To investigate the biological characteristics of field isolates of Borna disease 
virus (BDV), as well as to understand BDV infections outside endemic countries, 
we isolated the virus from brain samples of a heifer with Borna disease in 
Japan. We demonstrate that the brain lysate contained replication products of 
BDV and induced viral propagation in rat glioma cells, suggesting that a 
replication-competent BDV existed in the bovine brain. This field strain of BDV, 
named Bo/04w, showed efficient viral release and transmissibility and also 
displayed a distinct pattern of expression of viral phosphoprotein (P) during 
infection, as compared with laboratory-adapted BDV strains. Interestingly, we 
found the level of P to be significantly low in cells infected with Bo/04w, and 
the transcription of this isolate to be more efficient than that of laboratory 
strain of BDV. These results indicated that the field isolate may regulate the 
expression of P at an optimal level in infected cells. We also confirmed that 
Bo/04w maintains biological significance in neonatal gerbil brain. Sequencing 
revealed that despite the biological differences, the field isolate is closely 
related genetically to the laboratory strains of BDV. We discuss here the 
sequence similarities between BDV isolates from endemic and nonendemic 
countries.