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Scarlet fever is a disease caused by an infection with group A beta-hemolytic streptococcal bacteria. The rash initially appears on the neck and chest, then spreads over the body. While the rash is still red, the patient may develop Pastia's lines, bright red coloration of the creases under the arm and in the groin.

Scarlet fever (scarlatina) is an exotoxin-mediated disease arising from group A beta-hemolytic streptococcal infection. Ordinarily, scarlet fever evolves from a tonsillar/pharyngeal focus, although the rash develops in fewer than 10% of cases of "strep throat."  
Exotoxin-mediated streptococcal infections range from localized skin disorders (eg, bullous impetigo) to the systemic rash of scarlet fever to the uncommon but highly lethal streptococcal toxic shock syndrome.

Streptococci are gram-positive cocci that grow in chains. They are classified by their ability to produce a zone of hemolysis on blood agar and by differences in carbohydrate cell wall components.
Streptococci may be alpha-hemolytic (partial hemolysis), beta-hemolytic (complete hemolysis), or gamma-hemolytic (no hemolysis). Most streptococci excrete hemolyzing enzymes and toxins. Erythrogenic toxins cause the rash of scarlet fever. The erythema-producing toxin was discovered by Dick and Dick in 1924.
Group A streptococci are normal inhabitants of the nasopharynx. Group A streptococci can cause pharyngitis, skin infections, pneumonia, bacteremia, and lymphadenitis. Scarlet fever usually is associated with pharyngitis but, in rare cases, follows streptococcal infections at other sites.
Infections occur year-round, but the incidence of pharyngeal disease is highest in school-aged children (5-15 y) during winter and spring and in a setting of crowding and close contact. Person-to-person spread by respiratory droplets is the most common vector.
The incubation period for scarlet fever ranges from 12 hours to 7 days. Patients are contagious during the acute illness and during the subclinical phase.

In the US: Up to 10% of the population contracts group A streptococcal pharyngitis.
In the past century, the number of cases of scarlet fever has remained high, with marked decrease in case mortality rates secondary to widespread use of antibiotics.

The exanthem is diffusely erythematous; but, in some patients, it is more palpable than visible.
Exanthem usually has the texture of coarse sandpaper, and the erythema blanches with pressure.
The skin can be pruritic but usually is not painful.
A few days following generalization of the rash, it becomes more intense along skin folds and produces lines of confluent petechiae known as the Pastia sign. These lines are caused by increased capillary fragility.
The rash begins to fade 3-4 days after onset, and the desquamation phase begins. This phase begins with flakes peeling from the face. Peeling from the palms and around the fingers occurs about a week later and lasts for about a month after onset of the disease.

The appearance of the tongue also has a characteristic course in scarlet fever.
During the first 2 days of the disease, the tongue has a white coat through which the red and edematous papillae project. This is referred to as a white strawberry tongue.
After 2 days, the tongue also desquamates, resulting in a red tongue with prominent papillae called the red strawberry tongue..

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