Cutaneous Actinomycosis of the Perineum in an Immunocompetent Man.

We present a case of primary cutaneous actinomycosis of unclear pathogenesis. A 30-year-old-man with no significant medical or surgical history presented to the emergency department with a 2-week history of a tender perineal mass. The patient denied trauma or perforating injury to the area. Examination of the area revealed an indurated, nonfluctuant, erythematous papulonodule located 2 cm from the anus. The lesion was unresponsive to oral and topical antibiotics and was therefore excised. The excision specimen revealed a dense mixed infiltrate partially filling the reticular dermis and extending into the subcutaneous fat. The infiltrate surrounded grains of basophilic material with an outer rim of eosinophilic radiating Splendore-Hoeppli material. Within the grains, filamentous bacteria were highlighted with Periodic acid-Schiff and Grocott's methenamine silver. The organisms were gram-positive and acid-fast negative. Given the clinical and histopathologic findings, actinomycosis was diagnosed. Two weeks later, the patient reported resolution of symptoms. The patient was lost to follow-up. This case highlights an unusual presentation of actinomycosis and the crucial role histopathology plays in diagnosis.