Occult disseminated Nocardia farcinica infection with subsequent ANCA-negative crescentic glomerulonephritis: a diagnostic challenge—a case report

Nocardiosis may follow a prolonged and clinically nonspecific course, making diagnosis difficult even when an infectious etiology is actively investigated. We report an unusual case of occult disseminated Nocardia farcinica infection with subsequent antineutrophil cytoplasmic antibody (ANCA)-negative crescentic glomerulonephritis, a clinical evolution that mimicked an immune-mediated systemic disease. A 68-year-old White European man presented with persistent unexplained systemic inflammation, pulmonary ground-glass opacities, interstitial lung involvement, and mediastinal lymphadenopathy. Extensive infectious, autoimmune, hematological, and oncological investigations failed to establish a diagnosis. On retrospective reassessment, a febrile illness shortly after travel to Indonesia, together with potential environmental exposure during the trip, became a plausible epidemiological clue, although the source of infection could not be established. One year after the onset of systemic symptoms, he developed rapidly progressive kidney dysfunction with nephrotic-range proteinuria and microscopic hematuria. Kidney biopsy demonstrated crescentic glomerulonephritis with chronic tubulointerstitial inflammation, while serological testing for ANCA and anti-glomerular basement membrane antibodies was negative. Corticosteroid therapy for a presumed immune-mediated process resulted in transient clinical, renal, and radiologic improvement but may have facilitated progression of the underlying infection. Six months after initiation of corticosteroid therapy, cavitary pulmonary lesions and multiple deep abscesses developed. Prolonged culture of material aspirated from an iliacus abscess identified Nocardia farcinica . Targeted antimicrobial therapy resulted in regression of inflammatory lesions and improvement in kidney function. The chronology supported the interpretation of a probable infection-associated crescentic glomerulonephritis, although a causal relationship could not be definitively established. Disseminated nocardiosis may remain occult despite extensive diagnostic investigation and may mimic immune-mediated disease as its clinical phenotype evolves. In patients with crescentic glomerulonephritis accompanied by unexplained systemic or pulmonary manifestations, particularly when serological findings are negative or discordant, infection should remain an active consideration before and during immunosuppressive therapy, even after an initially unrevealing infectious workup. Retrospective reassessment of chronology and environmental exposures may provide critical diagnostic clues as new manifestations emerge.