Mechanism of Heart Disease in Scarlet Fever
Heart disease in scarlet fever occurs through the same autoimmune mechanism as rheumatic heart disease—the Group A Streptococcus (GAS) infection that causes scarlet fever triggers an abnormal autoimmune response in genetically susceptible individuals, where antibodies against streptococcal M-protein cross-react with cardiac tissue proteins, leading to inflammatory damage of heart valves and myocardium. 1, 2
The Autoimmune Cascade
Scarlet fever is caused by GAS pharyngitis, the same organism responsible for rheumatic fever and subsequent rheumatic heart disease. 1 The critical pathogenic sequence involves:
Molecular mimicry: The M-protein serotypes of GAS share structural similarities with human cardiac tissues, particularly cardiac myosin. 1, 3 This cross-reactivity is the fundamental mechanism driving cardiac damage.
Latent period: After the initial GAS pharyngitis (which manifests as scarlet fever), there is a symptom-free interval of 14-21 days before the autoimmune reaction begins. 1
Autoantibody formation: The immune system generates antibodies against multiple cardiac targets including cardiac myosin, heart valve fibroblasts, and smooth muscle. 3 These autoantibodies are present in higher titers in patients who develop carditis compared to those with uncomplicated streptococcal infections. 3
Cardiac Tissue Damage Mechanisms
The heart damage occurs through combined humoral and cell-mediated autoimmune mechanisms:
Valvular inflammation (valvulitis): The mitral valve is primarily affected, with the anterior leaflet tip showing abnormal coaptation and regurgitation. 1 Aortic valve involvement is less common and rarely occurs in isolation. 1
Myocardial involvement: Mononuclear cell infiltration into heart tissue causes myocarditis. 4, 5 Evidence suggests lymphocyte influx and oxygen-free radical generation contribute to myocardial damage. 5
Complement activation: Both complement and anti-M protein antibodies can activate cardiac endothelium, contributing to valvular inflammation. 4
Progressive Nature with Repeated Exposure
Recurrent GAS infections (including repeated scarlet fever episodes) accelerate cardiac damage:
Each subsequent GAS exposure enhances the anti-cardiac myosin antibody response, breaking immune tolerance further. 4
With each recurrent infection, more inflammatory cells infiltrate heart tissue, leading to progressive valvular damage. 4
Experimental models demonstrate that repetitive exposure to GAS M-protein causes P-R interval prolongation on ECG, indicating conduction system involvement. 4
Chronic Sequelae
Without prevention, the acute inflammation transitions to chronic structural damage:
Valvular fibrosis and calcification: Fusion of commissures with scarring and eventual calcification leads to permanent stenosis and/or regurgitation. 1, 2
Long-term complications: Include atrial fibrillation, heart failure, stroke from thromboembolic events, and infective endocarditis on damaged valves. 1, 2
Epidemiologic significance: After recovery from initial rheumatic fever, 60-65% of patients develop valvular heart disease. 1, 2
Critical Prevention Point
The window for preventing cardiac damage is narrow but definitive: Antibiotics can be started up to 9 days after symptom onset and still prevent rheumatic fever. 6 This underscores that treating scarlet fever promptly with appropriate antibiotics (Penicillin V 250-500 mg every 6-8 hours for 10 days) is essential for primary prevention of cardiac complications. 6
Secondary prophylaxis with continuous penicillin is crucial for patients who have had one episode of rheumatic fever to prevent recurrent GAS infections and further valvular damage. 1, 2
Important Clinical Caveat
The severity of scarlet fever itself has declined over 150 years, with reduced subsequent cases of rheumatic fever. 7 However, sporadic outbreaks with rheumatogenic GAS strains still occur, making vigilant treatment of all GAS pharyngitis (including scarlet fever) essential to prevent this devastating cardiac complication. 7