Chest wall tissue microsporidiosis associated with metagenomic next-generation sequencing detection of Encephalitozoon cuniculi: a case report

Encephalitozoon cuniculi is an obligate intracellular microsporidian that typically causes clinically apparent disease in immunocompromised hosts, whereas symptomatic infection in individuals without recognized immunodeficiency is uncommon. We report a 57-year-old man without known major immunodeficiency who presented with a tender mass in the right anterior chest wall. Color Doppler ultrasound revealed a predominantly cystic lesion between the right pectoralis major muscle and the adjacent costal cartilage. Histopathological examination of a core-needle biopsy showed a reactive spindle-cell proliferation with prominent inflammatory infiltrates but was not etiologically diagnostic. Metagenomic next-generation sequencing (mNGS) of the biopsy specimen detected Encephalitozoon cuniculi , with no other pathogens reaching the reporting threshold. Subsequent periodic acid–Schiff (PAS) and Grocott’s methenamine silver (GMS) staining highlighted small spore-like structures compatible with microsporidia. Based on the combined findings, tissue microsporidiosis associated with mNGS detection of E. cuniculi was considered the most likely diagnosis, and albendazole therapy was initiated. Albendazole was administered for 3 months and subsequently discontinued. At telephone follow-up after completion of the 3-month treatment course, the patient reported resolution of the palpable mass and tenderness. Follow-up contrast-enhanced chest CT on August 13, 2026 showed no residual right anterior chest wall lesion. This case highlights an unusual presentation of tissue microsporidiosis as a chest wall mass and underscores the diagnostic value of mNGS as an adjunct when conventional histopathology is inconclusive.