Dual-Positive ANCA-Associated Vasculitis Triggered by Parvovirus B19 and Vaccination in a Young Adult with Diffuse Alveolar Hemorrhage

Introduction: ANCA-associated vasculitis (AAV) typically presents in middle-aged patients with renal-pulmonary syndromes. Dual positivity for PR3 and MPO ANCA is rare and often associated with drug or infection related vasculitis. We describe a diagnostically challenging case of biopsy-proven microscopic polyangiitis (MPA) in a young adult with severe pulmonary involvement, preserved renal function, and recent immunologic triggers. Case Description: A 33-year-old male presented with fatigue, anemia and hypoxia four weeks after flu and mRNA COVID-19 boosters. Imaging revealed bilateral ground-glass opacities; bronchoscopy confirmed diffuse alveolar hemorrhage. Labs showed Hb 5.8 g/dL, positive PR3- and MPO-ANCA, and Parvovirus B19 IgM/IgG. Despite creatinine of 1.2 mg/dL and eGFR >60 ml/min, urinalysis revealed microscopic hematuria and proteinuria (UPCR ~300 mg/g). Kidney biopsy confirmed pauci-immune necrotizing glomerulonephritis. He received steroids and rituximab with marked clinical recovery. Discussion: This case elucidates under-characterized AAV phenotype: pulmonary-dominant disease with minimal renal impairment. Dual ANCA positivity, though uncommon, often suggests a complex or secondary autoimmune trigger. In this case, the co-occurrence of recent vaccinations and Parvovirus B19 IgM suggests synergistic immune priming—supporting the role of environmental stimuli in the pathogenesis of vasculitis. Despite preserved renal function, biopsy was critical in establishing diagnosis and guiding timely immunosuppression. DAH in the absence of hemoptysis or creatinine elevation may lead to diagnostic delays; this highlights the need for high clinical suspicion and early ANCA testing in similar pulmonary presentations. Therapeutically, this case supports rituximab as effective monotherapy for induction in non-severe renal AAV, consistent with RAVE and PEXIVAS findings. Avoidance of alkylating agents and plasmapheresis was possible due to early recognition and stabilization.