Parasitic infections in children: Viva

Branch 1: Recognising the diagnosis and its clinical pattern

The candidate should immediately recognise this as scabies based on the classic clinical triad of intense pruritus worse at night, burrows in the characteristic distribution of finger webs and wrists, and a household contact with the same itch. The mite Sarcoptes scabiei burrows into the stratum corneum, and the pruritus is a type four hypersensitivity response to mite products that typically develops two to six weeks after first infestation, though reinfestation provokes symptoms within days. ^[1]

The candidate should describe the distribution that distinguishes scabies from other pruritic conditions. In older children and adults, the burrows and papules favour the finger webs, flexor surfaces of the wrists, axillae, waistline, and genitalia. In infants and young children, the distribution is broader and includes the palms, soles, head, and neck, which is a key difference from the adult pattern. The presence of a household contact with itch, the nocturnal predominance, and the characteristic burrows make the diagnosis clinical, and a skin scraping showing mites, eggs, or scybala confirms it when there is doubt. ^[1]

The examiner will probe the differential diagnosis. The candidate should distinguish scabies from atopic dermatitis, which favours the flexural creases of elbows and knees and is associated with a personal or family atopic history; from contact dermatitis, which follows a pattern related to the offending agent; and from papular urticaria from insect bites. The keys to scabies are the household contact, the nocturnal predominance, and the burrows in finger webs. ^[2]