Rheumatic Fever and Post-Streptococcal Glomerulonephritis
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Direct answer
Rheumatic fever is an immune-mediated inflammatory disease that follows a pharyngeal group A streptococcal infection by about two to four weeks, through molecular mimicry between streptococcal M protein and human cardiac tissue. Post-streptococcal glomerulonephritis is a separate nephritic syndrome that follows pharyngitis or skin infection, and it is distinguished by low serum complement. Both are sequelae of streptococcal infection, but they differ in target organ and mechanism.
What you must remember
- Revised Jones criteria: two major criteria, or one major and two minor criteria, with evidence of a preceding streptococcal infection, establish a first attack of acute rheumatic fever (the 2015 revision adds subclinical carditis as a major criterion and monoarthritis or polyarthralgia as major criteria in moderate- to high-risk populations such as India).
- Major criteria: carditis, polyarthritis, Sydenham chorea, erythema marginatum and subcutaneous nodules.
- Minor criteria: fever, arthralgia, raised ESR or CRP, and a prolonged PR interval on the ECG.
- Evidence of infection: a raised or rising ASO titre, a positive throat culture or rapid antigen test, or a raised anti-DNase B titre.
- Carditis: pancarditis with the mitral valve most often affected, and Aschoff bodies in the myocardium.
- Secondary prophylaxis: deep intramuscular benzathine penicillin G every 3 to 4 weeks prevents recurrence in rheumatic heart disease, and it is central to rheumatic heart disease control in India.
- Post-streptococcal glomerulonephritis: haematuria, oedema and hypertension after pharyngitis or impetigo, with low C3 that returns to normal within about eight weeks.
Comparing the two streptococcal sequelae
Both diseases follow a streptococcal trigger, but the target organ and the complement pattern differ. The table sets out the features that exam questions most often test.
| Feature | Rheumatic fever | Post-streptococcal GN |
|---|---|---|
| Preceding infection | Pharyngitis | Pharyngitis or impetigo |
| Latent period | About 2 to 4 weeks | 1 to 2 weeks after throat, 3 to 6 weeks after skin |
| Mechanism | Molecular mimicry, autoimmunity | Immune complex deposition |
| Serum complement | Normal | Low C3 |
| Main features | Carditis, arthritis, chorea | Haematuria, oedema, hypertension |
Both conditions are delayed immune responses, with the streptococcus usually cleared by the time symptoms begin. The low C3 of post-streptococcal glomerulonephritis is the key discriminator in exam questions, because rheumatic fever leaves complement levels unchanged. A nephritic picture with low complement after a sore throat should bring this diagnosis to mind first.
Haematuria, puffy eyes and raised blood pressure in a child one to two weeks after a sore throat form the classic set. Urine microscopy showing red cell casts supports the diagnosis.
Where the exam tries to catch you
Streptococcal sequelae questions test the diagnostic criteria and the complement pattern.
- Trap: using the ASO titre alone to diagnose rheumatic fever Correct: ASO shows past streptococcal infection, so it supports the Jones diagnosis rather than proving it, and anti-DNase B helps after skin infection.
- Trap: expecting low complement in rheumatic fever Correct: complement is normal in rheumatic fever, whereas low C3 is the hallmark of post-streptococcal glomerulonephritis.
- Trap: expecting antibiotics to reverse established glomerulonephritis Correct: penicillin may clear any active infection, but it does not reverse glomerular injury, so treatment is supportive.
Frequently asked questions
What are the major Jones criteria for acute rheumatic fever?
The major Jones criteria are carditis, polyarthritis, Sydenham chorea, erythema marginatum and subcutaneous nodules. Acute rheumatic fever is diagnosed with two major criteria, or one major plus two minor criteria, together with evidence of a preceding group A streptococcal infection.
Why is low serum C3 typical of post-streptococcal glomerulonephritis?
Low serum C3 is typical of post-streptococcal glomerulonephritis because immune complexes activate and consume complement in the glomerulus. C3 usually returns to normal within about eight weeks, so a persistently low value should prompt a search for another cause.
Why does rheumatic fever cause valve damage?
Rheumatic fever causes valve damage through molecular mimicry, in which antibodies and T cells against streptococcal M protein cross-react with cardiac tissue. The mitral valve is most commonly affected, and the resulting inflammation leads to fibrosis, regurgitation or stenosis over time.
What does a raised ASO titre prove in rheumatic fever?
A raised ASO titre proves only that a group A streptococcal infection occurred in the recent past, not that rheumatic fever is present. ASO titres usually peak about three to five weeks after infection, so they support the Jones diagnosis rather than confirming it alone.