Highly specific serological diagnosis of Borna disease virus 1 (BoDV-1) and variegated squirrel bornavirus 1 (VSBV-1) encephalitis by novel antibody isotype assay with multiple viral antigens

BACKGROUND: Human bornavirus encephalitis is an emerging, severe and nearly uniformly fatal zoonotic disease in Germany. The etiological pathogens so far encompass the Borna disease virus 1 (BoDV-1) and the variegated squirrel bornavirus 1 (VSBV-1). While BoDV-1 is at least harbored by the white-toothed shrew (Crocidura leucodon) as a natural reservoir and autochthonous in Germany, VSBV-1 has been detected in captive exotic squirrels with an unknown geographical origin. Clinically, a rapid progression is typical for both forms of bornavirus encephalitis, however, medical awareness is low, and therefore treatment attempts are notably delayed. Diagnosis relies on symptomatology, epidemiology, imaging, and virologic testing. One cornerstone of laboratory diagnosis is serology with limitations in sensitivity and specificity. OBJECTIVES: Here, we describe a newly developed spot immunoassay using recombinant BoDV-1 nucleoprotein (N), phosphoprotein (P), accessory protein X (X), and glycoprotein (GP) to detect bornavirus-reactive IgG, IgM and IgA antibodies. STUDY DESIGN: A comparatively large cohort encompassing 14 patients with BoDV-1 encephalitis and one individual with VSBV-1 encephalitis were tested. In addition, 241 patients with encephalitis of unknown etiology, 58 interference samples, as well as 40 blood donor samples were analyzed. RESULTS: The combined use of different antibody isotype-specific conjugates with four different BoDV-1-specific proteins (N/P/X/GP) while employing a newly developed evaluation scheme enabled a highly specific (97-100 %) diagnosis in patients with either form of bornavirus encephalitis, with a sensitivity of up to 92 %. CONCLUSIONS: The novel spot immunoassay is an easy-to-use approach for the specific and sensitive serological diagnosis of human bornavirus encephalitis.