A 5 year old, 26kg, mixed breed dog was presented to Cornell’s Emergency Service after treatment for ongoing lethargy, hyporexia, and hypersalivation was unsuccessful. Physical examination revealed a shifting leg lameness, diffuse muscle atrophy, and a new systolic heart murmur. Clinicopathological testing revealed mild anemia, hyperglobulinemia, hyperfibrinogenemia, and elevated D-dimers. An echocardiogram showed a vegetative lesion on the pulmonic valve and mild pulmonic insufficiency; subsequent blood cultures revealed no growth, and a Bartonella PCR was negative. Synovial fluid analysis was consistent with mild, mixed inflammation. The patient was presumptively diagnosed with endocarditis, and treated with amoxicillin/ clavulanic acid, enrofloxacin, and doxycycline. The patient’s clinical presentation did not improve at his subsequent recheck, and prednisone was started after the dog developed ventral edema; following steroid administration, the dog improved significantly. This report will discuss the physical exam findings, diagnostic testing, and pathophysiology of endocarditis with secondary immune-mediated polyarthritis.