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 (>38.0°C [>100.4°F]), 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 aged 1 to 10 years, and it is most common in those aged 3 to 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.[17][18] Both conditions have similar epidemiology, evaluation, and treatment. See Acute pharyngitis.

There is a statutory requirement to notify all suspected cases of scarlet fever in China, South Korea, and many European countries including England, Wales, Northern Ireland, Austria, and Poland.

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.[17][18]

    • 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 aged <5 years, the scarlatiniform rash and fever can present prior to or independent of symptoms of pharyngitis.[18]

  • 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.[13] The incubation period is approximately 2 to 5 days.[13] 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).[19] See Complications. 

  • Children aged 1 to 10 years

    • Scarlet fever can occur at any age but mainly affects children aged 1 to 10 years, and it is most common in children aged 3 to 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 Europe and Asia, scarlet fever is most common during the winter and spring.[20]

  • Crowded environment

    • Crowding, such as found in schools and nurseries, increases the risk of disease spread.[13]

  • 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, nursery workers, paediatric medical staff). This is reflected in higher rates of scarlet fever reported in women than men.[5][13]​​​​

  • Patients with non-pharyngitis 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][18]​​

  • Rash

  • Sore throat

  • Fever

  • Headache

  • Nausea, vomiting, and abdominal pain.

Physical examination

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][13][18]​​​ The features of the rash include:[3][13]

  • 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's lines.[27]

  • 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; 362 :k3005. [Citation ends].com.bmj.content.model.Caption@394f555

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

Patients may present with an inflamed tongue with a white coating and prominent papillae ('strawberry tongue').[13][Figure caption and citation for the preceding image starts]: Flushed bilateral cheeks with circumoral pallor in a child with scarlet feverFrom https://dermnetnz.org/topics/scarlet-fever used with permission [Citation ends].com.bmj.content.model.Caption@7349dabf

[Figure caption and citation for the preceding image starts]: ​Strawberry tongue in a child with scarlet feverBMJ 2018; 362 :k3005 [Citation ends].com.bmj.content.model.Caption@2f4c1824

Skin desquamation is a late finding (3 to 4 days after scarlatiniform rash), consisting of fine peeling of the skin that starts on the head and progresses downwards.[13][27]

[Figure caption and citation for the preceding image starts]: Peeling phase of scarlatiniform rash in a patient with scarlet feverBMJ 2018; 362 :k3005 [Citation ends].com.bmj.content.model.Caption@6d538059

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

  • Fever (>38°C [>100.4°F])

  • 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, rhinorrhoea, 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.[28] See Differentials.

The UK, France, Ireland, the Netherlands, and Sweden are currently experiencing an increased incidence of scarlet fever and iGAS disease. The increase has been particularly marked during the second half of 2022.[29]

Be aware, therefore, of the potential for an increase in iGAS infection following outbreaks of scarlet fever. Have a high index of suspicion especially in patients with recent chickenpox, respiratory virus symptoms, those who are immunosuppressed, and women in the puerperal period. Early recognition and treatment of iGAS infection can be lifesaving.[25] One in 40 patients are admitted to hospital for the management of scarlet fever or potential complications.[5] See Complications.

Risk assessment

Clinical prediction scores for distinguishing between viral and GAS pharyngitis (such as Centor, McIsaac, and FeverPain) are available, but are not recommended to guide decisions on testing and treatment in patients with suspected scarlet fever in many countries, such as the UK.[25]​​ [ Sore Throat (Pharyngitis) Evaluation and Treatment Criteria (McIsaac) Opens in new window ]

Where they are used, international guidelines suggest that the McIsaac score is the preferred risk assessment tool for patients with GAS pharyngitis.[30] The McIsaac (modified Centor) and the Centor score correlate directly with the risk of a positive throat culture for GAS.[31]

In children aged 3-14 years, clinical scoring criteria such as the McIsaac score may provide guidance to clinicians, but should be used with caution given the mixed evidence of their use in ruling out infection. Several studies have demonstrated that in children, no scoring system is sensitive enough to determine who should be tested for GAS pharyngitis.[32][33][34][35] According to a meta-analysis, a McIsaac score of ≥3 would provide a sensitivity of only 69%, and a Centor score of ≥3 a sensitivity of 54%, which does not rule out GAS pharyngitis in children.[36]

Initial investigations

Recommendations on testing for scarlet fever vary between countries. Refer to your local protocol for specific guidance in your area.

In countries such as the UK, where rapid antigen detection tests (RADTs) for scarlet fever are not readily available, test confirmation of GAS infection is not required before starting treatment in patients with a clinical diagnosis of scarlet fever.[25]

In countries where RADTs for scarlet fever are available, a positive test result may be required before starting antibiotics. For example, a RADT for GAS from a throat swab is recommended in:

  • All children aged 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, by the Infectious Diseases Society of America and the American Academy of Pediatrics.[2]

  • Patients ≥15 years old with a McIsaac score or a Centor score of ≥3, by international guidelines.[30]

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][37][38] These tests may have a lower specificity in children recently treated for GAS.[39]

Other investigations

In countries such as the UK where RADTs for scarlet fever are not routinely used or recommended, consider taking a throat swab for culture of GAS in the following circumstances:[25]

  • Uncertainty about the clinical diagnosis

  • Suspected case as part of an outbreak: the local health protection team should advise primary care if a local outbreak is suspected and when testing is appropriate

  • True allergy to penicillin, to determine antimicrobial susceptibility, depending on clinical judgement. GAS can be resistant to non-penicillin options such as macrolides and clindamycin

  • Regular contact with vulnerable people who are at high risk of complications, such as healthcare workers.

Take swabs before starting antibiotics but do not delay starting treatment while waiting for culture results. See Management.

In countries such as the US where RADTs for scarlet fever are available, a culture of throat swab is recommended in children and adolescents (3 to 15 years of age) who have a negative RADT result.[2][3]

  • Throat cultures are not recommended 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, is very low in adults with acute pharyngitis.[2]

A culture of other body sites (e.g., superficial skin lesion, open wound) is indicated as the initial test in patients with suspected non-pharyngitis 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 non-focal 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 anti-streptococcal antibodies (antistreptolysin O [ASO] and anti-DNase B) tests as these reflect past but not current infection.[2][25] However, ASO may be helpful in the diagnosis of post-infection complications, such as acute rheumatic fever or glomerulonephritis.[25]

Use of this content is subject to our disclaimer