Childhood Viral Exanthems & Paramyxoviruses
Microbiology · Virology · lean revision notes
Childhood Viral Exanthems & Paramyxoviruses
A rash plus fever in a child is a recurring NEET PG image-based and one-liner favourite. This note consolidates the "classic six" childhood exanthems with a sharp focus on measles, mumps and rubella (the paramyxovirus-heavy MMR triad) plus roseola (HHV-6), arming you with the eponyms, cut-offs and vaccine facts that examiners love.
Definition & the "Classic Six" exanthems
An exanthem is a widespread rash that erupts as a sign of systemic (usually viral) infection; the matching mucous-membrane eruption is an enanthem (e.g. Koplik spots, Forchheimer spots). Historically six classic childhood exanthems were numbered:
| No. | Disease | Causative agent | Family |
|---|---|---|---|
| First | Measles (rubeola) | Measles virus | Paramyxoviridae (Morbillivirus) |
| Second | Scarlet fever | Streptococcus pyogenes (erythrogenic toxin) | Bacterial (not viral) |
| Third | Rubella (German/3-day measles) | Rubella virus | Matonaviridae (formerly Togaviridae) |
| Fourth | "Filatov-Dukes" disease | Disputed/obsolete entity | — |
| Fifth | Erythema infectiosum | Parvovirus B19 | Parvoviridae |
| Sixth | Roseola infantum (exanthem subitum) | HHV-6 (and HHV-7) | Herpesviridae |
High-yield: Measles, mumps and rubella are the exam-darlings, but note the family trap — measles and mumps are paramyxoviruses (enveloped, ssRNA, negative-sense, helical), whereas rubella is NOT a paramyxovirus (it is a positive-sense RNA virus, now classified under Matonaviridae). Examiners frequently club rubella with measles to mislead you.
Paramyxoviridae snapshot
Paramyxoviruses are enveloped, non-segmented, negative-sense, single-stranded RNA viruses with helical nucleocapsids. Medically important members: Measles, Mumps, Parainfluenza, RSV, Nipah, Hendra. Two key surface glycoprotein patterns:
- Measles (Morbillivirus): H (haemagglutinin, no neuraminidase) + F (fusion) protein. No neuraminidase.
- Mumps & Parainfluenza (Rubulavirus/Respirovirus): HN (haemagglutinin-neuraminidase) + F protein.
- RSV (Pneumovirus): only F protein, no haemagglutinin, no neuraminidase.
The F (fusion) protein mediates cell-to-cell fusion → multinucleated giant (syncytial) cells, the morphologic hallmark across this family.
Measles (Rubeola)
Pathophysiology
Measles virus enters via the respiratory tract, replicates in lymphoid tissue, and causes a primary then secondary viraemia. The receptor is CD150 (SLAM) on immune cells and nectin-4 on epithelium. It induces profound, transient immunosuppression ("immune amnesia") that predicts secondary bacterial infections.
Clinical features — the 3 C's
Cough, Coryza, Conjunctivitis (the "3 C's") + high fever in the prodrome. Then:
- Koplik spots → pathognomonic enanthem: tiny bluish-white/grey specks on an erythematous base on the buccal mucosa opposite the second molars, appearing 1–2 days before the rash.
- Rash → erythematous maculopapular, begins behind the ears/at the hairline → face → spreads cephalocaudally (head to trunk to limbs), becomes confluent, and clears with brownish staining and fine desquamation.
Disease flow: Exposure → incubation 10–14 days → prodrome (3 C's + fever + Koplik spots) → rash on day 3–4 → cephalocaudal spread → resolution with desquamation.
Histology / cytology
Warthin-Finkeldey giant cells — multinucleated giant cells (reticuloendothelial syncytial cells with up to 50–100 nuclei) found in lymphoid tissue (tonsils, appendix, lymph nodes). These are the cytologic eponym of measles.
High-yield: Koplik spots = pathognomonic enanthem; Warthin-Finkeldey cells = pathognomonic histology; rash spreads cephalocaudally.
Complications
- Otitis media — most common complication overall.
- Pneumonia — most common cause of measles death; may be Hecht's giant cell pneumonia (in immunocompromised).
- Acute post-measles encephalomyelitis (1 in 1000, days–weeks later, immune-mediated/ADEM-like).
- Subacute sclerosing panencephalitis (SSPE) — a slow virus infection.
SSPE (very high yield)
A late, progressive, fatal degenerative CNS disease appearing ~7–10 years after measles (often after natural infection before age 2). Defective M (matrix) protein prevents viral budding → persistent intracellular virus.
| Feature | SSPE finding |
|---|---|
| EEG | Periodic burst-suppression complexes (Radermecker complexes) |
| CSF | Raised anti-measles antibody (oligoclonal IgG); high CSF:serum antibody ratio |
| Histology | Intranuclear & intracytoplasmic inclusions (Dawson's inclusion bodies) |
| Clinical | Myoclonic jerks, cognitive decline, dementia, death |
High-yield: SSPE → defective M protein → high anti-measles antibody titres in CSF + periodic EEG complexes. Mnemonic: "M for Matrix, M for Measles, M for Myoclonus."
Diagnosis & management
- Investigation of choice: Serology — measles-specific IgM (positive within 1–3 days of rash) or a 4-fold IgG rise. RT-PCR for RNA also used.
- Management: Supportive. Vitamin A reduces morbidity and mortality (WHO recommends two doses on consecutive days). Vaccine: live attenuated (in MMR), given at 9 months (India schedule) and a second dose at 15–18 months.
Mumps
Pathophysiology & features
A paramyxovirus (Rubulavirus, HN + F proteins) transmitted by droplets. After respiratory replication and viraemia it shows tropism for glandular and neural tissue.
- Parotitis — bilateral (or unilateral) painful swelling; classically the angle of the jaw is obscured and the ear lobe is pushed up and out. Eating sour food worsens pain (Stensen's duct orifice may be erythematous).
- Orchitis — most common extra-salivary manifestation in post-pubertal males (15–30%); usually unilateral; sterility is rare despite the exam folklore.
- Aseptic meningitis / meningoencephalitis — mumps is a leading viral cause of aseptic meningitis where vaccination is poor; CSF shows lymphocytic pleocytosis, and characteristically LOW CSF glucose may occur (a pitfall mimicking bacterial meningitis).
- Pancreatitis, oophoritis, deafness (mumps is a cause of acquired unilateral sensorineural deafness), myocarditis.
High-yield: Mumps orchitis is usually unilateral and rarely causes sterility; mumps is a classic cause of aseptic meningitis with occasionally low CSF sugar.
Diagnosis & management
- Investigation of choice: Serum mumps IgM; rising IgG; RT-PCR of saliva/CSF. Serum amylase is raised (from parotid and/or pancreas).
- Management: Supportive. Live attenuated vaccine in MMR. Jeryl Lynn strain is the classic mumps vaccine strain.
Rubella (German measles / 3-day measles)
Virology & postnatal disease
Positive-sense ssRNA, enveloped virus, family Matonaviridae (reclassified out of Togaviridae). Mild self-limiting illness in children:
- Tender post-auricular, suboccipital and posterior cervical lymphadenopathy — a near-signature.
- Forchheimer spots — petechiae on the soft palate (enanthem; not specific, also seen in scarlet fever).
- Pink maculopapular rash that, like measles, starts on the face and spreads down, but is milder, lasts ~3 days, and does not coalesce/desquamate.
Congenital Rubella Syndrome (CRS) — extremely high yield
Maternal infection in the first trimester is teratogenic (risk highest <12 weeks, ~80–90%). Classic Gregg's triad:
| Component of Gregg triad | Lesion |
|---|---|
| Eye | Cataract (salt-and-pepper retinopathy), microphthalmia, glaucoma |
| Ear | Sensorineural deafness (most common single defect overall) |
| Heart | Patent ductus arteriosus (PDA) (and peripheral pulmonary artery stenosis) |
Other features: "blueberry muffin" rash (dermal extramedullary haematopoiesis), hepatosplenomegaly, thrombocytopenia, microcephaly, intellectual disability, "celery-stalking" of long bones.
TORCH context flow: Maternal rubella in 1st trimester → transplacental spread → Gregg triad (cataract + SNHL + PDA) → blueberry muffin baby; infants shed virus for months and are infectious.
High-yield: SNHL is the most common single defect in CRS; PDA is the most common cardiac lesion. The "blueberry muffin baby" appearance is shared with congenital CMV (and other congenital infections) — not unique to rubella.
Diagnosis & management
- Investigation of choice (acquired): Rubella IgM serology.
- Congenital: IgM in neonatal serum, or persistent/rising IgG beyond expected maternal decline; viral RNA from throat/urine.
- Management: Supportive; no specific antiviral. Prevention via live attenuated MMR. Vaccine is contraindicated in pregnancy and pregnancy should be avoided for 4 weeks after vaccination (though no CRS case has ever been documented from the vaccine).
Roseola Infantum (Exanthem Subitum / Sixth Disease)
Caused by HHV-6 (chiefly variant B; HHV-7 causes a minority) — a Herpesvirus (dsDNA, enveloped), not a paramyxovirus.
- Affects infants 6 months–2 years.
- Classic course: 3–5 days of HIGH fever in a well-looking child → fever drops abruptly → THEN a rose-pink maculopapular rash erupts on the trunk (rash appears after defervescence — hence "subitum"=sudden).
- Most common identifiable cause of febrile seizures in this age group.
- Nagayama spots — papules on the soft palate/uvula (enanthem).
- HHV-6 establishes latency; reactivation matters in transplant recipients.
High-yield: Roseola = fever first, rash later (rash appears when fever breaks). Leading viral cause of febrile convulsions in infants.
Comparison table: the three "measles-like" rashes
| Feature | Measles | Rubella | Roseola |
|---|---|---|---|
| Agent | Measles virus (Paramyxo) | Rubella virus (Matonaviridae) | HHV-6 (Herpes) |
| Genome | (−) ssRNA | (+) ssRNA | dsDNA |
| Prodrome | Severe (3 C's, high fever) | Mild | High fever, child well |
| Pathognomonic sign | Koplik spots | Post-auricular nodes; Forchheimer spots | Rash after fever; Nagayama spots |
| Rash spread | Cephalocaudal, confluent, stains | Cephalocaudal, mild, 3 days | Trunk → periphery, after fever |
| Lymphadenopathy | Generalised | Post-auricular/suboccipital | Mild |
| Key complication | SSPE, pneumonia | CRS in pregnancy | Febrile seizures |
| Vaccine | MMR (live) | MMR (live) | None |
Erythema infectiosum (Parvovirus B19) — quick differential
Although not in the MMR group, B19 is a frequent distractor.
- "Slapped-cheek" appearance then a lacy/reticular rash on limbs.
- Binds the P antigen (globoside) receptor on erythroid precursors → transient aplastic crisis in chronic haemolysis (e.g. sickle cell, hereditary spherocytosis).
- In pregnancy → hydrops fetalis; in immunocompromised → pure red cell aplasia.
- Causes arthropathy in adults.
High-yield: Parvovirus B19 → slapped cheek, aplastic crisis in haemolytic anaemia, hydrops fetalis, single-stranded DNA, non-enveloped — the only DNA virus that is ssDNA.
Hand-foot-and-mouth & other vesicular mimics (differentials)
- Hand-foot-mouth disease — Coxsackievirus A16 / Enterovirus 71; oral ulcers + vesicles on palms/soles. EV71 causes more severe CNS disease.
- Varicella (chickenpox) — VZV; centripetal, pruritic, vesicles in different stages simultaneously ("dew drop on rose petal"); contrast with smallpox (centrifugal, same-stage lesions).
- Scarlet fever — sandpaper rash, strawberry tongue, perioral pallor, Pastia's lines; bacterial (GAS).
High-yield: Distinguish varicella (lesions in different stages, centripetal) from smallpox (same stage, centrifugal) — a perennial MCQ.
Diagnosis & investigation of choice — consolidated
- Measles / Rubella / Mumps: Specific IgM serology is the practical investigation of choice for acute diagnosis; RT-PCR confirms and is used for outbreak genotyping.
- SSPE: elevated CSF anti-measles antibody + periodic EEG complexes.
- CRS: neonatal rubella IgM / persistent IgG / RNA from urine-throat.
- Roseola: clinical; HHV-6 PCR if needed.
Management / prevention — consolidated
- No specific antiviral for measles, mumps or rubella; care is supportive.
- Vitamin A for measles (mortality benefit, esp. malnourished).
- MMR is live attenuated → contraindicated in pregnancy and significant immunosuppression; can be given in HIV if not severely immunosuppressed.
- India National Immunisation Schedule: Measles/MR at 9–12 months and 16–24 months (second dose); measles-rubella (MR) campaign targets rubella elimination.
High-yield: A live vaccine in pregnancy is a classic "wrong answer to choose" — MMR, varicella, OPV, BCG, yellow fever are all live and contraindicated in pregnancy.
Recently asked / exam angle
- Image of buccal mucosa with white spots → Koplik spots → Measles (commonest image MCQ in this topic).
- "Defective M protein, periodic EEG, high CSF antibody" → SSPE.
- Most common single defect / commonest cardiac defect in CRS → SNHL / PDA.
- "Rash appears after fever subsides in an infant" → Roseola/HHV-6, and HHV-6 as the commonest cause of febrile seizures.
- Family identification: which is NOT a paramyxovirus among Measles/Mumps/RSV/Rubella → Rubella.
- RSV has only the F protein (no H, no N) — frequently paired with bronchiolitis in paediatrics MCQs.
- Warthin-Finkeldey giant cells asked as a stand-alone histology recall for measles.
- Jeryl Lynn = mumps vaccine strain; Edmonston = measles vaccine lineage.
- Blueberry muffin baby differential: congenital rubella vs congenital CMV.
- Parvovirus B19 → transient aplastic crisis in a sickle-cell child.
Rapid revision
- Paramyxoviruses = enveloped, negative-sense ssRNA, helical; F protein → syncytia. Members: measles, mumps, parainfluenza, RSV, Nipah.
- Rubella is NOT a paramyxovirus — it is positive-sense RNA, family Matonaviridae.
- Measles 3 C's = Cough, Coryza, Conjunctivitis; Koplik spots are pathognomonic; rash spreads cephalocaudally.
- Warthin-Finkeldey cells = measles histology; measles receptor = CD150 (SLAM) + nectin-4.
- SSPE = defective M protein, 7–10 yrs later, high CSF anti-measles antibody, periodic EEG complexes, Dawson's inclusion bodies.
- Vitamin A reduces measles mortality; commonest measles complication = otitis media; commonest cause of death = pneumonia.
- Mumps = Rubulavirus (HN+F); orchitis (post-pubertal, usually unilateral, rarely sterile), aseptic meningitis (may have low CSF sugar); Jeryl Lynn vaccine strain.
- Rubella = post-auricular nodes, Forchheimer spots, 3-day mild rash.
- CRS Gregg triad = Cataract + SNHL + PDA; SNHL = commonest single defect; PDA = commonest cardiac lesion; blueberry muffin rash.
- Roseola (HHV-6, dsDNA herpesvirus) = high fever first, rash after defervescence; commonest cause of febrile seizures; Nagayama spots.
- Parvovirus B19 (ssDNA, non-enveloped) = slapped cheek, aplastic crisis, hydrops fetalis, P-antigen receptor.
- MMR is live attenuated → contraindicated in pregnancy and severe immunosuppression; India gives doses at 9 mo and 16–24 mo.