Genetic vaccination of rainbow trout against viral haemorrhagic septicaemia virus: small amounts of plasmid DNA protect against a heterologous serotype

Viral haemorrhagic septicaemia (VHS) is known as one of the most important diseases in cultured rainbow trout in Europe. An efficient vaccine is highl y desirable, but so far only limited success has been obtained with traditi onal products based on killed or attenuated virus. Genetic immunization wit h a plasmid vector containing the VHS virus glycoprotein gene under the con trol of a cytomegalovirus promoter has recently been shown to induce high l evels of protection against the homologous virus isolate. Expressed glycopr otein could be detected immunohistochemically in fish muscle and about 70% of the vaccinated animals had neutralizing antibodies in their serum. To fu rther evaluate the potential of the DNA Vaccine technology for prophylaxis of VHS, a vaccination trial including lower doses of DNA and different viru s isolates was performed. Eight weeks after injection, rainbow trout were c hallenged by immersion with the homologous virus isolate or with a serologi cally different isolate. Cumulative mortalities demonstrated that even the lowest dose of DNA tested (0.1 mu g per fish) induced protective immunity a gainst both virus isolates. Virus neutralization tests in cell culture indi cated that trout sera neutralized VHS virus isolates independently of serot ypes defined with mammalian mono- and polyclonal antibodies. No protecion w as observed following vaccination with a plasmid construct carrying the VHS virus nucleocapsid-protein gene. (C) 1999 Elsevier Science B.V. All rights reserved.