BARTONELLA ENDOCARDITIS MIMICKING ANCA-ASSOCIATED VASCULITIS

Infective endocarditis (IE) and ANCA-associated vasculitis (AAV) both cause systemic inflammation and multi-organ involvement, often making differential diagnosis difficult. Although the Duke's criteria emphasize positive blood cultures, up to 30% of IE cases remain culture-negative. Bartonella species are important causative organisms in such cases, particularly in patients with cat exposure or valvular disease.

Case Presentation:
A 76-year-old man underwent biological aortic valve replacement for severe aortic stenosis. Six months later, he developed acute kidney injury (Cr 4.06 mg/dL), hematuria, and proteinuria. Laboratory tests showed PR3-ANCA 82.3 U/mL, hypocomplementemia (C3 50 mg/dL, C4 5 mg/dL), and elevated CRP 7.85 mg/dL. Blood cultures were negative. Echocardiography revealed new vegetations on the prosthetic valve. Renal biopsy showed immune complex-mediated glomerulonephritis with IgM, C3, and C1q deposits, inconsistent with pauci-immune AAV. Given his cat exposure, Bartonella endocarditis was suspected, and antibiotic therapy with rifampicin, doxycycline, and ceftriaxone was initiated. Despite therapy, vegetations enlarged, and CT/MRI demonstrated Infectious aneurysms. Emergency valve replacement was performed, and PCR of the excised valve confirmed Bartonella henselae infection.

Discussion:
Bartonella IE frequently presents with ANCA positivity (≈60% of ANCA-positive IE), likely due to molecular mimicry between bacterial peptides and PR3 or TLR9-mediated B-cell activation. Elevated dsDNA antibody levels may also occur without lupus manifestations. These immune responses complicate differentiation from autoimmune vasculitis.

Conclusion:
Bartonella henselae endocarditis can mimic ANCA-associated vasculitis with immune-complex nephritis. Thorough clinical evaluation and molecular diagnostic testing are essential to avoid misdiagnosis and to guide appropriate antimicrobial therapy.