B71-13 Not All That Glitters Is Tb: Legionella Micdadei Mimicking Miliary Tuberculosis

Abstract Introduction Legionnaires’ disease is a severe pneumonia caused by Legionella species, typically L. pneumophila serogroup 1, acquired through inhalation of contaminated aerosols from water sources. Non-pneumophila species such as Legionella micdadei are rare human pathogens and can exhibit weak acid-fast staining, often leading to confusion with mycobacterial infection. The usual radiographic findings include focal or multilobar consolidation, while miliary nodules are not characteristic. We report a case of L. micdadei pneumonia presenting with innumerable miliary nodules, closely mimicking miliary tuberculosis, illustrating an uncommon radiologic manifestation and diagnostic challenge. Case Presentation A 77-year-old man with a history of hyperlipidemia, benign prostatic hyperplasia, and gastroesophageal reflux disease presented with three weeks of intermittent fevers, chills, cough, headache, anorexia, unintentional weight loss, and progressive weakness following travel to Pakistan two months prior. He was afebrile and hemodynamically stable but appeared cachectic. Laboratory testing revealed leukocytosis (14.6 × 109/L), thrombocytosis (746 × 109/L), C-reactive protein 112 mg/L, and erythrocyte sedimentation rate 53 mm/h. Chest radiography showed diffuse small nodular densities; contrast-enhanced CT demonstrated innumerable miliary nodules throughout both lungs. The differential included tuberculosis, fungal infection, metastasis, and sarcoidosis. Given the radiographic pattern, empiric antitubercular therapy (RIPE) was initiated. Blood cultures, Quantiferon-TB, sputum acid-fast stains, and HIV testing were negative. Extensive infectious workup, including Pneumocystis, Aspergillus, Histoplasma, Blastomyces, and Legionella cultures, was unrevealing. Bronchoscopy with cultures and cytology was nondiagnostic. Subsequent microbial cell-free DNA sequencing (Karius test) detected Legionella species, confirming disseminated Legionella infection. RIPE therapy was discontinued, and he was discharged with oral Levofloxacin 750 mg every 48 hours for three weeks. The patient’s symptoms resolved, and he remained clinically stable on follow-up. Conclusion This case highlights an unusual radiologic presentation of L. micdadei infection manifesting as diffuse milliary nodules, a pattern typically associated with tuberculosis. Recognition of this atypical manifestation and the use of next-generation sequencing were critical for diagnosis and targeted therapy. Clinicians should maintain a broad differential when evaluating miliary patterns on imaging, especially when conventional microbiologic studies are negative. Early consideration of atypical Legionella species may prevent unnecessary prolonged antitubercular therapy and guide appropriate antimicrobial management. This abstract is funded by: none