Etiology
Scarlet fever is caused by pyrogenic exotoxin-producing strains of Streptococcus pyogenes (also called group A streptococcus, or GAS). S pyogenes are gram-positive cocci that grow in chains; they can be found in the throat or on the skin of infected patients. S pyogenes does not usually cause problems in asymptomatic carriers.[10] However, some virulent forms produce pyrogenic exotoxins, also known as streptococcal superantigens, that cause the characteristic scarlatiniform rash, strawberry tongue, and desquamation of the skin seen in scarlet fever.[11][12]
Pathophysiology
The pathognomonic scarlatiniform rash of scarlet fever is caused by the circulating pyrogenic exotoxin produced by Streptococcus pyogenes, the infectious agent, as a result of local production of inflammatory mediators and alteration of the cutaneous cytokine milieu. This results in a sparse inflammatory response and blood vessel dilation.[12]
The mechanisms by which these exotoxins may contribute to the symptoms of scarlet fever are not well studied, but they have been found to alter immunoglobulin G production and reticuloendothelial clearance function, induce T-cell proliferation, and enhance skin reactivity.[13]
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