Infective Endocarditis Due to Coxiella Burnetii in a Patient With Prosthetic Valve: A Diagnostic Challenge in Culture-Negative Endocarditis

A 56-year-old woman with a biological mitral prosthesis implanted in July 2023 was admitted with an insidious course of low-grade fever for approximately two months. She reported close contact with stray cats and a history of cutaneous sporotrichosis. Transthoracic and transesophageal echocardiography revealed an image suggestive of vegetation on the prosthetic ring. Given the high suspicion of prosthetic-valve infective endocarditis (IE), PET-CT was performed and showed periprosthetic hyperuptake compatible with active infection. Despite these suggestive findings, serial blood cultures remained negative. In this context, the etiologic investigation was expanded to include fastidious microorganisms such as Bartonella, Brucella, fungi, and Coxiella burnetii. Considering her contact with animals, a zoonotic agent was strongly suspected. Molecular testing revealed a positive PCR for C. burnetii, confirming the diagnosis of chronic Q fever with IE. The chronic form of this zoonosis manifests predominantly as endocarditis, especially in individuals with preexisting valvular disease or prosthetic valves. Diagnosis is based on PCR and serology, with PCR now considered a major microbiological criterion in the Duke criteria updated in 2023. The presence of prosthetic vegetations associated with abnormal uptake on PET-CT also constitutes a major criterion of endocardial involvement. Negative blood cultures are expected because C. burnetii is an obligate intracellular bacterium that is difficult to culture. Treatment requires prolonged doxycycline plus hydroxychloroquine, with periodic serologic monitoring. Although ruminants are the main source of infection, infected cats are also recognized as urban reservoirs. Transmission occurs primarily via inhalation of aerosols contaminated with feces, urine, or birth products. This case underscores the importance of broadened clinical reasoning in IE with negative cultures, particularly in patients with risk exposures for zoonoses, and highlights the role of complementary methods such as PET-CT and molecular testing in achieving a precise diagnosis.