Increase of virus yields and releases of Borna disease virus from persistently infected cells

Borna disease virus grows to low titres in persistently infected cells with an infectious particle to cell ratio of 0.01 to 0.05. Inclusion of n-butyrate in the growth medium enhances infectivity yields up to 1 log. This effect is time and concentration dependent. In hypertonic medium with an excess of NaCl, KCl or Na2SO4 up to 50% of the total infectious virus yield is released from the cells. Released supernatant virus (buoyant density in sucrose rho = 1.22 g/cm3) is more heat stabile than cell-bound virus (rho = 1.18 g/cm3). The access to cell-free (released) virus opens new possibilities for the characterization of this neurotropic agent.