A bright, red sharply demarcated rash that is caused by group A-beta-hemolytic streptococci, Perianal Streptococcal Dermatitis. Symptoms include itching, rectal pain, and in one third of patients, blood-streaked stools. It primarily affects children between six months and 10 years and is often initially misdiagnosed. A rapid strep test of the perianal area may confirm the diagnosis. Treat with amoxicillin 40 mg/kg/day divided into three doses and/or topical applications of mupirocin (Bactroban) tid x 10 days.
Source
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Brilliant, L. MD. "Perianal Streptococcal Dermatitis" American Family Physician. 12 Jan 2000.