Capnocytophaga canis is a zoonotic pathogen not previously linked to infective endocarditis (IE). We report the first case of recurrent IE caused by C. canis, initially presumed to be culture-negative. The diagnosis was established through prolonged blood culture and 16S rRNA-based analysis.

A 66-year-old man had two hospitalizations within six months for culture-negative IE. No causative pathogen was identified, as all 16 blood culture sets remained negative. Two months after discharge, he presented again with fever and peripheral embolic signs. Echocardiography showed vegetations on both the mitral and aortic valves, consistent with recurrent IE. Prolonged incubation of repeat blood cultures yielded Gram-negative rods on the seventh day. Conventional methods failed to identify the organism. Subsequent 16S rRNA-based analysis at a reference laboratory identified C. canis. The patient had close daily contact with a domestic cat, suggesting a zoonotic source. He underwent double valve replacement and received six weeks of intravenous antibiotics, with a favorable outcome.

This case provides some clinical insights. First, C. canis should be considered as a potential pathogen in culture-negative IE, particularly in patients with animal exposure. Second, C. canis IE may present with a subacute or recurrent course with minimal valvular damage, making early recognition difficult. Careful history-taking, prolonged blood cultures, and molecular diagnostic methods are essential for accurate evaluation of culture-negative IE.