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 n ecrosis virus (IHNV) cause severe epizootics among most salmonid fish as we ll as a number of marine species. The role of the marine reservoir in the e pidemiology is unclear. This epidemiological situation, i.e. the wide distr ibution among a lot of species, gives rise to the belief that it seems to b e unsuccessful to control the disease by regulatory methods in regions in w hich the virus is enzootic, i.e. where feral fish carry the virus. Therefor e, the development of a vaccine seems to be more promising. There is now on e live vaccine against VHS available in Germany. For identification of attenuated viral strains as vaccines, molecular marke rs are needed. The G-genes of several VHSV strains, including two attenuate d strains, were sequenced to define marker for specific recognition of the attenuated virus. The deduced amino acid sequences were compared to elucida te the molecular basis of attenuation. For rapid differentiation of the vac cine-strain from wild-type viruses, RT-PCR was used with primers specific f or the attenuated virus. The PCR-product is a fragment of 251 bp which is u sed for identification of the vaccine after isolation of a virus from the f ield. The administration of an inactivated vaccine seems to be unsuitable for rai nbow trout fry. Therefore, recent studies have focused on the use of recomb inant viral proteins as candidates for vaccines, with particular attention given to the glycoprotein. Rainbow trout vaccinated by immersion with live Aeromonas salmonicida bacte ria carrying VHS-G-gene and IHN-G-gene fragments were differentiated from s urvivors after infection with wild-type virus by western blot: sera from va ccinated fish react only with G-protein and not with other structural compo nents of the virus.