4.2 Viral Exanthems & Common Childhood Infections
Key Takeaways
- Koplik spots on the buccal mucosa are pathognomonic for measles and appear 1-2 days before the cephalocaudal exanthem begins.
- Roseola infantum classically presents as several days of high fever in a well-appearing infant, with the rash appearing only after the fever abruptly resolves.
- By the time the slapped-cheek rash of fifth disease (parvovirus B19) appears, the child is generally no longer contagious, because the rash is immune-mediated rather than a sign of active viremia.
- Parvovirus B19 can cause a transient aplastic crisis in children with underlying hemolytic disease and fetal hydrops if a pregnant woman is infected.
- Scarlet fever's sandpaper rash and strawberry tongue reflect Group A Streptococcus infection, and antibiotics are given primarily to prevent acute rheumatic fever.
Approaching the Classic Childhood Exanthems
A viral exanthem is a widespread skin eruption associated with a systemic viral (or, in the case of scarlet fever, bacterial) illness. Arab Board Pediatrics exams reliably test the ability to match a short clinical vignette (fever pattern + rash timing + associated signs) to the correct diagnosis, so the fastest way to prepare is to memorize the distinguishing feature of each classic exanthem rather than a full description.
Quick-Reference Table
| Disease | Organism | Rash timing | Classic clue |
|---|---|---|---|
| Measles (rubeola) | Measles virus (paramyxovirus) | Rash starts 3-5 days after fever begins | Koplik spots (grey-white spots on buccal mucosa) precede the rash; cephalocaudal spread |
| Rubella (German measles) | Rubella virus | Rash appears with or just after fever, resolves in about 3 days | Mild illness; postauricular/suboccipital lymphadenopathy; Forchheimer spots |
| Roseola infantum (sixth disease) | Human herpesvirus 6 (HHV-6) | Rash appears as the fever suddenly resolves | High fever (often 39-40C) for 3-5 days in an otherwise well-looking infant, then abrupt defervescence with rash |
| Erythema infectiosum (fifth disease) | Parvovirus B19 | Facial rash first, then lacy/reticular rash on limbs | 'Slapped cheek' appearance; child is often no longer contagious by the time the rash appears |
| Hand-foot-mouth disease (HFMD) | Coxsackievirus A16, Enterovirus 71 | Vesicles/papules on palms, soles, and oral mucosa | Painful oral ulcers plus peripheral vesicular lesions; usually mild fever |
| Varicella (chickenpox) | Varicella-zoster virus (VZV) | Vesicles appear in crops over several days; centripetal spread (trunk to extremities) | 'Dewdrop on a rose petal' vesicles on an erythematous base; intensely pruritic; contagious until all lesions are crusted |
| Scarlet fever | Streptococcus pyogenes (Group A Streptococcus, GAS) -- a bacterial exotoxin-mediated illness | Rash begins 1-2 days after pharyngitis/fever | 'Sandpaper' rash, strawberry tongue, circumoral pallor, Pastia lines in skin folds |
Measles: The 3 C's and Koplik Spots
Measles remains an exam favorite because of resurgence in under-vaccinated populations. The prodrome is remembered as the 3 C's: cough, coryza, and conjunctivitis, accompanied by high fever. Koplik spots -- small grey-white lesions on an erythematous base on the buccal mucosa opposite the molars -- are pathognomonic and appear 1-2 days before the rash. The rash itself is an erythematous, maculopapular eruption that begins on the face/hairline and spreads cephalocaudally (head to trunk to extremities), coinciding with a second fever spike. Complications tested on exams include otitis media (the most common complication), pneumonia (the leading cause of measles death in children), and the rare but fatal delayed complication subacute sclerosing panencephalitis (SSPE), which presents years after infection with progressive neurologic decline.
Rubella: The Mild Mimic With a Devastating Congenital Form
Rubella causes a mild febrile illness with a fine, pink maculopapular rash that spreads quickly and fades within about three days ('three-day measles'), plus tender postauricular and suboccipital lymphadenopathy. The postnatal illness is usually trivial; the reason rubella is heavily tested is its teratogenic potential in congenital rubella syndrome (covered in the TORCH section of this chapter) when a non-immune woman is infected during pregnancy.
Roseola: Fever First, Rash After
Roseola infantum, caused by HHV-6 (and sometimes HHV-7), classically affects infants 6-24 months old. The pattern is distinctive: 3-5 days of high fever with a well-appearing child and no rash, followed by abrupt defervescence and the appearance of a pink macular/maculopapular rash on the trunk that spreads outward. This fever pattern is a classic setup for febrile seizures, and examiners frequently pair roseola with a febrile seizure vignette in a toddler.
Fifth Disease: Slapped Cheeks and Hidden Danger
Erythema infectiosum, caused by parvovirus B19, gets its 'fifth disease' name from historical exanthem numbering. The rash begins as bright red cheeks ('slapped cheek' appearance) with circumoral pallor, followed 1-4 days later by a lacy, reticular (net-like) rash on the extremities that can recur with heat, sun exposure, or exercise for weeks. A key exam trap: by the time the rash appears, the child is no longer viremic or contagious, because the rash is immune-complex mediated rather than a direct effect of active viral replication. Parvovirus B19 has two important complications to know: it can cause a transient aplastic crisis in children with underlying hemolytic conditions (such as sickle cell disease or hereditary spherocytosis) because the virus lyses erythroid precursor cells, and infection during pregnancy can cause fetal hydrops from severe fetal anemia.
Hand-Foot-Mouth Disease
HFMD, most often from Coxsackievirus A16 or Enterovirus 71, causes painful vesicular/ulcerative lesions in the mouth plus vesicular or maculopapular lesions on the palms and soles (and sometimes buttocks). Illness is usually self-limited over 7-10 days; the main clinical concern is poor oral intake and dehydration from painful oral lesions. Enterovirus 71 outbreaks are associated with rarer but more severe neurologic complications (brainstem encephalitis), worth knowing as a distractor for severe presentations.
Varicella (Chickenpox)
Varicella, caused by varicella-zoster virus (VZV), presents with a pruritic vesicular rash that appears in successive crops over several days, giving lesions at different stages (macules, papules, vesicles, crusts) on the same body area. The rash spreads centripetally (trunk to extremities), the opposite of measles's cephalocaudal pattern. Classic lesions are described as a 'dewdrop on a rose petal' -- a clear vesicle on an erythematous base. The child is contagious from 1-2 days before rash onset until all lesions have crusted over, typically about 5-7 days after rash appears. Complications tested on exams include secondary bacterial skin infection, pneumonia (more common in adolescents and immunocompromised hosts), and cerebellar ataxia. Routine childhood varicella vaccination has sharply reduced disease burden in countries with universal programs.
Scarlet Fever: The Bacterial Exception
Scarlet fever is caused by erythrogenic (pyrogenic) exotoxin-producing strains of Group A Streptococcus (S. pyogenes), usually complicating streptococcal pharyngitis. The rash has a fine, papular, 'sandpaper' texture, is accentuated in skin folds (Pastia lines), and spares the area around the mouth (circumoral pallor). The tongue progresses from a white coating with red papillae ('white strawberry tongue') to a beefy-red 'strawberry tongue'. Treatment is penicillin or amoxicillin, primarily to prevent rheumatic fever, not to shorten the rash.
URI, Otitis Media, and Pharyngitis Basics
Most childhood fevers are due to routine upper respiratory infections (URIs), viral in the great majority of cases. Acute otitis media (AOM) is diagnosed by a bulging, erythematous tympanic membrane with reduced mobility, typically following a viral URI, and the leading bacterial causes are Streptococcus pneumoniae, non-typeable Haemophilus influenzae, and Moraxella catarrhalis. Pharyngitis requires distinguishing viral causes (most common, often with cough/coryza/conjunctivitis favoring a viral source) from GAS pharyngitis (favored by sudden-onset sore throat, fever, tonsillar exudate, tender anterior cervical nodes, and absence of cough -- the basis of Centor-type clinical scoring), because untreated GAS pharyngitis is the precursor to both scarlet fever and rheumatic fever.
An 18-month-old has had a temperature of 39.5C for four days, appears well between fever spikes, and has no rash. On day 5, the fever resolves abruptly and a pink macular rash appears on the trunk. What is the most likely diagnosis?
A 6-year-old with sickle cell disease presents with fever, pallor, and a bright red 'slapped cheek' rash. Two days later, a hemoglobin drop to 4 g/dL is noted. Which mechanism best explains this complication?
Which finding is considered pathognomonic for measles and typically precedes the exanthem by 1-2 days?
A 5-year-old has sudden-onset sore throat, fever, tonsillar exudate, and tender anterior cervical lymph nodes, without cough or coryza. One day later a fine sandpaper-textured rash develops with accentuation in the axillary and inguinal folds. What is the most important reason to treat this child with antibiotics?