Scabies, lice and infestations — viva

Opening (2 minutes)

The candidate should recognise the classic picture of scabies: a nocturnal itch out of proportion to the visible lesions, serpiginous burrows in the finger-webs and wrists, the characteristic distribution around the waist and genitalia, secondary impetiginisation, and simultaneous onset across several household members. The diagnosis is clinical, and the candidate should commit to it and to excluding the common mimics rather than waiting for a scraping in an obvious case. [1]

Branch 1 — diagnosis and pathophysiology

Examiner: "How do you confirm this diagnosis and what causes the itch?" The expected answer applies the 2020 International Alliance for the Control of Scabies consensus criteria: this child meets clinical scabies, defined by typical lesions in a typical distribution together with itch and a clear epidemiological link to affected contacts. Confirmed scabies would require mites, eggs or faecal pellets on scraping, dermoscopy or polymerase chain reaction, but clinical criteria are sufficient to treat. [1]

The itch is a delayed type four hypersensitivity to mite faeces, eggs and saliva rather than to the act of burrowing. In a first infestation, sensitisation takes two to six weeks, which is why the family became itchy only after weeks of silent infestation and why asymptomatic contacts are already infested; on re-infestation the itch returns within one to four days. The fertilised female mite burrows into the stratum corneum and lays two to three eggs a day, surviving one to two months on the host. A strong candidate names the organism — Sarcoptes scabiei var hominis — and links the incubation period to the need to treat contacts. [3]

Branch 2 — differential diagnosis and complications

Examiner: "What else could this be, and what complications should you look for?" The candidate should distinguish scabies from atopic dermatitis, which favours the flexures and cheeks and spares the finger-webs and groin; from contact dermatitis, whose distribution matches a contactant; and from papular urticaria and arthropod bites, which are episodic and lack burrows. The honey-coloured crusting is secondary impetigo, often streptococcal or staphylococcal, rather than a separate diagnosis. [3]

The complications to seek are secondary bacterial infection and, in an endemic or remote setting, its renal and rheumatic consequences. Scabies infestation sharply increases the risk of streptococcal skin sores, and these in turn drive acute post-streptococcal glomerulonephritis and contribute to acute rheumatic fever risk. A candidate who examines for fever, checks the urine for haematuria and measures the blood pressure demonstrates the skin-to-kidney awareness that marks a higher-level answer. [6]

Branch 3 — management and the household principle

Examiner: "Lay out your management plan for the child and the family." The plan rests on permethrin five percent cream applied to the whole body from the neck down — in this three-year-old the head and neck are included because toddlers carry lesions on the scalp and face — left on overnight for eight to fourteen hours, washed off, and repeated after seven to fourteen days. The critical step, which the candidate must name unprompted, is to treat every household and close contact on the same day, whether or not they itch, because the incubation period means they are already infested. [3]

Oral ivermectin at 200 micrograms per kilogram, repeated after seven to fourteen days, is the alternative when topical therapy is impractical or in crusted disease, and is avoided in small children weighing less than fifteen kilograms and in pregnancy. Linen and clothing used in the previous two to three days are washed in a hot machine cycle and unwashable items sealed for at least seventy-two hours. The candidate should close by reassuring the family that post-scabies itch may persist for several weeks and is not evidence of treatment failure, and by arranging review at two weeks. [3]

Closing (1 minute)

Summarise the plan: confirm the clinical diagnosis under the 2020 IACS criteria, treat the child and every contact on the same day with permethrin five percent cream applied correctly and repeated, launder linen, treat secondary impetigo, check for renal complications in an endemic setting, and review at two weeks while counselling that persistent itch reflects clearing antigen rather than living mites. [1]