18 YEARS OF VACCINATION AGAINST VIRAL HEMORRHAGIC SEPTICEMIA IN FRANCE

Viral haemorrhagic septicaemia (VHS) has been considered for many year s to be a major cause of loss in the French trout industry. The high p revalence of VHS in certain geographic areas made a control strategy b ased on control policy unfeasible. This provided the impetus for immun oprophylaxis development that resulted in 3 successive types of vaccin es: inactivated, live attenuated and recombinant vaccines. When delive red by intraperitoneal injection, the 2 propiolactone-inactivated VHS virus was immunogenic and/or protective for trout all of sizes, but it was not suitable for the practical immunization of alevin, the trout life stage that is the most sensitive to VHS. A carp cell-passed, atte nuated variant of the VHS virus was effective after both immersion or injection delivery and met the practical requirements of juvenile vacc ination. However, this vaccine was discarded because it retained some virulence that discouraged the launching of its commercialization. The n came the era of genetically engineered vaccines. The recombinant gly coprotein of VHSV produced in Escherichia coli or in Saccharomyces cer evisiae failed to protect fish whatever the route of delivery. A recom binant baculovirus vaccine was found to be immunogenic and protective against VHS, but only when delivered by injection. Due to its cost and route of delivery, the latter vaccine was not licensed. Simultaneousl y, the sudden occurrence of another rhabdovirosis, infectious haematop oietic necrosis (IHN), in France, rendered vaccination against VHS que stionable. Indeed, no cross-protection between these 2 rhabdoviroses e xists. If vaccination is still believed to be an effective control met hod for VHS, it should be based in the future upon an autoreplicative vaccine. Protection against IHN will also have to be taken into consid eration. It is also possible that certain technical devices will, some day, render the injection of inactivated IHN and VHS viruses acceptab le for the trout farming industry.