Red flags
- Non-blanching rash + fever - meningococcaemia until excluded, urgent bloods/antibiotics, do not wait for confirmation
- Mucosal involvement, skin pain out of proportion, or blistering/desquamation - think SJS/TEN or staphylococcal scalded skin, not a simple viral exanthem
- Rash + haemodynamic instability/hypotension - toxic shock syndrome
- Pregnant contact exposed to a rash illness - parvovirus B19 (hydrops fetalis) or rubella (congenital rubella syndrome) risk - urgent serology
- Immunosuppressed host with a vesicular rash - disseminated varicella/zoster risk, treat empirically pending confirmation
Differential by mechanism
Classic childhood exanthems
- Measles - fever, cough/coryza/conjunctivitis, Koplik spots precede rash by ~1-2 days, cephalocaudal spread, maculopapular, confluent
- Rubella - mild fever, posterior auricular/suboccipital lymphadenopathy, pink maculopapular rash, fades quickly ("3-day measles") - teratogenic in pregnancy
- Parvovirus B19 (fifth disease) - "slapped cheek" facial erythema then reticulate/lacy limb rash; arthropathy in adults; aplastic crisis risk in haemolytic disease; hydrops fetalis if maternal infection
- Roseola (HHV-6/7) - high fever then rash on defervescence, infants; febrile seizure risk during the fever phase
- Hand, foot and mouth (Coxsackie A16/EV71) - oral ulcers + vesicles on palms/soles, young children, highly contagious
Vesicular/exanthems with systemic significance
- Varicella (chickenpox) - centripetal, crops of lesions at different stages (papule-vesicle-crust simultaneously)
- Herpes zoster - dermatomal, reactivation not primary infection
- EBV/CMV - maculopapular rash, esp. if given aminopenicillin (amoxicillin) - classic trap, not a true penicillin allergy
Non-viral mimics to exclude
- Meningococcaemia - petechial/purpuric, non-blanching, systemically unwell
- Scarlet fever (group A strep) - sandpaper rash, strawberry tongue, circumoral pallor
- Kawasaki disease - fever >=5 days + rash + conjunctival injection + mucosal changes + extremity changes + cervical lymphadenopathy
- Drug eruption - temporally linked to a new medication, can mimic a viral exanthem closely
Focused history
- Prodrome sequence and timing of rash onset relative to fever (rash-with-fever vs rash-on-defervescence is discriminating)
- Vaccination history (measles/rubella/varicella)
- Sick contacts, childcare/school exposure, recent travel
- Pregnancy status/contacts, immunosuppression
- Any new medication in preceding 2-3 weeks (drug eruption mimic)
Focused examination
- Rash morphology and distribution, evolution pattern (single-stage vs crops at different stages)
- Blanching test - always check non-blanching status with a glass/finger pressure
- Mucosal surfaces (oral, conjunctival, genital) - involvement suggests a more significant process (SJS/TEN, Kawasaki, HFMD)
- Lymphadenopathy pattern (posterior auricular - rubella; cervical - Kawasaki/EBV)
- Signs of systemic illness - hydration, perfusion, level of consciousness
Investigation strategy
- Most classic childhood exanthems are clinical diagnoses - serology/PCR reserved for atypical presentation, immunosuppressed host, pregnancy exposure, or public health notification requirement
- Notifiable diseases (measles, rubella) require urgent public health notification and, where measles suspected, isolation and infection control before confirmation
- Serology (IgM/IgG) or PCR for parvovirus B19, measles, rubella as indicated
- FBE/blood cultures/meningococcal PCR if red flags for sepsis/meningococcaemia present
Management
Most exanthems - supportive
- Antipyretics, fluids, rest; most are self-limiting over days
Specific interventions
- Measles - notify public health, isolate, vitamin A in selected cases (WHO recommendation, esp. developing settings/severe disease), post-exposure prophylaxis (MMR vaccine within 72h or immunoglobulin for high-risk contacts within 6 days)
- Varicella - oral aciclovir if >12 years, immunosuppressed, or severe; VZIG for susceptible high-risk contacts (pregnant, immunosuppressed, neonates) post-exposure
- Parvovirus B19 exposure in pregnancy - refer for serology and monitoring (risk of fetal anaemia/hydrops), fetal medicine input if seroconversion confirmed
- Rubella exposure in pregnancy - urgent serology, specialist referral given congenital rubella syndrome risk if non-immune and infected <20 weeks gestation
Traps
- Amoxicillin given for presumed bacterial tonsillitis in EBV infectious mononucleosis produces a widespread rash that is not a true penicillin allergy - do not label it as one
- Missing meningococcaemia because an early petechial/purpuric rash is dismissed as "probably viral" - always check for non-blanching lesions
- Assuming a rash-with-fever pattern is roseola when the child is systemically unwell - roseola children are typically well between fever spikes
- Forgetting pregnancy screening questions when assessing any patient (or household contact) with a viral exanthem
Talk track
1. Blanch it before you reassure
- "I always press on the rash before calling it viral - a non-blanching petechial rash with fever is meningococcaemia until proven otherwise."
2. Ask who else lives in the house
- "A viral exanthem isn't just about this patient - pregnancy exposure and immunosuppressed contacts change the urgency completely."
8 of 8 sections written · drafted 2026-09-14