Our previous study reported that persistently infected (PI) cattle of bovine viral diarrhea virus (BVDV) have co-infected with BVDV/END and /END that promote and inhibit host's type-I interferon (IFN) production, respectively. However, the relationship between co-infection of immunologically distinct BVDVs and persistent infection as well as the biological significance of END viruses remains unknown. Experiments using cultured cells revealed that END virus, which is unable to propagate in situations where the host's immune response is induced by IFN-α addition, is able to propagate under those conditions when co-infecting with END virus. These results indicate that BVDV/END can coexist with BVDV/END and that co-infection with END viruses supports the propagation of END viruses. Our in vitro experiments strongly suggest that co-infection with END virus is involved in the maintenance of persistent infection of BVDV.