Viral exanthems

Meet the patient

A 10-month-old has had three days of fever to 39.5 degrees and has, against all expectation, kept playing between the spikes; this morning the fever has gone and a faint pink rash has bloomed across his trunk. His mother is terrified; the registrar is reaching for antibiotics. The consultant's first sentence is the one that settles it: "The rash appearing as the fever falls, in a well child, is the diagnosis, not a complication."[5][6]

Hold that vignette, because it captures the single highest-yield skill in the topic — reading the rash against the fever. Roseola's rash is recovery; measles' rash marks the child at their worst; varicella's crops ride the fever up and down. Pair that skill with two obligations — protect the at-risk groups (pregnancy, immunocompromise, haemolytic anaemia, the neonate) and never miss a dangerous mimic — and the topic is yours.[1][10]

The numbered exanthems — a historical scaffold that still earns marks

From the sixteenth to the twentieth century, clinicians numbered the rash illnesses of childhood. The original six classic exanthems were measles (first), scarlet fever (second), rubella (third), the now-disputed Filatov-Dukes disease (fourth), erythema infectiosum (fifth, parvovirus B19), and roseola infantum (sixth, HHV-6). Scarlet fever is bacterial and has its own topic, but examiners still reference the numbering.[1][9]

Today the working catalogue adds the non-numbered exanthems — varicella, mumps, hand-foot-and-mouth disease, infectious mononucleosis and Gianotti-Crosti syndrome — and the clinician's job at the bedside is to sort the eruption by morphology and tempo into the right shortlist.[1][15]

Fever against rash — the discriminator that earns marks

The timing of the rash relative to the fever is the single most useful clinical clue, and examiners test it relentlessly. Two structural principles organise recall: morphology tracks mechanism (a maculopapular rash is immune-mediated, a true vesicle is direct viral cytopathic effect), and timing tracks the immune response.[5][12]

[1] [5]

Why the rash happens — mostly you, not the virus

Most viral exanthems share one cascade: mucosal entry, primary replication, primary viraemia to the reticuloendothelial system, then a larger secondary viraemia that seeds the skin. The visible rash is largely a host immune response — cytotoxic T cells and immune complexes attacking virus-laden dermal endothelium — not the virus eating the skin.[1][9]

The exceptions are the vesicular diseases. In varicella and the enteroviruses, the virus itself replicates in keratinocytes, causing ballooning degeneration and a true intraepidermal vesicle — which is why these lesions run the papule-to-vesicle-to-pustule-to-crust course. The timing trick follows directly: in roseola the rash is the immune response clearing virus (so the child is better when it appears), while in measles the prodromal viraemia has been building for days (so the child is at their worst).[5][12]

The diseases — each in one discriminating breath

Measles (rubeola, first disease)

The 3 Cs plus Koplik spots plus a cephalocaudal rash. Three to four days of fever, cough, coryza and conjunctivitis precede the rash, and Koplik spots — pathognomonic bluish-white specks on an erythematous buccal base, "grains of salt on a red background", opposite the lower molars — appear one to two days before the rash. The eruption begins behind the ears and hairline on day 4, marches cephalocaudally, becomes confluent, then desquamates. Measles is among the most contagious infections known, with an R0 of 12 to 18, and the virus lingers in a room's air for up to two hours.[1][13]

Rubella (German measles, third disease)

A mild illness defined by lymphadenopathy, dangerous only in pregnancy. A brief prodrome is followed by postauricular, suboccipital and cervical lymphadenopathy and a fine pink maculopapular rash spreading cephalocaudally over one to three days; the child is comparatively well. Forchheimer spots on the soft palate support but do not confirm it. The clinical danger lies entirely in the first trimester, where congenital rubella syndrome follows.[2]

Erythema infectiosum (fifth disease) — parvovirus B19

The slapped cheek, then the lacy rash — and the child is no longer infectious once the rash appears. Bright red confluent cheek erythema with circumoral sparing is followed one to four days later by a reticulated (lacy) rash on the trunk and extensors that recurs for weeks with heat, sunlight, exercise or bathing. Two at-risk presentations dominate stems: arthropathy in up to 60 percent of adult women, and transient aplastic crisis in chronic haemolytic anaemia.[3][4]

Roseola infantum (sixth disease) — human herpesvirus 6

High fever in a well infant, then rash as the fever drops. Three to five days of fever, often 39 to 40 degrees, in a child who looks remarkably well between spikes, then abrupt defervescence as a rose-pink maculopapular rash appears on the trunk. The age range is six months to three years, and it is the commonest cause of febrile seizures under two years; HHV-6 (less often HHV-7) is the cause.[5][6]

Varicella (chickenpox) — varicella-zoster virus

Vesicles in successive crops — papules, vesicles, pustules and crusts all present at once. The rash begins on the face, scalp and trunk and spreads centripetally, also involving mucosa; new crops appear for three to five days and the child is infectious until every lesion has crusted (around five to seven days after the last new vesicle). Incubation is 14 to 21 days. Herpes zoster is the later reactivation of latent VZV in a dermatome.[12]

Mumps — paramyxovirus

Fever and painful parotitis, with the ear lobe pushed up and out. Usually bilateral, often starting on one side; the submandibular glands may join. Orchitis affects about 20 to 30 percent of postpubertal males (rarely causing infertility), aseptic meningitis is common (1 to 10 percent, usually mild), pancreatitis can occur, and sensorineural deafness is the classical rare complication. Mumps is prevented by MMR and is notifiable.[14]

Hand-foot-and-mouth disease — coxsackievirus A16, enterovirus 71

Painful oral ulcers plus acral vesicles in a young child. Oval grey-white vesicles on an erythematous base, aligned with skin lines, sit on the hands, feet and buttocks alongside painful mouth ulcers that cause drooling and refusal to drink. Most cases are mild, but enterovirus 71 strains carry neurological complications — meningitis, brainstem encephalitis, acute flaccid paralysis — and neurogenic pulmonary oedema, especially in young children in the Asia-Pacific region.[7][8]

Infectious mononucleosis — Epstein-Barr virus

Fever, pharyngitis and lymphadenopathy, with splenomegaly in about half — and the amoxicillin rash is the exam favourite. A maculopapular rash appears in a minority untreated, but ampicillin or amoxicillin given during the illness triggers a dramatic morbilliform rash in roughly 90 percent. This is not a true penicillin allergy — it reflects polyclonal T-cell activation by EBV — and patients usually tolerate penicillins later. Counsel against contact sports for three to four weeks because of splenic rupture.[16]

Gianotti-Crosti syndrome (papular acrodermatitis of childhood)

Symmetrical monomorphic flat-topped papules on the face, buttocks and extensors — characteristically sparing the trunk. It affects children from six months to twelve years, lasts two to eight weeks, and is a paraviral reaction to many viruses — historically hepatitis B, now more often EBV, CMV, coxsackievirus, parainfluenza and RSV — and may follow vaccination. In endemic regions, the distinctive distribution still warrants a hepatitis screen.[15]

The dangerous mimics — exclude these before you settle on "viral"

The highest-yield exam task is to separate the benign self-limiting exanthem from its killers. The single commonest mimic is the drug eruption, and the single most dangerous is meningococcaemia; both must be actively excluded in every febrile child with a new rash.[9][10]

The central bedside distinction — viral exanthem versus drug eruption — deserves its own framework, because it is asked repeatedly. A drug eruption is more pruritic, lacks the characteristic viral prodrome, appears days after exposure, and often shows eosinophilia; a viral exanthem has a recognisable prodrome and morphology, is less itchy, and follows the characteristic course of the virus. The exposure history is decisive.[10]

The bedside round — morphology, mucosa and nodes

The assessment is morphology-driven: a focused full-skin and mucosal examination usually yields the diagnosis before any test is sent. Read the rash for morphology (macular, papular, vesicular, petechial), distribution (cephalocaudal versus centripetal versus acral) and evolution (successive crops versus a single wave).[9]

Examine the mucosa and the nodes. Koplik spots point to measles; Forchheimer spots and palatal petechiae to rubella and scarlet fever respectively; oral vesicles and ulcers to HFMD and herpangina; a strawberry tongue to scarlet fever and Kawasaki. Postauricular and suboccipital nodes suggest rubella; tender cervical adenopathy suggests EBV and group A strep; a unilateral large cervical node is a Kawasaki criterion. Always take a drug history (the mimic), a vaccination history (MMR, varicella), a pregnancy status in any woman of reproductive age with a maculopapular rash, and a travel and contact history.[9][10]

Investigations — clinical first, confirm when it changes management

The diagnosis is clinical in classic, uncomplicated cases, and that is the expected board answer for most stems. Laboratory confirmation is reserved for the atypical case, the pregnant woman, the immunocompromised patient, the severe case, the outbreak requiring public-health action, and the notifiable disease.[1][3]

Notifiable diseases must be reported to public health without waiting for confirmation — measles, rubella (and congenital rubella syndrome) and mumps are notifiable in most jurisdictions, and public-health teams arrange confirmation, contact tracing and post-exposure prophylaxis.[1][14]

Management — mostly supportive, with a few specific interventions

Most viral exanthems need no resuscitation and no specific therapy — reassurance, antipyretics and hydration is the expected answer for a well child with roseola or fifth disease. Escalation is reserved for the complications that turn a benign illness into an emergency: measles pneumonia or encephalitis, varicella pneumonia (especially in pregnancy and adults), parvovirus aplastic crisis, HFMD neurological complications, febrile seizures in roseola, and the dangerous mimics.[1]

[1] [12]

Two principles recur across the whole topic and are heavily tested. First, aspirin must be avoided in children with acute viral illness — especially varicella and influenza — because of Reye syndrome, a hepatic encephalopathy with microvesicular fatty change; use paracetamol or ibuprofen instead. Second, vaccination prevents most of these diseases: the MMR vaccine in two doses and the varicella vaccine where routinely used have transformed the epidemiology of childhood exanthems.[12][17]

The scenarios that bite — pregnancy, the neonate, and haemolytic anaemia

Congenital rubella syndrome follows first-trimester infection: the classic tetrad is cataracts, sensorineural deafness (the commonest single manifestation), cardiac defects (especially PDA and peripheral pulmonary artery stenosis), and thrombocytopenia with a blueberry-muffin rash of dermal extramedullary haematopoiesis. Prevention is universal MMR plus antenatal screening.[2]

Parvovirus B19 in pregnancy carries roughly a 5 to 10 percent risk of fetal hydrops when acquired in the first and second trimesters, by inhibiting fetal erythropoiesis and causing high-output cardiac failure; risk peaks around 10 to 20 weeks. Manage with maternal serology (IgM and IgG with avidity), serial fetal ultrasound and middle cerebral artery Doppler for anaemia, and referral for intrauterine transfusion when fetal anaemia is confirmed.[4]

Varicella in pregnancy and the neonate risks maternal pneumonia (the leading cause of maternal death from varicella), congenital varicella syndrome (limb hypoplasia, cutaneous scarring, eye abnormalities, cortical atrophy) in the first and second trimester, and neonatal varicella when maternal infection falls from 5 days before to 2 days after delivery — the baby receives virus without antibody. Give VZIG to the neonate in that window and IV aciclovir if disease develops.[12]

Transient aplastic crisis strikes the child with sickle cell disease, hereditary spherocytosis or another chronic haemolytic anaemia: parvovirus B19, tropic for erythroid progenitors, drops the haemoglobin with profound reticulocytopenia. The child remains infectious — transfuse until the crisis resolves, and isolate from other haemolytic and pregnant patients.[4]

How febrile children with a rash come to harm — the preventable list

• Diagnosing viral exanthem in early meningococcaemia or Kawasaki — the preventable disaster.[9][11]
• Giving aspirin to a febrile child with varicella or influenza and precipitating Reye syndrome.[12]
• Failing to screen pregnancy status in a woman with a maculopapular rash, missing congenital rubella or parvovirus hydrops.[2][4]
• Mislabelling the amoxicillin-mono rash as penicillin allergy, depriving the patient of a first-line antibiotic class for life.[16]
• Missing varicella pneumonia in pregnancy or the immunocompromised, where oral or IV aciclovir changes the outcome.[12]
• Missing enterovirus 71 neurological complications in HFMD, where supportive monitoring in PICU is life-saving.[7][8]

Prognosis and disposition

Most viral exanthems in immunocompetent children are self-limiting with full recovery, and the disposition for an uncomplicated case is home with safety-net advice — return for breathing difficulty, drowsiness, petechiae, dehydration or persistent fever. The numbers that matter for vivas: measles case fatality 1 to 3 per 1000 in developed settings; untreated parvovirus fetal hydrops mortality up to 50 percent; varicella pneumonia mortality 10 to 30 percent in pregnancy and adults.[1][4]

Disposition is home for uncomplicated cases, hospital for dehydration, secondary bacterial infection, pneumonia, encephalitis, aplastic crisis and severe HFMD, and PICU or HDU for varicella pneumonia in pregnancy, EV-71 neurological disease and measles pneumonia with respiratory failure. Pregnant contacts of measles, rubella, parvovirus B19 and varicella need urgent serological assessment and fetal-medicine input.[1][12]

Special populations

Evidence, guidelines and regional differences

MMR vaccination schedules are the backbone of prevention. The US (CDC/ACIP) gives two MMR doses at 12 to 15 months and 4 to 6 years with routine two-dose varicella vaccination at the same visits. The UK (Green Book/JCVI) historically gave MMR at 12 to 13 months and a preschool booster at 3 years and 4 months, with varicella vaccine reserved for susceptible contacts. The WHO drives global measles-rubella elimination, and India (ICMR/IAP) runs MR catch-up campaigns.[1][17]

The WHO recommends vitamin A for all children with measles in two age-appropriate doses, repeated the next day and at 4 weeks, on mortality reduction shown in vitamin-A-deficient populations. Post-exposure prophylaxis — MMR within 72 hours of measles exposure, HNIG within 6 days for susceptible pregnant and immunocompromised contacts, and VZIG for varicella-susceptible high-risk contacts — is codified in the UK Green Book, CDC guidance and national immunisation manuals.[1][13]

The mantra, and the mnemonic

[1]

The mantra: read the rash against the fever, screen the pregnancy, never give aspirin — and exclude meningococcaemia and Kawasaki before you call it a virus.[1][9]

Ward-round test — three stems, thirty seconds each

Stem 1 — the well febrile infant from the top of the topic (answer)

A 10-month-old has three days of fever to 39.5 degrees but plays between spikes; today the fever has gone and a faint pink rash covers the trunk. What is this, and what do you do? Model: Roseola infantum (HHV-6, sixth disease). The diagnostic clue is the rash appearing as the fever defervesces in a well-looking infant — the classic fever-then-rash sequence, and the commonest cause of febrile seizures under two years. Management is supportive: antipyretics, and treat febrile seizures if they occur. Reassure the family; no tests, no antibiotics.[5][6]

Stem 2 — the unvaccinated child with cough, coryza and a rash (answer)

A 4-year-old whose parents declined vaccination has had four days of fever, cough, coryza and conjunctivitis; today a confluent maculopapular rash erupted behind his ears and is marching down his body. What is this, what is the pathognomonic sign, and what specific therapy does he get? Model: Measles (rubeola). The 3 Cs plus a cephalocaudal rash on day 4 is diagnostic; the pathognomonic sign is Koplik spots — bluish-white specks on an erythematous buccal base opposite the lower molars, seen in the prodrome. Isolate and notify public health, give supportive care, and give vitamin A (under 6 months 50,000 IU; 6 to 11 months 100,000 IU; 12 months or older 200,000 IU, repeated next day and at 4 weeks). Watch for pneumonia (the commonest cause of death), encephalitis, and SSPE years later. Offer MMR within 72 hours to susceptible contacts.[1][13]

Stem 3 — the teenager given amoxicillin for a sore throat (answer)

An 18-year-old with fever, sore throat and cervical lymphadenopathy was started on amoxicillin yesterday and has woken with a dramatic confluent morbilliform rash. What is happening, and what do you tell the patient? Model: The amoxicillin-triggered morbilliform rash of EBV infectious mononucleosis, occurring in about 90 percent of patients given aminopenicillins during EBV. It is not a true penicillin allergy — it reflects polyclonal T-cell activation by EBV. Stop the antibiotic, treat the EBV supportively, counsel against contact sports for three to four weeks because of splenic rupture risk, and reassure that penicillins can usually be tolerated in future.[16]

References

1. [1]Hübschen JM, Gouandjika-Vasilache I, Dina J. Measles Lancet, 2022.PMID 35093206
2. [2]Winter AK, Moss WJ. Rubella Lancet, 2022.PMID 35367004
3. [3]Leung AKC, Lam JM, Barankin B, et al. Erythema Infectiosum: A Narrative Review Curr Pediatr Rev, 2024.PMID 37132144
4. [4]Heegaard ED, Brown KE. Human parvovirus B19 Clin Microbiol Rev, 2002.PMID 12097253
5. [5]Leung AK, Lam JM, Barankin B, et al. Roseola Infantum: An Updated Review Curr Pediatr Rev, 2024.PMID 36411550
6. [6]Agut H, Bonnafous P, Gautheret-Dejean A. Laboratory and clinical aspects of human herpesvirus 6 infections Clin Microbiol Rev, 2015.PMID 25762531
7. [7]Leung AKC, Lam JM, Barankin B, et al. Hand, Foot, and Mouth Disease: A Narrative Review Recent Adv Inflamm Allergy Drug Discov, 2022.PMID 36284392
8. [8]Saguil A, Kane SF, Lauters R, et al. Hand-Foot-and-Mouth Disease: Rapid Evidence Review Am Fam Physician, 2019.PMID 31573162
9. [9]Haber JS, Cipriano SD, Oza VS. Morbilliform Eruptions in the Hospitalized Child Dermatol Clin, 2022.PMID 35366972
10. [10]Khandpur S, Ahuja R. Drug-Induced vs. Viral Maculopapular Exanthem-Resolving the Dilemma Dermatopathology (Basel), 2022.PMID 35645232
11. [11]Rife E, Gedalia A. Kawasaki Disease: an Update Curr Rheumatol Rep, 2020.PMID 32924089
12. [12]Kennedy PGE, Gershon AA. Clinical Features of Varicella-Zoster Virus Infection Viruses, 2018.PMID 30400213
13. [13]Moss WJ. Measles Lancet, 2017.PMID 28673424
14. [14]Su SB, Chang HL, Chen AK. Current Status of Mumps Virus Infection: Epidemiology, Pathogenesis, and Vaccine Int J Environ Res Public Health, 2020.PMID 32150969
15. [15]Leung AKC, Sergi CM, Lam JM, et al. Gianotti-Crosti syndrome (papular acrodermatitis of childhood) in the era of a viral recrudescence and vaccine opposition World J Pediatr, 2019.PMID 31134587
16. [16]Mergoum AM. Amoxicillin Rash in Infectious Mononucleosis N Engl J Med, 2021.PMID 34496177
17. [17]Lievano F, Galea SA, Thornton M, et al. Measles, mumps, and rubella virus vaccine (M-M-R II): a review of 32 years of clinical and postmarketing experience Vaccine, 2012.PMID 22959986