PREVENTION EFFICACY OF VACCINES AGAINST THE TRANSMISSION AND VIRULENCE EVOLUTION OF INFECTIOUS HEMATOPOIETIC NECROSIS VIRUS IN RAINBOW TROUT Oncorhynchus mykiss

Vaccine usage in aquaculture is rapidly expanding.   Many vaccines have proven to be  a powerful tool for managing clinical disease in aquatic animals.  However, vaccine impacts on pathogen transmission are rarely investigated and remain largely unknown.  In  the  few cases where studied, many vaccines have shown poor efficacy at blocking transmission, despite effectively reducing disease levels .   This is problematic because vaccines that do not prevent transmission can allow for the persistence of pathogens in host populations.  This is likely to greatly hinder eradication efforts and long-term disease management.  Furthermore, t heory predicts  that vaccines that  reduce clinical disease but do not block transmission, can select for the evolution of increased pathogen virulence.  Such vaccines may allow hosts infected with highly virulent pathogen strains to survive longer than unvaccinated host, thereby increasing the transmission duration of the pathogen.   Evidence of vaccine induced virulence evolution has been found in Marek's disease in the poultry industry.  However, investigation of this phenomenon is limited.

We investigated three vaccine types, DNA, inactivated, and attenuated, against infectious hematopoietic necrosis virus (IHNV) in rainbow trout.  Each vaccine was evaluated for its ability to prevent host clinical disease , viral shedding, transmission, and virulence evolution.  To assess disease prevention efficacy, vaccinated and unvaccinated fish were exposed in batch to a natural immersion challenge of IHNV and mortality tracked over course of infection.  To assess viral shedding, water samples were collected  from individual  infected  fish  at numerous time points then processed by q uantitative  PCR targeting the virus .  To assess transmission, the proportion of naïve fish infected after cohabitation with infected fish was quantified.   To assess virulence evolution, the virus was subjected to multiple rounds of transmission between fish in a serial passage design, then the virulence of the passaged virus  was  compared to the unpassaged virus.

We found that the  three vaccine types differed in their disease prevention efficacy, with the DNA vaccine being the most effective, followed by the inactivated vaccine.  All vaccines allowed for significant levels of viral shedding and transmission, despite strong disease protection (Figure 1).  Both increases and decreases in virulence were observed after passage of virus through vaccinated fish, although changes were small.  In conclusion, this work demonstrates the importance of assessing the transmission blocking efficacy of vaccines in addition to disease prevention.  This is likely to result in more effective and sustainable disease management.