Viral Rashes

A reference compilation of common childhood exanthems and related viral/ post-viral rashes. See also scarlet fever / staphylococcal scalded skin syndrome and acute rheumatic fever (a non-suppurative complication of the same group A strep that causes scarlet fever, below).

Chicken pox (varicella)

Generally benign, but death can occur from viral pneumonia/fulminant infection in the immunocompromised or neonates (rarely in others). Incubation 10–21 days; much less common now (vaccination).

• Management: rest until well; paracetamol for fever (avoid aspirin — Reye syndrome risk); calamine lotion/antihistamines for itch.
• Antivirals:
  • Neonates — specialist management, refer to ED.
  • Immunocompetent children, no underlying skin condition — no antivirals needed.
  • Immunocompetent children with significant skin disease (e.g. eczema) — aciclovir 20 mg/kg 5×/day, or valaciclovir 20 mg/kg TDS, for 7 days.
  • Immunocompetent adults — valaciclovir 1 g TDS for 7 days, started within 36 hrs of rash (non-pregnant) or 72 hrs (pregnant).
  • Immunocompromised — IV antivirals.

Measles

Highly contagious, airborne. Can be complicated by otitis media (9%), pneumonia (6%), diarrhoea (8%), encephalitis (0.1%); ~0.1% mortality in children. Subacute sclerosing panencephalitis — a fatal late complication, ~7 years after infection. Incubation 10–14 days.

• Prodrome: 3–4 days fever, malaise, ± diarrhoea, then the “3 Cs” — cough, coryza, conjunctivitis.
• Exanthem: blotchy red maculopapular rash, 4–5 days, starting on the face and spreading down, becoming confluent. Koplik spots (white spots near the molars) appear and disappear early. Recovery 7–10 days after rash onset.
• Investigations: measles IgM serology; nose/throat swab and first-pass urine for PCR.
• Management: supportive; isolate for ≥4 days after rash onset; notify public health. Contacts = anyone in the same room up to 30 min after the case left. Offer vaccination to susceptible contacts; immunoglobulin for infants <6 months and pregnant women.

Note: measles has seen a global (and some Australian) resurgence linked to declining vaccination coverage — isolation and public-health notification remain particularly relevant.

Rubella (German measles)

Mild in children/adults, but causes severe birth defects in the first trimester. Avoid pregnancy until 28 days post-vaccination; can vaccinate immediately postpartum if not previously immunised. ~⅓ asymptomatic; incubation 14–21 days.

• History: no prodrome, absent/low-grade fever, headache, myalgia, conjunctivitis, small-joint polyarthritis.
• Examination: discrete pale pink macules/papules, face → trunk/limbs, sometimes pruritic.
• Investigations: rubella IgM/IgG, throat swab PCR (women of childbearing age and adults).
• Infectious 1 week before to 4 days after rash onset. No specific treatment. Rare complications: arthralgia/arthritis (adults), thrombocytopenic purpura, otitis media, encephalitis.

Viral exanthem (fourth syndrome): a similar but non-pruritic rash, trunk only, fading in 48 hrs — the child is generally well.

Parvovirus B19 (erythema infectiosum / fifth disease / “slapped cheek”)

Mild fever, rhinorrhoea, nausea; arthralgia in adults. Cervical lymphadenopathy. 50% of household and 20% of classroom contacts become infected. No longer infectious by the time the rash appears. Avoid contact with pregnant women (fetal complications/miscarriage risk) — test pregnant contacts (IgG + IgM).

Roseola

Age 6 months–2 years. Sudden high fever (~40°C) for 3 days, rhinorrhoea — rash appears as the fever subsides. Red macular/maculopapular rash on the trunk, sparing the face. Symptomatic management only.

Hand, foot and mouth disease

Incubation 3–5 days; mainly children, can affect adults. Fever, headache, malaise, then a rash progressing from red macule → vesicle on the hands, palms, and soles (especially the lateral border) — can extend to limbs, buttocks, genitals. Infectious until blisters resolve/crust; virus shed in faeces/saliva for several weeks after.

Scarlet fever

Group A strep producing an erythrogenic toxin; age 4–8 years. Malaise, sore throat, fever precede the rash; can complicate impetigo.

• Examination: pinkish-red spots covering the body (“boiled lobster” appearance), sandpaper texture, strawberry-red tongue; desquamation (especially axillae/hands/feet) as the rash resolves.
• Management: phenoxymethylpenicillin BD for 10 days; antihistamines for itch. Can return to school 24 hrs after starting antibiotics if well.

Mumps

Paramyxovirus infecting salivary glands and meninges; ⅓ asymptomatic. Bilateral parotitis in ⅔. Unvaccinated: age 5–9; vaccinated: adolescents/ young adults.

• Fever, headache, malaise, painful (often bilateral) parotid swelling obscuring the jaw angle.
• Exclude from school/work until 9 days after fever onset. Avoid contact with pregnant women (spontaneous abortion risk in the first trimester).
• Complications: orchitis, abdominal pain, aseptic meningitis. Rare: oophoritis, encephalitis, arthritis, pancreatitis.

Oral herpes simplex (HSV-1)

Childhood infection classically causes gingivostomatitis (rare in adults). Painful oral lesions, classically on the lips. PCR for HSV-1/2; most adults are HSV-1 seropositive.

• Primary, minor: topical anaesthetics (Difflam rinse/spray, 2% viscous lidocaine).
• Primary, severe: valaciclovir 1 g BD.
• Recurrent, minor: topical aciclovir 5×/day (4-hourly while awake) for 5 days, or famciclovir 1500 mg PO.
• Recurrent, severe (treat within 48 hrs): famciclovir 1.5 g once, or valaciclovir 2 g 12-hourly for 1 day.
• Suppressive therapy: valaciclovir 500 mg daily for 6 months.

Genital herpes

STI caused by HSV-1/2; recurrent infection is usually HSV-2. Patients with undiagnosed HIV may present with an unusually severe first episode. Painful genital ulcer, may start as a vesicle. Swab for HSV-1/2 PCR. Antiviral therapy shortens the acute episode if started within 72 hrs.

• First episode: valaciclovir 500 mg BD for 10 days (can stop at 5 days if responding rapidly).
• Episodic therapy: valaciclovir 500 mg BD for 3 days, started at prodrome onset.
• Suppressive therapy (if several recurrences/year): valaciclovir 500 mg daily.

Shingles (herpes zoster)

Increasing incidence with age but possible in childhood. Dermatomal blistering rash, erupting over ~1 week then healing over ~2 weeks; immunocompromised patients can get a multi-dermatomal rash with systemic symptoms. PCR for VZV if diagnosis uncertain.

• If rash <72 hrs old: valaciclovir 1 g TDS for 7 days — reduces pain/ rash duration and viral shedding; unclear whether it reduces post-herpetic neuralgia rates.

Pityriasis rosea

Thought secondary to HHV-6/7 reactivation (the roseola viruses). Lasts 6–12 weeks; adolescents/young adults most affected, but any age possible.

• Herald patch (2–5 cm oval pink/red plaque, scale trailing just inside the edge) appears first, sometimes after a URTI prodrome.
• Secondary rash a few days later — smaller oval patches/plaques on chest and back, dry with an inner collarette of scale, following Langer’s lines.
• Management: no specific treatment; moisturisers; usually clears in 6–12 weeks.

EBV ampicillin/amoxicillin rash

Appears 7–10 days after ampicillin/amoxicillin in a patient with EBV infection — a hypersensitivity reaction, not a true allergy; won’t recur on antibiotic re-exposure in the absence of EBV. Widespread, blanching maculopapular rash on extensor surfaces and pressure areas.

Orf

Zoonotic viral infection from direct contact with sheep/goats (e.g. farmers, bottle-feeding lambs/kids) — not transmitted human-to-human. Solitary or few lesions on hands/forearms (can affect the face); a firm nodule develops into a flat-topped blister ± mild fever/lymphadenopathy. No specific treatment; usually resolves spontaneously over 6 weeks. Can be complicated by secondary bacterial infection or secondary erythema multiforme.

Milker’s nodules

Similar to orf but from cow teats (milkers, vets). Incubation 5–14 days, then red raised flat-topped spots progressing to blisters on hands/ fingers (also face). Same natural history and complications as orf.

Primary HIV (acute retroviral syndrome)

Occurs in 40–90% of people in the first weeks after HIV exposure (mostly sexual) — highly infectious at this stage due to high viral load.

• Fever (most common symptom), sore throat, lymphadenopathy, headache, anorexia, nausea/vomiting/diarrhoea; can include meningitis, encephalitis, peripheral neuropathy, myelopathy.
• Dermatological: symmetrical erythematous maculopapular rash (face, palms, soles, trunk, limbs); mucocutaneous ulceration (mouth, oesophagus, penis, anus — painful, only a small proportion of patients).