S5956 Mistaken Identity: A Rare Case of Hepatic Actinomycosis Initially Suspected as IgG4-Related Disease

Introduction: Hepatic actinomycosis is a rare bacterial infection caused by Actinomyces, often misdiagnosed due to its indolent course and nonspecific presentation. IgG4-related disease (IgG4-RD) is a fibroinflammatory condition that can affect the hepatobiliary system usually resulting in cholestasis, jaundice, and rarely, liver masses. We present a unique case of hepatic actinomycosis initially suspected as IgG4-RD. Case Description/Methods: A 71-year-old man presented in August 2024 with abdominal pain, night sweats, and 40-pound weight loss. Prior computed tomography (CT) abdomen in February 2024 revealed a 10 cm liver mass. Liver biopsy showed prominent inflammatory infiltrate. He was admitted to our institution in June 2024 for similar complaints. CT abdomen revealed large right hepatic hypodense mass with scattered small right hepatic hypodense lesions. Entamoeba, echinococcus, histoplasma, leishmania antibodies, AFP, CA 19-9, CEA, C3/4, ANA, syphilis were all unremarkable. Liver biopsy in June 2024 showed abundant mixed inflammatory cells and associated fibrosis. Spirochete, fungal, and acid-fast bacilli stains were negative. IgG and IgG4 immunostains showed a ratio of IgG4:IgG greater than 40% and IgG4 positive plasma cells were greater than 100/HPF. During admission in August 2024 for continued fever, chills and failure to thrive, CT abdomen showed findings similar to prior. MRI liver just 2 days later revealed interval new rim enhancing lesions. IgG level was 3,220 mg/dL (649 mg/dL subclass 4). Rheumatology was consulted for IgG4-RD and remarked that this would be a very unusual presentation of IgG4-RD. The patient continued to have fever and little clinical improvement despite being on piperacillin/tazobactam. Liver biopsy in August 2024 showed mixed inflammation, IgG4:IgG more than 30%, IgG4 positive plasma cells greater than 50/HPF and, after meticulous review, sulfur granules. Sulfur granules are suggestive of Actinomyces infection. Rheumatology advised against immunosuppression given low likelihood of IgG4-RD. The patient was started on treatment for actinomycosis with augmentin. Outpatient CT abdomen in December 2024 showed interval improvement of abscesses with no rim enhancement. Discussion: This is a rare case of hepatic actinomycosis mimicking IgG4-related hepatobiliary disease. Actinomyces infection may induce an immune response that leads to elevated IgG4 levels. This case contributes to the limited number of available case reports on hepatic actinomycosis and highlights potential diagnostic pitfalls.