First reported case of Capnocytophaga canimorsus pituitary abscess.

Background: Capnocytophaga canimorsus is a rare human pathogen, typically associated with exposure to dog saliva. It is a slow-growing, Gram-negative anaerobe, with clinical manifestations being sepsis, meningitis, or endocarditis. Pituitary abscesses are themselves uncommon, accounting for <1% of all pituitary lesions, and are often misdiagnosed preoperatively due to their nonspecific presentation and frequent culture negativity. We present the first documented case of a pituitary abscess caused by C. canimorsus, highlighting the diagnostic value of 16S ribosomal RNA (rRNA) polymerase chain reaction (PCR) when conventional microbiological methods are inconclusive. Case Description: A 69-year-old woman presented following a collapse with symptoms including hypernatremia, new-onset atrial fibrillation, and thyrotoxicosis. Initial sepsis workup-including cerebrospinal fluid analysis-suggested possible encephalitis. Brain magnetic resonance imaging revealed a cystic, contrast-enhancing pituitary lesion with central diffusion restriction. The patient underwent endoscopic transsphenoidal surgery, with intraoperative findings consistent with abscess formation. Despite direct culture yielding no growth, 16S rRNA PCR of the aspirated fluid identified C. canimorsus. Histology revealed xanthogranulomatous inflammation. The patient was treated with a 6-week course of intravenous ceftriaxone, with subsequent clinical and radiological recovery. Importantly, she had no known exposure to animal bites or saliva and no underlying immunosuppressive condition. Conclusion: This case is the first to implicate C. canimorsus in the formation of a pituitary abscess, expanding the spectrum of its potential clinical presentations. It underscores the diagnostic limitations of conventional microbiological techniques in deep-seated intracranial infections and supports the role of 16S rRNA PCR as a critical adjunct when routine cultures are negative. Clinicians should maintain a high index of suspicion for atypical pathogens in sellar lesions and consider molecular diagnostics early, particularly when histopathology reveals inflammatory features suggestive of infection without an identifiable organism.