Infectious Disease and Microbiology – Scarlet Fever

Scarlet fever is a toxin-mediated illness caused by group A Streptococcus (GAS), Streptococcus pyogenes. It is characterized by an erythematous, finely papular rash associated most commonly with streptococcal pharyngitis. The rash results from streptococcal pyrogenic (erythrogenic) exotoxins.

Scarlet fever occurs predominantly in children, particularly school-aged children, and is uncommon in adults. Its epidemiology generally parallels that of group A streptococcal pharyngitis, with infections occurring most frequently during late fall, winter, and early spring in temperate climates.

Transmission usually occurs through respiratory droplets and close contact with an infected person. Household contacts, classmates, roommates, and people living in crowded environments therefore have increased exposure.

Although pharyngitis is the classic underlying infection, toxin-producing GAS infections at other sites can occasionally produce a scarlet-fever syndrome. These include infected wounds, burns, skin infections, surgical wounds, and puerperal infections.

Scarlet fever develops when an infecting strain of GAS produces streptococcal pyrogenic exotoxins in a susceptible host. These exotoxins are responsible for the characteristic rash and other systemic manifestations.

The incubation period is usually a few days. Illness commonly begins abruptly with fever and sore throat. Associated manifestations can include chills, headache, malaise, myalgia, nausea, vomiting, and abdominal pain, particularly in children.

Examination of the throat commonly demonstrates erythematous pharyngitis or tonsillitis, sometimes with tonsillar exudates. Palatal petechiae may be present, and tender anterior cervical lymphadenopathy is common.

The characteristic rash generally appears within the first few days of illness. It usually begins on the neck, upper chest, or trunk and subsequently spreads to the remainder of the trunk and extremities.

The rash consists of numerous small erythematous papules that produce the classic fine “sandpaper” texture. It typically blanches with pressure. The palms and soles are usually spared during the initial exanthem.

The face may appear flushed, but the region immediately surrounding the mouth characteristically remains pale. This finding is known as circumoral or perioral pallor.

Another characteristic finding is Pastia’s lines. These are areas of accentuated erythema within skin folds, particularly in the antecubital fossae, axillae, and groin. They result from increased concentration of the rash and petechiae in flexural areas.

The tongue may initially have a white coating with prominent erythematous papillae, producing the appearance traditionally called a white strawberry tongue. After several days, the coating disappears and the tongue becomes intensely erythematous with prominent papillae—the classic red strawberry or raspberry tongue.

The exanthem generally resolves within approximately a week. As the rash fades, desquamation may occur. Peeling can persist for several weeks and may become particularly noticeable around the fingertips, toes, hands, and feet.

Diagnosis is based on the compatible clinical syndrome together with evidence of group A streptococcal infection. Because several illnesses can produce a scarlatiniform rash, microbiologic confirmation of GAS pharyngitis is important when appropriate.

A rapid antigen detection test can identify GAS from a throat specimen. A throat culture can also establish the diagnosis and is particularly useful when additional confirmation is required. Current testing strategies depend on the patient’s age and local clinical guidelines.

Leukocytosis may occur during acute disease, but routine blood counts are not required to establish uncomplicated scarlet fever. Antistreptococcal antibody tests such as antistreptolysin O (ASO) and anti-DNase B reflect previous streptococcal exposure and are generally more useful for evaluating delayed post-streptococcal complications than for diagnosing acute pharyngitis.

The differential diagnosis includes viral exanthems, drug eruptions, Kawasaki disease, toxic shock syndrome, staphylococcal toxin-mediated disease, measles, rubella, and other causes of a diffuse erythematous rash. Clinical context and microbiologic testing help distinguish these disorders.

Penicillin or amoxicillin is standard first-line treatment for uncomplicated GAS pharyngitis associated with scarlet fever. A conventional oral penicillin or amoxicillin course is generally given for 10 days.

Appropriate alternatives are available for patients with penicillin allergy, with the choice depending partly on whether the allergy represents immediate hypersensitivity. Options can include selected cephalosporins, clindamycin, azithromycin, or clarithromycin. Local macrolide and clindamycin resistance should be considered when selecting therapy.

Antibiotic treatment shortens the period of contagiousness, reduces transmission, improves symptoms, and—when appropriately administered for GAS pharyngitis—helps prevent acute rheumatic fever.

Patients should receive adequate fluids, rest, and symptomatic treatment for fever and throat discomfort when necessary. Severe systemic illness, hemodynamic instability, rapidly progressive soft-tissue infection, or multisystem involvement should raise concern for invasive GAS infection or streptococcal toxic shock syndrome rather than uncomplicated scarlet fever.

Patients with uncomplicated scarlet fever generally become substantially less contagious after at least 12–24 hours of appropriate antibiotic therapy and when clinically improving. Good hand hygiene, respiratory etiquette, and avoidance of sharing eating utensils can help limit household and school transmission.

The prognosis is excellent with appropriate antibiotic treatment. The fever, pharyngitis, and rash usually resolve without permanent consequences, although skin desquamation can continue after the systemic illness has resolved.

Suppurative complications of the underlying GAS infection can include peritonsillar or retropharyngeal abscess, cervical lymphadenitis, otitis media, mastoiditis, pneumonia, bacteremia, and other invasive infections.

Important delayed immune-mediated complications include acute rheumatic fever and post-streptococcal glomerulonephritis. Acute rheumatic fever typically develops several weeks after GAS pharyngitis, whereas post-streptococcal glomerulonephritis may follow either pharyngeal or certain skin infections.

A useful clinical pattern to remember is:

GAS pharyngitis + fever → sandpaper-like rash → Pastia’s lines + circumoral pallor + strawberry tongue → later desquamation = Scarlet fever.