Impetigo and bacterial skin infection — formative SAQs

SAQ 1 (10 marks)

A 14-month-old infant is brought to the emergency department with a two-day history of flaccid, translucent bullae on the trunk and neck, several of which have ruptured to leave a thin collarette of scale and a moist base. The infant is febrile to 38.6 degrees Celsius, irritable, and has widespread, tender erythema extending beyond the bullae, with wrinkling of the skin around the neck that shears with gentle wiping. [8]

Question: Outline the diagnosis, immediate management, and the stepwise treatment of this infant, including how you would distinguish this from other blistering disorders. (10 marks) [8]

Model answer

Diagnosis and recognition (2 marks). The most likely diagnosis is staphylococcal scalded skin syndrome, the circulating-toxin extreme of the same exfoliative-toxin mechanism that causes bullous impetigo. The widespread tender erythema, flaccid bullae, sheet-like desquamation with a positive Nikolsky sign, fever and irritability in an infant are characteristic. This is a hospital emergency: admit for intravenous therapy and supportive care. [8]

Immediate management and resuscitation (2 marks). Admit, establish intravenous access, and treat as a burn: start intravenous anti-staphylococcal therapy with flucloxacillin (vancomycin or clindamycin if the child is critical or MRSA is likely), provide meticulous fluid, electrolyte and temperature management, apply non-adherent wound care and adequate analgesia, and monitor for secondary infection and heat and fluid loss. Identify and treat the primary staphylococcal focus, often a conjunctival, umbilical or skin site. [8]

Distinguishing the blistering disorders (2 marks). The decisive features are the spared mucous membranes and the superficial split, which separate staphylococcal scalded skin syndrome from Stevens-Johnson syndrome and toxic epidermal necrolysis (which involve the mucosae and follow a drug exposure) and from toxic shock syndrome. Mucosae are spared because mucosal epithelium co-expresses desmoglein-3, which compensates when desmoglein-1 is cleaved by the exfoliative toxin. A skin biopsy is reserved for the genuinely ambiguous case. [8]

Stepwise treatment of the broader disease (2 marks). For bullous impetigo in a well child, an oral anti-staphylococcal antibiotic such as flucloxacillin for five days is first-line; for the toxin-mediated extreme here, intravenous therapy and burns-style care are required. State the principle: localised toxin gives bullous impetigo, circulating toxin gives staphylococcal scalded skin syndrome, and the severity of the child dictates the setting. [8]

Disposition and safety-netting (2 marks). Admit the infant to a monitored bed, continue supportive care and intravenous therapy, and de-escalate to oral therapy as the child recovers. On discharge, give a clear safety-net for spreading redness, fever, lethargy and poor feeding, and reinforce skin care. The outcome in children is generally good with prompt treatment, in contrast to the high mortality in adults. [8]

SAQ 2 (10 marks)

Question: A 4-year-old child in a remote Aboriginal community presents with multiple crusted, weeping sores on the limbs of several weeks' duration, and a sibling has similar lesions. There is also clinical scabies. (a) What is the diagnosis and how is it confirmed? (b) Outline the treatment and public-health management. (c) What complication must you screen for over the following weeks, and what key distinction guides whether rheumatic-fever prophylaxis is needed? (10 marks) [2]

Model answer

(a) Diagnosis and confirmation (3 marks). The likely diagnosis is non-bullous impetigo (pyoderma), endemic in this setting and likely streptococcal given the remote-community context, complicated by scabies as the portal of entry. The diagnosis is clinical — the honey-coloured crust on an erythematous base is characteristic. Swab the active lesion only because the disease is recurrent and in a high-endemic setting, to identify the organism and sensitivities; do not delay treatment while waiting for the result. [2] [3]

(b) Treatment and public-health management (4 marks). In a remote endemic community, the National Healthy Skin Guideline approach supports a short three-day oral course of co-trimoxazole (trimethoprim 8 mg/kg/day in two divided doses) or a single weight-based intramuscular dose of benzathine benzylpenicillin for streptococcal impetigo, with the non-inferiority trial evidence supporting oral co-trimoxazole as a practical alternative to injection. Treat the scabies concurrently, because scabies is the portal of entry driving recurrence. Screen and treat household contacts, exclude from school or childcare until lesions are dry or healed (or 24 hours into effective treatment), and address environmental determinants such as crowding and water access. [3] [2]

(c) The complication and the key distinction (3 marks). Screen for post-streptococcal glomerulonephritis, which follows skin infection with nephritogenic group A streptococcal strains one to three weeks later, presenting with haematuria (tea- or cola-coloured urine), oedema, hypertension and oliguria. Check blood pressure, urinalysis, renal function, complement C3 and streptococcal serology if the child presents with these features. The key distinction is that skin-derived group A streptococcal infection is nephritogenic but not rheumatogenic: impetigo can cause glomerulonephritis but not acute rheumatic fever, which follows throat infection alone, so a child with impetigo does not need rheumatic-fever prophylaxis. [2]