Immunity to viral haemorrhagic septicaemia (VHS) following DNA vaccination of rainbow trout at an early life-stage

Rainbow trout fry of average weight 0.5 g were vaccinated against viral haemorrhagic septicaemia (VHS) by intramuscular injection of 1 microg of plasmid DNA encoding the VHS virus glycoprotein gene. Challenge with a lethal dose of virus at two different time points, 9 and 71 days post-vaccination respectively, revealed that a highly protective and lasting immunity was established shortly after vaccination, in accordance with earlier experiments with larger fish. The defence mechanisms activated by the DNA vaccine are thus functional at an early life-stage in rainbow trout.