Staph or Strep toxin: high fever, diffuse "sunburn" rash, hypotension, multi-organ failure, later peeling of the palms and soles; tampons, burns, wounds, surgery — remove the source; IV antibiotics (with clindamycin); ICU.
• Scarlet fever — group A strep: sandpaper rash, strawberry tongue, flushed face with circumoral pallor; phenoxymethylpenicillin for 10 days; notifiable (see the Paediatric Infections chapter).
• Erythrasma — Corynebacterium minutissimum: brown, slightly scaly patches in the groin, armpits or toe webs; coral-red fluorescence under Wood's lamp; topical fusidic acid or clindamycin, oral clarithromycin if extensive.
• Pitted keratolysis — smelly feet with small pits on the soles (sweaty feet); topical antibiotics, antiperspirants.
• Syphilis — painless chancre; secondary syphilis rash including palms and soles; serology (see the Sexual Health chapter).
• Leprosy (Hansen's disease) — rare in the UK (migrants, travellers): hypopigmented or red patches with loss of sensation, thickened peripheral nerves; multidrug therapy; refer to specialist services.
• Cutaneous TB (lupus vulgaris), atypical mycobacteria (fish tank granuloma — Mycobacterium marinum; nodules along the lymphatics of the hand).

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Animal and Human Bites

• Assess — type of bite, site (hands, feet, face, genitals, near joints or tendons are high risk), depth (puncture wounds), time since bite, immune status (asplenia, diabetes, immunosuppression, cirrhosis), tetanus status; for bites abroad or by bats — rabies risk.
• Wound care — irrigate thoroughly; debride; don't close puncture wounds or bites to the hand (delayed closure); X-ray for foreign bodies or fractures; refer hand bites with tendon or joint involvement (including "fight bites" over the knuckles).
• Antibiotic prophylaxis (NICE NG184) — co-amoxiclav for 3 days:
  • Human bites — if the skin is broken and blood drawn (consider if broken without blood in high-risk sites or people).
  • Cat bites — if the skin is broken (Pasteurella multocida; deep punctures).
  • Dog bites — if the skin is broken and blood drawn, especially in high-risk sites or people.
  • Penicillin allergy — doxycycline with metronidazole (adults; not in pregnancy or under 12).
• Infected bite — co-amoxiclav for 5 days (longer if severe); IV if severe.
• Human bites — also assess risk of hepatitis B, hepatitis C and HIV (blood-borne virus prophylaxis); safeguarding (bites in children).
• Tetanus — vaccine and/or immunoglobulin as per the Green Book.
• Rabies — post-exposure vaccination (and immunoglobulin) after bites or scratches from mammals in rabies-endemic countries or from bats in the UK — urgent UKHSA advice.

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Lyme Disease

• Cause — Borrelia burgdorferi, spread by ticks (deer and sheep areas — New Forest, Scottish Highlands, Lake District, Exmoor, parks); tick bites are often unnoticed.
• Erythema migrans — a red, expanding rash (often a "bull's-eye" with central clearing), usually over 5 cm, appearing 3–30 days after the bite; not usually itchy or painful; may have flu-like symptoms.
• Later features — facial nerve palsy (bilateral in some), meningitis, radiculopathy, heart block (carditis), arthritis (large joints, especially the knee), acrodermatitis chronica atrophicans.
• Diagnosis (NICE NG95) — erythema migrans is diagnosed clinically (treat without testing); for other symptoms — ELISA, then immunoblot if positive (may be negative early — repeat).
• Treatment — oral doxycycline for 21 days (erythema migrans); amoxicillin if doxycycline is unsuitable (pregnancy, children under 9 — amoxicillin is usual); IV ceftriaxone for some neurological or cardiac disease.
• Tick removal — fine-tipped tweezers or a tick removal tool, close to the skin, pull steadily; don't squeeze or burn; prophylactic antibiotics are not recommended after a tick bite.
• Prevention — cover skin, insect repellent, check for ticks after walking.

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Tinea (Dermatophyte) Infections

• Cause — dermatophyte fungi (Trichophyton, Microsporum, Epidermophyton) that live on keratin — skin, hair, nails. From people, animals (cats, dogs, guinea pigs, cattle) or soil.
• Types:
  • Tinea corporis (body, "ringworm") — annular scaly patches with a raised, scaly, active edge and central clearing; itchy; spreading.
  • Tinea cruris (groin) — red, scaly, well-defined patch on the inner thighs extending outwards; scrotum usually spared (unlike candida).
  • Tinea pedis (athlete's foot) — itchy, peeling, macerated skin between the toes (fourth web space); or dry scaling on the soles ("moccasin"); entry point for cellulitis.
  • Tinea manuum — usually one hand ("one hand, two feet").
  • Tinea faciei, tinea barbae (beard).
  • Tinea incognito — tinea treated with topical steroids: less scaly, more widespread, ill-defined — confusing appearance.
• Diagnosis — skin scrapings for microscopy and culture if the diagnosis is uncertain, extensive, or before oral treatment.
• Treatment:
  • Topical — terbinafine cream (1–2 weeks) or an imidazole (clotrimazole, miconazole — 2–4 weeks, continue 1–2 weeks after clearing).
  • Oral — terbinafine (or itraconazole) for extensive, severe or resistant infection, or if topical treatment fails.
  • Avoid topical steroids alone; a short combined steroid–antifungal can help inflamed tinea.
  • Keep skin dry; treat feet and groin together; wash towels; treat infected pets (vet).
• Trichophyton indotineae — an emerging, terbinafine-resistant dermatophyte (linked to travel to South Asia and steroid creams): widespread, inflamed, persistent tinea of the groin, buttocks and trunk; send scrapings with travel history; usually needs itraconazole (specialist).

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Fungal Nail and Scalp Infections

• Fungal nail infection (onychomycosis):
  • Thick, discoloured (white, yellow, brown), crumbly nails with subungual debris and onycholysis; toenails more than fingernails; older people, diabetes, peripheral vascular disease, athletes.
  • Confirm with nail clippings (microscopy and culture) before oral treatment — psoriasis and trauma look similar.
  • Treatment — optional if mild and asymptomatic; oral terbinafine — 6 weeks for fingernails, 12 weeks for toenails (check LFTs before; taste disturbance, liver toxicity, rash); itraconazole (pulsed) for candida or non-dermatophyte moulds; topical amorolfine nail lacquer for limited distal disease.
  • Nails take 6–12 months to look normal; recurrence is common.
• Tinea capitis (scalp ringworm):
  • Mainly children; in the UK most commonly Trichophyton tonsurans (spread person to person, especially in children of African-Caribbean heritage); Microsporum canis (from cats and dogs).
  • Scaly patches with broken hairs, "black dots", diffuse dandruff-like scale; enlarged neck lymph nodes.
  • Kerion — a painful, boggy, pus-filled swelling (inflammatory reaction) — don't incise; can cause scarring hair loss.
  • Diagnosis — scalp scrapings, plucked hairs or brushings for culture; Wood's lamp (Microsporum fluoresces green; Trichophyton doesn't).
  • Treatment — oral antifungal is needed (topical alone doesn't work): terbinafine (Trichophyton) or griseofulvin (Microsporum) — specialist advice; plus ketoconazole or selenium sulfide shampoo for the child and household to reduce spread; check family members and pets.
  • Children can attend school once treatment has started.

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Candida

• Candidal intertrigo — red, moist, shiny rash in skin folds (groin, under the breasts, abdominal folds, between fingers) with satellite lesions (small papules and pustules beyond the edge); involves the scrotum; itching and soreness.
• Risk factors — diabetes, obesity, heat and sweating, incontinence, antibiotics, steroids, immunosuppression, pregnancy.
• Other sites — oral thrush (white plaques that scrape off — inhaled steroids, dentures, babies, HIV), angular cheilitis, nappy rash, vulvovaginal candidiasis, balanitis, chronic paronychia.
• Treatment — keep skin folds dry; topical imidazole (clotrimazole, miconazole) for 2 weeks (with mild steroid if inflamed); oral fluconazole for extensive or resistant infection; check glucose or HbA1c if recurrent.
• Oral thrush — miconazole oral gel (warfarin interaction) or nystatin; fluconazole if severe; rinse mouth after inhaled steroids.

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Pityriasis Versicolor

• Cause — overgrowth of Malassezia yeasts (normal skin flora); young adults; heat, humidity, sweating.
• Features — many small, slightly scaly, round or oval patches on the trunk, shoulders and upper arms that are paler (hypopigmented) or darker (pink or brown) than surrounding skin; more noticeable after a tan; mild itch.
• Diagnosis — clinical; skin scrapings ("spaghetti and meatballs" on microscopy); Wood's lamp — yellow-green.
• Treatment — ketoconazole 2% shampoo used as a body wash (leave on 5 minutes, daily for 5 days), or selenium sulfide; topical imidazoles for small areas; oral itraconazole or fluconazole if extensive or resistant (not terbinafine — oral form doesn't work).
• Advice — pigment can take months to return to normal; recurrences are common (prophylactic shampoo before summer).

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Herpes Simplex

• HSV-1 (mostly oral) and HSV-2 (mostly genital) — both can affect any site; the virus stays latent in sensory ganglia and reactivates.
• Primary infection — often in childhood: herpetic gingivostomatitis (fever, painful mouth ulcers, swollen gums, drooling, poor drinking — risk of dehydration); paracetamol, fluids, oral aciclovir if severe or immunocompromised.
• Cold sores (herpes labialis) — tingling, then clustered vesicles on the lip that crust and heal in 7–10 days; triggered by sunlight, fever, stress, menstruation. Topical aciclovir at the first tingle (modest benefit); oral antivirals if frequent or severe; sunblock for the lips.
• Herpetic whitlow — painful vesicles on a finger (children sucking thumbs; healthcare workers) — don't incise; oral aciclovir if early or severe.
• Eczema herpeticum — emergency — see the Skin Basics, Eczema and Psoriasis chapter.
• Herpes gladiatorum — contact sports (wrestling, rugby).
• Herpes simplex keratitis — dendritic corneal ulcer — urgent ophthalmology (see the Red Eye chapter).
• Recurrent erythema multiforme — often triggered by HSV; suppressive aciclovir.
• Neonatal herpes and HSV encephalitis — IV aciclovir urgently.
• Immunocompromised — extensive, chronic ulcers; oral or IV aciclovir.

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Shingles and Chickenpox

• Shingles (herpes zoster) — reactivation of varicella zoster virus in a dorsal root or cranial nerve ganglion; more common with age and immunosuppression.
  • Prodrome — burning, tingling or pain in a dermatome for a few days (can mimic angina, gallstones or renal colic).
  • Rash — clusters of vesicles on a red base in a dermatomal band, not crossing the midline; crusts in 7–10 days; usually thoracic.
  • Herpes zoster ophthalmicus — ophthalmic division of the trigeminal nerve; Hutchinson's sign (vesicles on the tip of the nose) means a high risk of eye involvement — same-day ophthalmology.
  • Ramsay Hunt syndrome — facial nerve palsy with vesicles in the ear canal or palate, hearing loss, vertigo (see the Ear chapter).
  • Disseminated zoster (widespread or several dermatomes) — immunosuppression; IV aciclovir.
• Treatment:
  • Oral antiviral within 72 hours of rash onset — aciclovir 800 mg five times a day, valaciclovir or famciclovir for 7 days — for people over 50, immunocompromised, non-truncal involvement (face, neck, limbs, genitals), moderate or severe pain or rash, or eczema. Ophthalmic and immunocompromised patients — treat even after 72 hours.
  • Analgesia — paracetamol, NSAIDs; amitriptyline, gabapentin or pregabalin for neuropathic pain.
  • Infection control — infectious (as chickenpox) to people who aren't immune until lesions crust; cover the rash; avoid pregnant women, newborns and immunosuppressed people.
• Postherpetic neuralgia — pain persisting more than 3 months after the rash (common over 60); amitriptyline, duloxetine, gabapentin or pregabalin (NICE CG173); capsaicin cream; lidocaine plasters (restricted use).
• Shingles vaccine (Shingrix) — 2 doses (non-live); routinely offered to people turning 65 and 70 (and older adults up to 79 under earlier cohorts), and from September 2025 to all severely immunosuppressed adults aged 18 and over.
• Chickenpox in adults — more severe (pneumonia, especially in smokers; hepatitis; encephalitis); oral aciclovir if presenting within 24 hours (adults and teenagers); pregnancy — risk of maternal pneumonia and fetal varicella syndrome: urgent advice; VZIG or antivirals for non-immune pregnant contacts.

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Warts and Molluscum

• Viral warts (human papillomavirus):
  • Common warts — rough, raised papules on the hands and knees; plantar warts (verrucae) — flat, painful on the soles, with black dots (thrombosed capillaries) — unlike corns, skin lines pass around them; plane warts — flat, small, on the face and backs of hands.
  • Most resolve without treatment — about half within a year and two-thirds within 2 years in children.
  • Treatment (if wanted) — salicylic acid (pare down first, daily for up to 12 weeks; avoid on the face and in diabetes or peripheral neuropathy), cryotherapy (liquid nitrogen, repeated every 2–3 weeks), duct tape occlusion; specialist options for resistant warts.
  • Immunosuppressed (e.g. organ transplant) — many, persistent warts; risk of squamous cell carcinoma.
  • Anogenital warts — see the Sexual Health chapter; in children consider sexual abuse.
• Molluscum contagiosum (poxvirus):
  • Small, firm, pearly, dome-shaped papules with a central dimple (umbilication); often in clusters; children (trunk, armpits, behind the knees) and in adults genital (sexually transmitted).
  • Often surrounded by eczema ("molluscum dermatitis"); inflamed lesions often signal resolution.
  • Resolves spontaneously — usually within 18 months; avoid squeezing (spreads, scars); no need to stay off school or swimming (avoid sharing towels).
  • Treatment if troublesome — cryotherapy, curettage, potassium hydroxide solution, hydrogen peroxide cream; eyelid lesions with red eye — ophthalmology.
  • Large or extensive molluscum in an adult — think HIV or other immunosuppression.

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Other Viral Rashes

• Hand, foot and mouth disease (Coxsackie) — mouth ulcers and vesicles on the hands, feet and buttocks; young children; self-limiting; no exclusion needed (see the Paediatric Infections chapter).
• Measles, rubella, parvovirus B19 (slapped cheek), roseola — see the Paediatric Infections chapter.
• Viral exanthems — widespread maculopapular rash with fever in many viral infections (EBV — especially a rash after amoxicillin; HIV seroconversion; COVID-19).
• Mpox — fever, lymphadenopathy, and vesicles and pustules that become umbilicated and crusted (often genital, perianal or oral in recent outbreaks); notifiable — isolate, test, contact UKHSA; vaccination for at-risk groups.
• Orf — a red, then pustular nodule on the hand after handling sheep or goats (farmers, lambing); self-limiting.

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Scabies

• Cause — the mite Sarcoptes scabiei, which burrows into the skin; spread by prolonged skin-to-skin contact (families, sexual partners, care homes, nurseries).
• Features:
  • Intense itch, worse at night; others in the household itchy too.
  • Starts 2–6 weeks after the first infestation (hypersensitivity reaction) — within days on re-infestation.
  • Burrows — short, wavy, grey lines in the finger webs, wrists, sides of fingers; also elbows, armpits, waist, buttocks, genitals (papules on the penis and scrotum are very suggestive), nipples.
  • Widespread itchy papules, excoriations; secondary eczema or impetigo.
  • Infants — may involve the scalp, face, palms and soles.
• Diagnosis — clinical; dermoscopy ("delta wing jet" sign); skin scraping to see the mite.
• Treatment:
  • Permethrin 5% cream — apply to the whole body from the neck down (including scalp, face, neck and ears in children under 2, older people and the immunosuppressed), especially between fingers and toes, under nails, and genitals; leave for 8–12 hours, then wash off; repeat after 7 days.
  • Malathion 0.5% aqueous liquid if permethrin is unsuitable (allergy).
  • Treat all household members and close contacts at the same time (even without symptoms).
  • Wash clothes, towels and bedding at 50 °C or above on the day of treatment (or seal in a bag for 3 days).
  • Itch can persist for 2–4 weeks after successful treatment — crotamiton, emollients, mild steroid, antihistamine; doesn't mean treatment failure.
  • Babies under 2 months — specialist advice.
• Crusted (Norwegian) scabies — huge numbers of mites: thick, crusted, scaly plaques (hands, feet, scalp), may not itch much; older people, care home residents, immunosuppressed, learning disability; highly contagious — outbreaks. Specialist treatment: oral ivermectin plus topical permethrin, keratolytics; infection control and contact tracing (UKHSA).

💡 Clinical Pearl: Scabies treatment fails most often because contacts weren't treated at the same time, the cream wasn't applied to the whole body (including between fingers, under nails and genitals), or the second dose wasn't given a week later.

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Head Lice and Other Infestations

• Head lice (Pediculus capitis) — common in primary school children; spread by head-to-head contact:
  • Itchy scalp (or none); diagnose only when a live louse is found by detection combing (nits — empty egg cases — alone don't confirm active infection).
  • Treatment — wet combing with conditioner and a fine-toothed comb (every 4 days for at least 2 weeks), or a physical insecticide such as dimeticone 4% lotion (2 applications 7 days apart); check family members and treat only those with live lice.
  • No school exclusion.
• Pubic lice ("crabs") — sexually transmitted; itch in the pubic area; also eyelashes; permethrin or malathion; screen for other STIs.
• Body lice — homelessness, poor access to washing; live in clothing seams; transmit trench fever (Bartonella) and typhus; wash clothing and bedding at high temperatures.
• Bed bugs — itchy bites in lines or clusters on exposed skin ("breakfast, lunch and dinner"), blood spots on sheets; professional pest control.
• Flea bites — itchy papules on the lower legs; treat pets and the home.
• Cutaneous larva migrans — itchy, wandering red track (serpiginous) on the feet or buttocks after walking barefoot on tropical beaches (dog hookworm larvae); oral ivermectin or albendazole.
• Myiasis — fly larvae in the skin (boil-like lesion with a central pore, sensation of movement) after travel to the tropics; occlude and remove.
• Delusional infestation — fixed belief of being infested despite no evidence (brings "specimens" — the "matchbox sign"); consider drugs (stimulants) and psychiatric illness; empathetic approach, antipsychotics (see the Psychosis chapter).

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Drug Reference Table

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Spot the Diagnosis

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Exam-style Scenarios

1. A 62-year-old man with type 2 diabetes has a hot, red, swollen left lower leg for 2 days. Temperature 38.1 °C, heart rate 96, BP normal. He has cracked skin between his toes.
Answer: Cellulitis (Eron class II) with tinea pedis as the entry point.

• Oral flucloxacillin for 5–7 days (consider IV if worsening); mark the edge; elevate the leg.
• Treat the tinea pedis (terbinafine cream).
• Review in 48–72 hours; exclude DVT and abscess if not improving.

2. A 40-year-old woman has severe pain in her thigh 12 hours after a minor cut. There's only mild redness, but she is tachycardic, hypotensive and confused.
Answer: Suspected necrotising fasciitis with septic shock.

• Sepsis bundle: IV fluids, blood cultures, lactate; IV broad-spectrum antibiotics with clindamycin.
• Immediate surgical referral for exploration and debridement.
• ICU; IVIG if streptococcal toxic shock; notify if invasive group A strep.

3. A 4-year-old has a few patches of golden crusting around her mouth. She is otherwise well.
Answer: Localised non-bullous impetigo.

• Hydrogen peroxide 1% cream for 5 days (topical fusidic acid if unsuitable).
• Hygiene advice; don't share towels.
• Stay off nursery until lesions crust and heal, or 48 hours after starting antibiotics.