Scarlet fever

Streptococcus pyogenes strains carrying a lysogenic bacteriophage produce erythrogenic exotoxins A, B, and C, which act as superantigens, cross-linking MHC class II to the T-cell receptor beta chain outside the antigen groove and causing massive polyclonal T-cell activation with IL-2, IFN-gamma, and TNF release. Clinically a child with fever and exudative pharyngitis develops a blanching, finely papular rash beginning on the trunk and spreading outward, rough like sandpaper, accentuated in skin folds as Pastia lines, with flushed cheeks and circumoral pallor; the tongue is first coated white then bright red and papillated. Desquamation of hands and feet follows in one to two weeks. Diagnosis is by rapid antigen detection with throat culture backup in children. Treatment with penicillin or amoxicillin shortens illness, reduces transmission, and prevents acute rheumatic fever, although it does not prevent post-streptococcal glomerulonephritis. Differential includes Kawasaki disease, staphylococcal scalded skin syndrome, toxic shock syndrome, and drug eruption.