First reported case of Capnocytophaga cynodegmi infective endocarditis: A diagnostic odyssey.

Capnocytophaga cynodegmi, a commensal of canine and feline oral flora, is rarely implicated in human infections, with most cases limited to localized soft tissue infections. We present the first case of C. cynodegmi-associated infective endocarditis (IE) in a 39-year-old man with bicuspid aortic valve and alcohol use disorder. The patient presented with sepsis, aortic valve vegetations, and systemic complications, including heart failure and shock liver. Despite negative blood and valve cultures, metagenomic sequencing of plasma (Karius test) initially detected Capnocytophaga canimorsus, while targeted Next-Generation Sequencing (NGS) of explanted valve tissue confirmed C. cynodegmi (100% match). The patient underwent valve replacement and completed a 6-week course of ampicillin-sulbactam with clinical recovery. This case underscores the diagnostic challenges of fastidious pathogens and demonstrates the potential of C. cynodegmi to cause life-threatening IE. It highlights the necessity of advanced molecular diagnostics, such as NGS, in atypical cases of IE. Clinicians should consider zoonotic Capnocytophaga spp. in culture-negative IE, particularly in high-risk patients with animal exposure or valvular abnormalities.