Diagnosis of Pelvic Actinomycosis and Fusobacterium Infection Using 16S rRNA Gene Analysis for Formalin-Fixed Paraffin-Embedded (FFPE) Tissue After Intrauterine Device (IUD) Removal

Severe pelvic infections present a diagnostic challenge owing to the clinical and radiological resemblance to malignancy. Pelvic actinomycosis has been implicated in the exacerbation of other anaerobic infections. Herein, we present the case of a 65-year-old woman who developed severe pelvic actinomycosis and <i>Fusobacterium</i> infection eight months after intrauterine device (IUD) removal. The patient presented with lower abdominal pain. Imaging revealed an enlarged uterus with extensive parametrial invasion, bilateral hydronephrosis, and rectal stenosis, mimicking uterine sarcoma. Laboratory tests revealed elevation in the levels of inflammatory markers and renal dysfunction. Surgery was performed under suspicion of malignancy; however, histopathological examination revealed no malignant cells. Grocott staining identified filamentous organisms consistent with <i>Actinomyces</i>; however, conventional cultures of ascites were negative. Next-generation sequencing (NGS) of 16S ribosomal RNA (rRNA) genes extracted from the formalin-fixed paraffin-embedded (FFPE) tissue revealed a mixed infection of <i>Actinomyces israelii</i> and <i>Fusobacterium nucleatum</i>. The hydronephrosis resolved after treatment, with no recurrence by the six-month follow-up. This case highlights the fact that pelvic actinomycosis and <i>Fusobacterium</i> infection can develop even after IUD removal, with cases potentially mimicking gynecological malignancy. To our knowledge, this is the first reported case of pelvic actinomycosis and <i>Fusobacterium</i> infection diagnosed by 16S rRNA gene analysis for FFPE tissue using NGS in a patient after IUD removal. Mixed infection involving <i>Actinomyces israelii</i> and <i>Fusobacterium nucleatum</i> may increase pathogenicity through synergistic interactions. When conventional cultures fail to identify the causative organism, molecular techniques, such as 16S rRNA gene analysis, could provide a definitive diagnosis, thereby enabling appropriate therapies.