How to obtaint high titre BRSV? - (Dec/12/2018 )

Hi all, I am trying to cultivate bovine respiratory syncytial virus (BRSV) on MDBK cells but my TCID50 is still low (around 5.6 no more). I need to cultivate it in high titre (ideally around 8.0).

Do you have any idea how to do this? I´ve tried a lot of things - different time of collection, different cultivation temperature, different composition of growing medium - but the result was always the same - not so high titre.

I have also found a references in the literature that mention that most BRSV viruses remain associated with sycytial cells. This could be the cause of a low virus titer in the collection. Please, does anyone have experience with high titre BRSV cultivation?

Nowbody routinelly cultivate BRSV of human respiratory syncytial virus?

Please, I need in high titre, but everything what I´ve done was without effect.