Histoplasma capsulatum Prosthetic Valve Endocarditis Diagnosed via Cell-Free DNA Sequencing

<i>Histoplasma capsulatum</i> (<i>H. capsulatum</i>), a dimorphic fungus endemic to the Mississippi and Ohio River Valleys, is a rare cause of fungal infective endocarditis. Here, we present a case of prosthetic valve endocarditis due to <i>H. capsulatum</i> diagnosed via microbial cell-free DNA (cfDNA) sequencing when traditional methods, such as urine antigen and blood cultures, fail. A 56-year-old male patient with a history of a bioprosthetic mitral valve replacement presented with chest pain, confusion, and constitutional symptoms. Initial laboratory results, negative blood cultures, and echocardiographic findings were consistent with a diagnosis of culture-negative endocarditis. To identify a potential pathogen not detected by blood cultures, both urine <i>Histoplasma</i> antigen testing and the Karius Test®, a next-generation sequencing-based cfDNA assay, were performed concurrently. While traditional methods, including blood cultures and urine antigen testing, were negative, the Karius Test® identified <i>H. capsulatum</i>, prompting initiation of antifungal therapy. The patient subsequently showed clinical improvement, evidenced by the resolution of symptoms and a reduction in the size of the valvular lesion. This case highlights the diagnostic challenges of fungal endocarditis and underscores the utility of cfDNA sequencing in identifying elusive pathogens when conventional tests are inconclusive. Early recognition and targeted antifungal therapy are crucial for improving patient outcomes in culture-negative endocarditis cases.