Approach

Have a high index of clinical suspicion for scarlet fever in a child or adolescent who presents with the triad of sore throat, fever (>100.4°F [>38.0°C]), and a scarlatiniform abdominal rash (i.e., a diffuse, finely papular [sandpaper-like], erythematous rash that blanches with pressure).[2][3] Be aware that the scarlet fever rash can be confused with measles. See Differentials.

  • Scarlet fever can occur at any age but mainly affects children ages 1-10 years, and it is most common in those ages 3-6 years. It is uncommon in children <1 year old and in adults.[5][6][7]

  • Around 90% of children and adolescents with scarlet fever present with group A streptococcus (GAS) (Streptococcus pyogenes) pharyngitis.[14][15]​ Both conditions have similar epidemiology, evaluation, and treatment. See Acute pharyngitis.

History

Take a detailed history. Suspect scarlet fever in:

  • Patients with GAS pharyngitis

    • Most cases of scarlet fever occur in conjunction with GAS pharyngitis.[3] While <10% of patients with GAS pharyngitis develop scarlet fever, around 90% of children and adolescents with scarlet fever also present with GAS pharyngitis; therefore, the presence of symptoms (rash, fever, and sore throat) and a diagnosis of GAS pharyngitis are the most important clinical indicators of scarlet fever.[14][15]

    • Note that GAS infection in children <3 years old is often associated with fever, mucopurulent rhinitis, excoriated nares, and diffuse adenopathy, and that exudative pharyngitis is rare in this age group.[2]

    • Occasionally, especially in children ages <5 years, the scarlatiniform rash and fever can present prior to or independent of symptoms of pharyngitis.[15]

  • Close contacts of a person with scarlet fever or other GAS presentation (e.g., GAS pharyngitis)

    • This is the most common risk factor. Clusters or outbreaks of scarlet fever are commonly reported in endemic settings.[10] The incubation period is approximately 2-5 days.[10] Household contacts of a person with scarlet fever have been found to have an increased risk of invasive GAS (iGAS) infection in the 2 months following scarlet fever onset, although the risk is relatively low (35.3 cases/100,000 person-years).[16] See Complications.

  • Children ages 1-10 years

    • Scarlet fever can occur at any age but mainly affects children ages 1-10 years, and it is most common in children ages 3-6 years. Scarlet fever is uncommon in children <1 year old and in adults.[5][6][7]

Other risk factors include:

  • Winter and spring seasons

    • In North America, scarlet fever is most common during the winter and spring.[17]

  • Crowded environment

    • Crowding, such as found in schools and daycare centers, increases the risk of disease spread.[10]

  • Close contact with young children

    • Scarlet fever is uncommon in adults, but more common in adults living or working with young children (e.g., teachers, daycare workers, pediatric medical staff). This is reflected in higher rates of scarlet fever reported in women than in men.[5][10]

  • Patients with nonpharyngitis GAS infection

    • Scarlet fever may also occur less commonly with GAS infection of the skin, soft tissue, and wounds.[3]

Symptoms may include (in order of relevance and frequency in scarlet fever):[2][15]

  • Rash

  • Sore throat

  • Fever

  • Headache

  • Nausea, vomiting, and abdominal pain.

Physical exam

Look for the presence of the characteristic scarlatiniform rash, which is the key diagnostic element of scarlet fever, and is present in 89% of children.[2][3][10][15]​​ The features of the rash include:[3][10]

  • Diffuse, finely papular (sandpaper-like), erythematous rash that blanches with pressure

  • Accentuated in flexor creases (i.e., under the arm, in the groin, in the elbows) often with petechiae, producing red streaks known as Pastia lines[22]

  • Starts on the trunk and may spread to the limbs (sparing palms and soles)

  • Flushed "scarlet" bilateral cheeks with circumoral pallor. In patients with more darkly pigmented skin, flushed bilateral cheeks may appear "sunburnt".

[Figure caption and citation for the preceding image starts]: Typical scarlatiniform rash in a child with scarlet feverBMJ 2018 Aug 30;362:k3005 [Citation ends].com.bmj.content.model.Caption@23266ee8

In patients with more darkly pigmented skin, the rash has the same characteristic raised sandpaper quality, but it may not appear erythematous.

[Figure caption and citation for the preceding image starts]: Flushed bilateral cheeks with circumoral pallor in a child with scarlet feverDermNet; used with permission [Citation ends].com.bmj.content.model.Caption@6505d08f

Patients may present with an inflamed tongue with a white coating and prominent papillae (strawberry tongue).[10]

[Figure caption and citation for the preceding image starts]: Strawberry tongue in a child with scarlet feverBMJ 2018 Aug 30;362:k3005 [Citation ends].com.bmj.content.model.Caption@194e5a2

Skin desquamation is a late finding (3-4 days after scarlatiniform rash), consisting of fine peeling of the skin that starts on the head and progresses downward.[10][22]

[Figure caption and citation for the preceding image starts]: Peeling phase of scarlatiniform rash in a patient with scarlet feverBMJ. 2018 Aug 30;362:k3005 [Citation ends].com.bmj.content.model.Caption@3017bde5

Patients typically present with GAS pharyngitis, either before or within 1-2 days after presentation of the rash. Signs include (in order of relevance and frequency in scarlet fever):[2]

  • Fever (>100.4°F [>38°C])

  • Tonsillopharyngeal inflammation

  • Patchy tonsillopharyngeal exudates

  • Palatal petechiae

  • Tender and enlarged anterior cervical lymph nodes.

See Acute pharyngitis.

In patients with a scarlatiniform rash without symptoms and signs of GAS pharyngitis, perform a thorough examination for potential skin or soft-tissue GAS infections, including impetigo and surgical wound infections. These patients may present with pyoderma.

Suspect an alternative diagnosis in patients presenting with viral features (e.g., cough, rhinorrhea, hoarseness, mouth ulcers). These are unlikely to be present in scarlet fever.[3]

Consider other diagnoses in patients presenting with rashes and fever, including Kawasaki disease, erythema infectiosum (fifth disease due to parvovirus B19), rubella, measles (rubeola), infectious mononucleosis (Epstein-Barr virus), enteroviral infection, rat-bite fever (Streptobacillus moniliformis infection), staphylococcal toxic shock syndrome, and staphylococcal scalded skin syndrome.[23] See Differentials.

Risk assessment

Use the McIsaac (modified Centor) score or the Centor score in patients ages ≥15 years to estimate the probability of GAS pharyngitis to guide decisions on testing.[24] [ Sore Throat (Pharyngitis) Evaluation and Treatment Criteria (McIsaac) Opens in new window ]

  • According to international guidelines, the McIsaac (modified Centor) score is the preferred clinical screening tool in patients with suspected GAS pharyngitis.[24]

  • The McIsaac (modified Centor) and the Centor score correlate directly with the risk of a positive throat culture for GAS.[25]

In children ages 3-14 years, clinical scoring criteria such as the McIsaac score may provide guidance to clinicians, but should be used with caution given mixed evidence of their utility in ruling out infection.

  • Several studies have demonstrated that, in children, no scoring system is sensitive enough to determine who should and should not be tested for GAS pharyngitis.[26][27][28][29]

  • According to a meta-analysis, a McIsaac score of ≥3 would provide a sensitivity of only 69%, and a Centor score of ≥3 would provide a sensitivity of 54%, which does not rule out GAS pharyngitis in children.[30]

Initial investigations

Perform a rapid antigen detection test (RADT) for GAS from a throat swab in:

  • All children ages 3-14 years who present with GAS pharyngitis (e.g., sudden onset of sore throat, tonsillopharyngeal inflammation as noted by erythema, pharyngeal exudates, swelling, palatal petechiae) and a scarlatiniform rash.[2] However, do not give antibiotics until a RADT or a throat culture is positive.[2] See Management.

  • Patients ≥15 years old with a McIsaac score or a Centor score of ≥3.[24]

RADTs for GAS offer the advantage of immediate point-of-care testing and are 70% to 90% sensitive and 95% specific compared with throat culture.[2][31][32] These tests may have a lower specificity in children recently treated for GAS.[33]

Testing is not routinely recommended in:

  • Children with obvious viral clinical and epidemiologic features (e.g., rhinorrhea, hoarseness, mouth ulcers).[2] The presence of multiple symptoms is more helpful in ruling out GAS pharyngitis than the presence of an individual symptom.[26][27][34]

  • Children <3 years old. Testing should be considered only in those with clinical symptoms and a household contact with microbiologically confirmed GAS pharyngitis.[2][3][24]

  • Adults with a McIsaac or Centor score of <3. These patients are at low risk for GAS pharyngitis.[24]

  • Asymptomatic contacts of those with GAS pharyngitis.[2]

Other investigations

Order a culture of throat swab in children and adolescents (ages 3-15 years) who have a negative RADT result.[2][3]

  • Do not order throat cultures in adults with a negative RADT result for GAS.[2][3] The incidence of GAS pharyngitis and the risk of subsequent complications, such as rheumatic fever, are very low in adults with acute pharyngitis.[2]

Order a culture of other body sites (e.g., superficial skin lesion, open wound) as the initial test in patients with suspected nonpharyngitis GAS infection (e.g, skin and soft-tissue infection, sepsis, or streptococcosis in children <3 years old).[3]

  • S pyogenes strains that cause scarlet fever may also cause other focal (e.g., skin and soft tissue) and nonfocal infections. 

Polymerase chain reaction (PCR) testing for S pyogenes has a high sensitivity and specificity comparable to that for throat culture, but it is more expensive and less readily available than RADTs for GAS.[3]

Do not routinely order antistreptococcal antibodies (antistreptolysin O [ASO] and anti-DNase B) tests as these reflect past but not current infection.[2]

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