Avian bornaviruses (ABV), representing a new genus within the family Bornaviridae, were recently discovered in parrots from North America and Israel with proventricular dilatation disease (PDD). We show here that closely related viruses are also present in captive European parrots of various species with PDD. The six ABV strains that we identified in clinically diseased birds are new members of the previously defined ABV genotypes 2 and 4. Viruses of both genotypes readily established persistent, noncytolytic infections in quail and chicken cell lines but did not grow in cultured mammalian cells in which classical Borna disease virus strains replicate very efficiently. ABV antigens were present in both the cytoplasm and nucleus of infected cells, suggesting nuclear replication of ABV. The genome organization of avian and mammalian bornaviruses is highly conserved except that ABV lacks a distinct control element in the 5' noncoding region of the bicistronic mRNA encoding the viral proteins X and P. Reverse transcription-PCR analysis demonstrated the presence of virus in many, if not all, organs of birds with PDD. Viral nucleic acid was also found in feces of diseased birds, suggesting virus transmission by the fecal-oronasal route. Immunohistochemical analysis of organs from birds with PDD revealed that infection with ABV is not restricted to cells of the nervous system. Thus, ABV exhibits a broad tissue and cell tropism that is strikingly different from classical Borna disease virus.