Demam Scarlet

01Pathophysiology

Scarlet fever is not a primary skin infection. It is a toxin-mediated complication of Group A Streptococcal pharyngitis. The disease begins when Streptococcus pyogenes colonizes the pharynx and produces streptococcal pyrogenic exotoxins (SPE A, B, or C). These exotoxins are encoded by bacteriophages, which is why not all GAS strains cause scarlet fever; only lysogenized strains carrying the toxin gene can produce the clinical syndrome.

The pyrogenic exotoxins function as superantigens. Rather than undergoing conventional antigen processing and MHC-restricted presentation, superantigens directly cross-link the MHC class II molecule on antigen-presenting cells with the V-beta region of the T-cell receptor. This bypasses normal antigen processing and activates up to 20% of the total T-cell population simultaneously (compared to less than 0.01% in a conventional immune response). The result is a flood of pro-inflammatory cytokines, including IL-1, IL-6, TNF-alpha, and IFN-gamma.

This cytokine storm produces the high fever, malaise, and systemic toxicity. The diffuse erythematous rash results from widespread capillary dilation and damage in the dermis, driven by cytokine-mediated endothelial injury. The characteristic sandpaper texture of the rash is caused by 1 to 2 mm papules from perifollicular inflammation and obstruction of sweat glands. The rash appears first on the trunk and neck, then spreads centrifugally to the extremities, typically sparing the palms and soles early in the course.

Pastia's lines occur because the fragile capillaries in flexural skin folds (antecubital fossae, axillae, inguinal creases) rupture under mechanical stress, producing linear petechiae that do not blanch with pressure and persist even after the rash resolves. Circumoral pallor results from sparing of the perioral skin while the rest of the face is flushed due to vasodilation. The strawberry tongue reflects papillary hypertrophy: initially, the white coating over edematous red papillae creates the "white strawberry tongue," and as the coating sloughs off by day 4 to 5, the beefy-red papillae become prominent, producing the classic "red strawberry tongue."

Late-phase desquamation, especially of the fingertips and toes, occurs 1 to 2 weeks after the rash fades. This happens because the intense inflammatory infiltrate in the dermal-epidermal junction disrupts keratinocyte adhesion, causing sheets of skin to peel. This desquamation pattern closely resembles that of Kawasaki disease and is a frequent source of confusion on exams.

02Classification and Clinical Manifestation

Scarlet fever progresses through three recognizable clinical phases. The table below organizes the findings by phase, which helps you identify the disease regardless of when in the illness the vignette is set.

A vignette may also describe the rash as having a "sunburn with goosebumps" appearance. The rash typically fades within 3 to 5 days even without treatment. Abdominal pain may be prominent in young children, sometimes mimicking acute abdomen; the key discriminator is the concurrent pharyngitis and rash. The severity of scarlet fever is generally classified by older literature into mild, moderate, and severe (toxic) forms, but this grading has limited clinical utility today and is rarely tested. What matters for exam purposes is recognizing the triad of pharyngitis + sandpaper rash + strawberry tongue.

03Diagnostic Workup

In a child presenting with sore throat, fever, and a sandpaper-textured rash, the diagnosis of scarlet fever is largely clinical. However, confirming the GAS etiology is essential because it dictates antibiotic therapy and prevention of complications.

The best initial test is the RADT performed on a posterior pharyngeal and tonsillar swab. If the RADT is positive, you treat immediately. The critical rule for pediatric patients: if the RADT is negative in a child, you must follow up with a throat culture because the sensitivity of RADT is not sufficient to rule out GAS in children, and the consequences of missed GAS pharyngitis (namely, acute rheumatic fever) are too serious. In adults, a negative RADT alone is generally sufficient to exclude GAS because the risk of rheumatic fever is significantly lower.

You do not order ASO titers during the acute illness. ASO titers take 1 to 3 weeks to rise and peak at 3 to 5 weeks. They are ordered only when a patient presents weeks later with signs of acute rheumatic fever or post-streptococcal glomerulonephritis, and you need retrospective evidence of recent streptococcal infection.

A clinical pearl that distinguishes scarlet fever from viral exanthems on workup: GAS pharyngitis does not cause cough, rhinorrhea, or conjunctivitis. A vignette that includes any of these symptoms should steer you away from GAS and toward a viral etiology. This negative finding (absence of coryza) is a frequently tested discriminator.

04Management and Treatment

The cornerstone of treatment is a full 10-day course of penicillin or amoxicillin, regardless of clinical improvement. Patients typically feel better within 48 to 72 hours, but the full course is mandatory to eradicate the organism from the pharynx and prevent acute rheumatic fever. Amoxicillin is often preferred in children because it tastes better and can be given once daily, improving adherence.

If you suspect the patient or caregiver will not complete a 10-day oral course, Benzathine Penicillin G as a single intramuscular injection guarantees therapeutic levels for approximately 3 to 4 weeks and is the definitive way to ensure compliance.

For patients with a documented penicillin allergy, the next best step depends on the type of allergy. If the reaction was a mild rash (non-anaphylactic), a first-generation cephalosporin such as cephalexin is appropriate because cross-reactivity between penicillins and first-generation cephalosporins is low (approximately 1 to 2%). If the allergy involved anaphylaxis, angioedema, or urticaria, cephalosporins are avoided entirely, and azithromycin or clindamycin is used instead.

Macrolide resistance in GAS is an increasingly recognized issue in certain regions. If local resistance rates exceed 10%, clindamycin is preferred over azithromycin.

Regarding complications prevention: antibiotics prevent acute rheumatic fever (ARF) if started within 9 days of symptom onset. However, antibiotics do not prevent post-streptococcal glomerulonephritis (PSGN). This is one of the most commonly tested distinctions. Antibiotics also shorten the duration of symptoms, reduce contagiousness (patients are considered non-contagious after 24 hours of appropriate antibiotics), and decrease suppurative complications such as peritonsillar abscess, retropharyngeal abscess, and cervical lymphadenitis.

There is no role for corticosteroids, antihistamines for the rash, or topical treatment of the exanthem. The rash resolves spontaneously as the toxin is cleared.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The most common trap with scarlet fever questions involves misidentifying it as Kawasaki disease. Both conditions feature fever, strawberry tongue, rash, cervical lymphadenopathy, and late desquamation. Students who rely on pattern-matching these overlapping features without reading the vignette carefully will select the wrong answer. The critical discriminator is the pharyngeal exudate and sandpaper-textured rash in scarlet fever versus bilateral conjunctival injection and extremity edema/erythema in Kawasaki. Additionally, Kawasaki disease requires fever lasting at least 5 days, while scarlet fever patients are typically treated and improving well before that threshold.

A second common error is ordering ASO titers to diagnose acute scarlet fever. ASO is a retrospective marker and has no role in the acute setting. The correct answer for initial diagnosis is always RADT first, then throat culture if negative in a child.

A third trap involves the penicillin-allergic patient. Students often select azithromycin reflexively. When the allergy is described as a "mild childhood rash," the correct choice is a first-generation cephalosporin, not a macrolide, because cephalosporins have better GAS eradication rates and the cross-reactivity risk is minimal.

Finally, remember the asymmetry between the two non-suppurative complications: antibiotics prevent rheumatic fever but do not prevent glomerulonephritis. A vignette describing a child who completed a full antibiotic course but still develops hematuria, periorbital edema, and hypertension 2 to 3 weeks later is testing your understanding that PSGN is an immune complex-mediated process that proceeds regardless of treatment. The core competency being tested across scarlet fever questions is your ability to distinguish toxin-mediated rash syndromes, sequence the diagnostic workup correctly, and understand the rationale behind a full 10-day antibiotic course even when the patient feels well by day 3.