Scabies, lice and infestations — short-answer questions

Question 1 (10 marks)

Describe the clinical diagnosis, the underlying pathophysiology of the itch, and the stepwise management of this child and her family. In your answer address the 2020 International Alliance for the Control of Scabies diagnostic category that applies. [1]

Question 1 — model answer

The clinical picture is classic scabies: a nocturnal itch that worsens after warmth, serpiginous burrows in the finger-webs and wrists, excoriated papules in the waist and genital distribution, secondary impetiginisation, and simultaneous onset in multiple household members. Under the 2020 IACS consensus criteria this meets clinical scabies, because she has typical lesions in a typical distribution together with itch and a clear epidemiological link to affected contacts, so treatment should not wait for a scraping. [1]

The itch is not caused by the burrowing itself but by a delayed type four hypersensitivity to mite faeces, eggs and salivary antigens. In a first infestation, sensitisation takes two to six weeks to develop, which explains why the whole family became itchy only after weeks of silent infestation; on re-infestation the established immune memory brings itch back 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. [3]

Management begins with permethrin five percent cream applied to the whole body from the neck down — in her baby brother the head and neck are included — left on overnight for eight to fourteen hours, washed off, and repeated after seven to fourteen days to kill mites hatching from surviving eggs. The decisive step is to treat every household and close contact on the same day, because the two-to-six-week incubation means the asymptomatic contacts are already infested. Linen and clothing used in the previous two to three days are washed in a hot machine cycle and unwashable items sealed in a bag for at least seventy-two hours. The honey-coloured crusting is treated with an antistaphylococcal and antistreptococcal agent, and the family is warned that post-scabies itch may persist for weeks and is not treatment failure. [3]

Because the family lives in a remote community with crowded housing, the clinician should also screen for secondary streptococcal skin disease and its renal complication: scabies infestation sharply increases the risk of skin sores, and acute post-streptococcal glomerulonephritis presents with haematuria, oedema and hypertension, so a urinalysis and blood pressure check are part of the assessment. Where scabies is endemic, referral to a community-based Healthy Skin programme and consideration of mass drug administration with ivermectin address the household and community transmission that individual treatment alone cannot break. [6]

Question 2 (10 marks)

A separate 8-year-old boy has an itchy scalp with cervical lymphadenopathy. His mother reports that over-the-counter permethrin head-lice lotion failed twice. Discuss the diagnosis of active head lice, the role of resistance, and a resistance-aware management plan including follow-up. [3]

Question 2 — model answer

The diagnosis of active head lice rests on finding a live louse or nymph rather than on the presence of nits, because empty egg cases persist for weeks after successful treatment. Wet combing with conditioner applied to damp hair, using a fine-toothed comb systematically over the whole scalp, detects live lice far more reliably than dry inspection. Nits glued more than a centimetre from the scalp are usually empty or hatched and do not by themselves prove active infestation. [3]

The two failures of over-the-counter permethrin lotion in this child point toward pyrethroid resistance, which is now widespread in head louse populations and is mediated by sodium-channel mutations that reduce susceptibility to neurotoxic agents. Resistance explains why permethrin one percent and related pyrethroids no longer work in many regions, and it shifts first-line therapy toward agents that act by physical rather than neurological means, to which resistance does not develop. [3]

A resistance-aware plan uses dimethicone four percent lotion as first line, a physical agent that coats and asphyxiates the louse with no neurotoxic resistance mechanism, supported by a randomised trial showing high efficacy. Alternatively, or where dimethicone is unavailable, systematic wet combing with conditioner every three to four days for two weeks physically removes lice and nits. Other agents include benzyl alcohol five percent lotion, spinosad and topical ivermectin 0.5 percent lotion. Any agent that is not fully ovicidal must be repeated after seven days to kill nymphs hatching from surviving eggs. [3]

Follow-up reviews the scalp at seven days and again at two weeks to confirm clearance, because re-infestation from an untreated contact is the commonest cause of apparent recurrence. All affected household members are treated, the family is shown correct application technique, and the child can return to school after the first treatment; excluding children with nits is not evidence-based. [3]