The Silent Mimic: Actinomyces and the Enigmatic Gastric Ulcer.

Background: Primary gastric actinomycosis is a rare infection caused by anaerobic Actinomyces species, normally colonizing the oral and gastrointestinal flora. Often misdiagnosed as malignancy due to atypical presentation and non-specific findings, it typically affects immunocompromised individuals or those with mucosal damage from surgery or trauma. Objective: The aim of this case report is to explore the diagnostic challenges and the necessity of high clinical suspicion for rare infectious causes of gastrointestinal pathology, especially given its atypical presentation. Case Presentation: A 63-year-old woman presented with hematemesis, melena, and epigastric pain. Laboratory tests revealed severe anaemia, hypoalbuminemia, and elevated cancer antigen 19-9 (CA 19-9). Gastroscopy revealed oesophageal erosions and a gastric ulcer with a wide, black-coloured base on a polypoid lesion, along with small erosions. Histopathological findings showed filamentous and rod-like bacteria, consistent with Actinomyces, in gastroscopic biopsies. The patient was then treated with high-dose intravenous penicillin G. Conclusion: This case highlights the diagnostic challenges of gastric actinomycosis, which can be mistaken for peptic ulcers or malignancy. Clinicians should consider gastric actinomycosis, especially in immunocompromised patients, when imaging or endoscopy reveals a submucosal lesion or ulcer.