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Borna disease virus (BDV), a unique genetically highly conserved RNA virus (Bornaviridae; Mononegavirales), preferentially targets neurons of limbic structures causing behavioral abnormalities in animals. Markers and virus in patients with affective disorders and schizophrenia have raised worldwide interest. A persistent infection was suggestive from follow-up studies, but inconstant detectability weakened a possible linkage.This study for the first time discloses that detection gaps are caused by BDV-specific circulating immune complexes (CIC), and their interplay with free antibodies and plasma antigens (p40/p24). Screening 3000 sera each from human and equine patients over the past 4 years by new enzyme immunoassays (EIAs) revealed that BDV-CICs indicate 10 times higher infection rates (up to 30% in controls, up to 100% in patients) than did previous serology. Persistence of high amounts of CICs and plasma antigens correlates with severity of depression. Even BDV RNA could be detected in plasma samples with strong antigenemia. Our discovery not only explains the course of persistent infection, but offers novel easy-to-use diagnostic tools by which new insights into BDV-related etiopathogenesis of disease and epidemiology are possible.