Group A Streptococcal Infections and Scarlet Fever
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Direct answer
Group A streptococcal infections are caused by Streptococcus pyogenes, a beta-haemolytic, bacitracin-sensitive organism that causes pharyngitis, scarlet fever, erysipelas and necrotising fasciitis. Scarlet fever is pharyngitis with a sandpaper rash driven by erythrogenic toxins, while rheumatic fever and post-streptococcal glomerulonephritis are the non-suppurative sequelae. Penicillin remains the first-line drug, and monthly or three-weekly benzathine penicillin is used for rheumatic heart disease prophylaxis.
What you must remember
- Identification: Streptococcus pyogenes is beta-haemolytic, PYR-positive, bacitracin-sensitive and Lancefield group A.
- Scarlet fever signs: sandpaper rash, circumoral pallor, strawberry tongue and Pastia lines in the skin folds, all driven by erythrogenic toxin.
- Major Jones criteria: carditis, polyarthritis, chorea, erythema marginatum and subcutaneous nodules, with two major or one major and two minor criteria needed; the 2015 revision for moderate-to-high-risk populations also counts monoarthritis, polyarthralgia and subclinical carditis.
- Post-streptococcal glomerulonephritis: a nephritic picture with low C3, appearing one to three weeks after pharyngitis or three to six weeks after skin infection.
- Necrotising fasciitis: pain out of proportion to the signs, rapid spread and toxic shock-like features, treated with surgical debridement plus penicillin and clindamycin.
- Erysipelas: a sharply demarcated, raised and painful red plaque of the face or leg, with penicillin as first-line therapy.
- Allergy and macrolides: clindamycin or a macrolide is used in penicillin allergy, though macrolide resistance is reported in parts of the world.
A stepwise approach to a sore throat in a child
Pharyngitis is the commonest streptococcal presentation, and most viral sore throats look similar on examination. The steps below separate the streptococcal case from the complications that need urgent action.
- Score the throat. Fever, tonsillar exudate, tender anterior cervical nodes and an absent cough raise suspicion, with age in childhood increasing the score.
- Confirm the test. A rapid antigen test is followed by throat culture when the antigen test is negative in children, since culture is the reference method. Culture takes about two days, so a high-risk child may be treated on clinical grounds while the result is awaited.
- Treat to prevent rheumatic fever. Penicillin V or amoxicillin is given for a full course, with a macrolide or clindamycin used only in penicillin allergy.
- Look for complications. Neck swelling, trismus or muffled voice suggests peritonsillar or retropharyngeal abscess, which needs drainage.
- Refer for sequelae. Suspected rheumatic fever needs echocardiography and secondary prophylaxis with benzathine penicillin G.
Traps in sore throat and rash stems
Most errors come from confusing a streptococcal rash, a viral sore throat and a post-infection syndrome.
- Trap: a child with sore throat and sandpaper rash is managed as a viral illness with no antibiotics. Correct: scarlet fever is streptococcal, and early antibiotics shorten illness and prevent rheumatic fever.
- Trap: antibiotics are started to treat established glomerulonephritis. Correct: post-streptococcal glomerulonephritis is immune-mediated after the infection, so management is supportive, with attention to blood pressure and fluid balance.
- Trap: a child with rheumatic fever is discharged after a single course of penicillin. Correct: rheumatic heart disease needs long-term secondary prophylaxis with benzathine penicillin G, because recurrences cause progressive valve damage.
Frequently asked questions
Which test confirms a streptococcal cause of sore throat?
A rapid antigen detection test or throat culture confirms Group A streptococcal pharyngitis, and culture is the reference method. Children with a negative rapid test often have a throat culture to exclude a false negative.
What are the major Jones criteria for rheumatic fever?
The major Jones criteria are carditis, polyarthritis, chorea, erythema marginatum and subcutaneous nodules. Diagnosis needs two major criteria, or one major and two minor criteria, along with evidence of a preceding streptococcal infection.
How does post-streptococcal glomerulonephritis differ from IgA nephropathy?
Post-streptococcal glomerulonephritis is a nephritic syndrome with low C3 that appears one to three weeks after pharyngitis. IgA nephropathy usually has normal C3 and presents with haematuria that coincides with an upper respiratory infection.
Which drug combination is used for necrotising fasciitis from Group A streptococci?
Surgical debridement is the cornerstone, with high-dose penicillin given alongside clindamycin. Clindamycin is added because it suppresses toxin synthesis in Group A streptococci, which lowers the superantigen load.
Why is benzathine penicillin used in rheumatic heart disease?
Benzathine penicillin G is a long-acting intramuscular penicillin given at regular intervals for secondary prophylaxis in rheumatic heart disease. It prevents recurrent streptococcal infection and further valve damage, which is why it continues for years.