Comparing the protection of attenuated and inactivated virus vaccines against pancreas disease and heart and skeletal muscle inflammation

Pancreas disease (PD), caused by Salmonid alphavirus (SAV), and Heart and skeletal muscle inflammation (HSMI), caused by Piscine orthoreovirus-1 (PRV-1), are important infectious causes behind the experienced 20 % loss of Norwegian farmed salmon during their grow out phase in the sea. In ViVaAct, experimental live vaccine models for these two diseases were tested. Better control and fewer disease outbreaks of PD and HSMI can be significant contributions to future sustainability of the aquaculture industry. Studies of how different experimental vaccines affect the host's immune system and if they provide protection throughout the production period in the sea are key. Vaccines for salmon in Norway are mostly administered by injection into the abdominal cavity, but it has not been mapped whether vaccination mobilize different subpopulations of B cells in the abdominal cavity than elsewhere. Researchers at NMBU, UiT and VI, with extensive experience in fish virology and immunology, are part of ViVaAct. Good vaccines against PD and HSMI should provide protection at the same level as achieved after natural infection. This involves stimulation of the innate and adaptive immune responses such that long-term protection is obtained. The use of live virus vaccines is common in mammals, including humans, and such vaccines provide long-term protection. We have developed live, attenuated variants of SAV and characterized different genetic variants of PRV. In two vaccine trials of short (10 weeks) and long (6 months) duration, respectively, we compared the protective effect of live vaccines. Immunization and challenge trials will not only study the protection, but also involves measuring immune responses. The short-term vaccine trial carried out against HSMI used live vaccine consisted of PRV-2 or PRV-3 which are PRV variants natural in brown trout and Coho salmon, respectively and do not cause disease in Atlantic salmon. That is, we used a virus from one animal species as a vaccine against another virus in another animal species. This principle is well known in vaccine science and was used, for example, when using the smallpox virus as a vaccine to protect people against smallpox. PRV cannot be grown in cell lines, and PRV-3 was therefore produced from blood from experimentally infected rainbow trout. In the short-term trial, the salmon vaccinated with PRV-3 were not only completely protected against HSMI, but in addition almost completely protected against infection with PRV-1. PRV-2 gave only partial protection. The short-term experiment against PD with live mutated SAV gave more variable results, as the SAV variants that were attenuated in cell culture were not completely attenuated in injected fish. In the long-term trial against PD, we therefore used three further weakened, mutated SAV variants. The immunization protected against clinical disease signs as well as pathological changes, and no mortality were observed. Vaccination with two of the variants did not prevent re-infection of virulent virus upon exposure after 20 wpi and these two variants were deemed to be too much attenuated. While the third variant protected better and was found to be a promising vaccine candidate. Immunization with attenuated virus gave better growth of the fish than that of the fish given virulent virus, but less growth than uninfected salmon. Live, attenuated viruses can theoretically mutate back into virulent variants. The experiments with live vaccines were therefore carefully investigated regarding mutations We found no reversion to virulence in any of the trials. Live, attenuated SAV vaccine may have a future as a field vaccine, but this requires a careful adjustment of virulence and excretion, and in addition genetic stability.

Vi har vist at en levende vaksine mot HSMB gir full beskyttelse og hindrer også infeksjon med PRV-1. Aberet er at vaksinen ikke kan dyrkes i cellekultur, men må produseres i regnbueørret. Dersom man hadde oppnådd å dyrke denne vaksinestammen i kultur ville veien til markedet vært veldig kort. Levende, genmodifisert SAV som vaksine mot PD har vi vist virker tildels bra mot sykdom, men vaksinen kan ikke hindre infeksjon med SAV. Den beskytter mot sykdom, men ikke infeksjon, og kan derfor ikke hindre påvisning av SAV og eventuelle restriksjoner som anlegg kan få derav. En genmodifisert SAV vil regnes som GMO, og det er nok fortsatt et mentalt hinder å passere før oppdrettsnæringen vil ta i bruk GMO vaksiner.

In order to ensure growth in Norwegian aquaculture, optimal disease control must be obtained. Two of the diseases causing outbreaks in a vast number of Norwegian fish farms are Pancreas disease (PD) caused by salmonid alphavirus (SAV2 and SAV3), and heart and skeletal muscle inflammation (HSMI) caused by Piscine orthoreovirus (PRV). Recent studies have demonstrated that functional vaccines may be developed against these diseases, but only suboptimal protection have been obtained with existing methods. Standard vaccines using inactivated viruses depend on targeted adjuvants for protective effects, but their ability to stimulate the immune system is less efficient compared to an infecting virus. To be able to fine-tune vaccine effects into optimal protection, more in depth understanding of the host-agent interaction and the pathogen-specific protective mechanisms are needed. Building on a solid background and scientific record on these specific viral infections and the Atlantic salmon immune system, using tools and methodology developed in the preceding ViVaFish project (237315/E40). The PIs behind ViVaACT aim to characterize the specific mechanisms that separate the host immune response triggered by attenuated or inactivated viruses and their protective potential, focusing primarily on SAV and PRV. This project may identify clues to protective vaccination against PD and HSMI and related viral diseases in fish.