Thereafter, virus suspensions obtained after a freeze–thaw cycle were inactivated using binary ethyleneimine (BEI) according to standard procedures[9]. Inactivated preparations were either mixed with PolygenTM (MVP Technologies, Omaha, USA), a co-polymer based adjuvant,or Emulsigen®-D (MVP Technologies, Omaha, USA), an oil-in-water emulsion with depot effect that incorporates dimethyldioctadecyl-ammonium bromide (DDA) as T-cell stimulant (20% v/v).For challenge infection, a spleen suspension was produced using spleen material from a previous animal trial.