Viral Exanthem

Viral exanthem is a skin eruption (rash) accompanying a viral infection, usually in children, caused by the virus itself, the host immune response, or both. Most are self-limiting and benign.

• Common patterns and causes:
  • Maculopapular (morbilliform): measles (rubeola), rubella, enteroviruses (echovirus, coxsackie), adenovirus, human herpesvirus 6 (roseola), parvovirus B19 (erythema infectiosum), Epstein-Barr virus, cytomegalovirus, dengue, chikungunya, Zika, SARS-CoV-2, human herpesvirus 7.
  • Vesicular: varicella zoster, herpes simplex, hand, foot and mouth disease (coxsackie A16, enterovirus 71), eczema herpeticum.
  • Papular purpuric: gloves and socks syndrome (parvovirus B19, other viruses).
  • Petechial or purpuric: enteroviruses, dengue, meningococcal disease (a bacterial mimic), Kawasaki disease (non-viral mimic).
  • Papulovesicular acral eruption: Gianotti-Crosti syndrome (hepatitis B, Epstein-Barr virus).
• Key task: separate benign viral exanthems from serious diseases that cause rash with fever: meningococcal septicaemia, measles, Kawasaki disease, scarlet fever (Group A streptococcus), staphylococcal scalded skin and toxic shock, drug reactions, rickettsial infection, malaria, and syphilis.
• Epidemiology: peak in spring and summer for enteroviruses, winter and spring for others; outbreaks in unvaccinated populations (measles).

• Preceding or accompanying fever, malaise, coryza, conjunctivitis, sore throat, cough, diarrhoea, lymphadenopathy; rash pink to red macules and papules that blanch, starting on the face or trunk and spreading; sometimes pruritic.
• Distinguishing clues:
  • Measles: high fever, cough, coryza, conjunctivitis, Koplik spots, rash begins on the face and spreads downward on day 3 to 4.
  • Rubella: mild fever, postauricular and occipital lymphadenopathy, rash lasting 3 days.
  • Roseola: high fever for 3 to 5 days, rash appears as fever resolves.
  • Erythema infectiosum: bright "slapped cheek" followed by lacy rash.
  • Enteroviral: summer, mouth ulcers, hand and foot lesions.
  • Varicella: crops of vesicles in different stages, centripetal.
• Red flags: non-blanching petechiae or purpura with fever (meningococcaemia, sepsis); rapidly spreading rash with toxic appearance; hypotension; mucosal involvement and blistering (Stevens-Johnson syndrome); fever for 5 days or more with conjunctivitis, cracked lips, strawberry tongue, extremity swelling and cervical node (Kawasaki); sandpaper rash with sore throat (scarlet fever); fever with rash in a returning traveller or immunocompromised child; rash in a pregnant woman or exposure of a pregnant contact; neonatal rash; stridor, respiratory distress, severe dehydration; new drug exposure.

• Clinical diagnosis from the pattern, timing and epidemiology; immunisation record and exposure history (measles, rubella, varicella, parvovirus).
• Investigations only if diagnosis is uncertain, the child is unwell, or to protect contacts:
  • Measles: IgM serology and throat or nasopharyngeal swab PCR (notifiable; do not delay public health notification).
  • Rubella and parvovirus B19: IgM serology, PCR (pregnant contacts: seek antenatal advice urgently).
  • Enterovirus PCR; EBV serology; HHV-6 PCR; dengue NS1 antigen or IgM/IgG; malaria rapid test and smear in endemic areas; throat swab for Group A streptococcus (scarlet fever) or rapid antigen test.
  • Full blood count, CRP, blood culture, electrolytes, urinalysis, liver function if unwell; coagulation screen for petechiae; lumbar puncture for suspected meningitis.

• Supportive care for benign viral exanthems: fluids, rest, paracetamol 15 mg/kg orally every 4 to 6 hours (maximum 4 doses in 24 hours), or ibuprofen 5 to 10 mg/kg orally every 6 to 8 hours (avoid in varicella, dehydration, asthma exacerbation); emollients and calamine lotion or cool baths for itch; short nails; cetirizine 2.5 mg twice daily (1 to 5 years) or 5 mg daily (6 to 11 years) for troublesome itch.
• Antibiotics are not indicated for viral exanthems; treat bacterial mimics promptly.
• Measles: vitamin A 200,000 IU orally (children 12 months and older), 100,000 IU (6 to 11 months), 50,000 IU (under 6 months) once daily for 2 consecutive days, with a third dose at 2 to 4 weeks if ocular signs of vitamin A deficiency; manage complications (otitis media, pneumonia, diarrhoea, encephalitis); isolate until 4 days after the rash starts; notify public health; vaccinate unimmunised contacts within 72 hours (MMR), or give immunoglobulin to high risk contacts within 6 days.
• Varicella: aciclovir 20 mg/kg orally four times daily (maximum 800 mg per dose) for 5 days for children over 12 years, immunocompromised children, or those with severe disease or chronic skin and lung conditions, started within 24 hours of the rash; intravenous aciclovir 10 mg/kg every 8 hours for immunocompromised children; avoid aspirin and NSAIDs; exclude from school until all lesions crusted (usually 5 days).
• Parvovirus B19: supportive; advise pregnant contacts to contact their obstetrician; risk of aplastic crisis in sickle cell disease and hereditary spherocytosis.
• Rubella: supportive; avoid contact with pregnant women; notify.
• Hand, foot and mouth: analgesia, fluids, oral hygiene.
• Meningococcal disease suspected (non-blanching rash with fever or ill appearance): give benzylpenicillin 300 mg (under 1 year), 600 mg (1 to 9 years), 1.2 g (10 years and over) intramuscularly or intravenously, or ceftriaxone 80 mg/kg (maximum 2 g) intravenously, and transfer to hospital immediately.
• Kawasaki disease suspected: immediate paediatric referral (intravenous immunoglobulin 2 g/kg and aspirin 30 to 50 mg/kg/day within 10 days of fever onset).
• School exclusion: according to the specific infection; most viral exanthems with no fever need no exclusion.
• Safety netting: return if the rash becomes non-blanching, breathing becomes fast or difficult, the child is drowsy or floppy, drinks poorly, passes little urine, develops a stiff neck, fits, or fever persists for 5 days.