Development of vaccines against VHS and IHN: oral application, molecular marker and discrimination of vaccinated fish from infected populations

Viral haemorrhagic septicaemia virus (VHSV) and infectious haematopoietic necrosis virus (IHNV) cause severe epizootics among most salmonid fish as well as a number of marine species. The role of the marine reservoir in the epidemiology is unclear. This epidemiological situation, i.e. the wide distribution among a lot of species, gives rise to the belief that it seems to be unsuccessful to control the disease by regulatory methods in regions in which the virus is enzootic, i.e. where feral fish carry the virus. Therefore, the development of a vaccine seems to be more promising. There is now one live vaccine against VHS available in Germany. For identification of attenuated viral strains as vaccines, molecular markers are needed. The G-genes of several VHSV strains, including two attenuated strains, were sequenced to define marker for specific recognition of the attenuated virus. The deduced amino acid sequences were compared to elucidate the molecular basis of attenuation. For rapid differentiation of the vaccine-strain from wild-type viruses, RT-PCR was used with primers specific for the attenuated virus. The PCR-product is a fragment of 251 bp which is used for identification of the vaccine after isolation of a virus from the field. The administration of an inactivated vaccine seems to be unsuitable for rainbow trout fry. Therefore, recent studies have focused on the use of recombinant viral proteins as candidates for vaccines, with particular attention given to the glycoprotein. Rainbow trout vaccinated by immersion with live Aeronionas salmonicida bacteria carrying VHS-G-gene and IHN-G-gene fragments were differentiated from survivors after infection with wild-type virus by western blot: sera from vaccinated fish react only with G-protein and not with other structural components of the virus.