Replication Defective Cyprinid Herpes Virus-3 (CyHV-3) as a Combined Prophylactic Vaccine in Carps

Aquacultured koi and common carp fish (Cyprinus carpio) are intensively bred as ornamental and food fish in many countries worldwide. Hatcheries of carp and koi have recently suffered massive financial damages due to two viral diseases caused by the Cyprinid herpesvirus-3 (CyHV-3), previously designated as Carp Interstitial Nephritis and Gill Necrosis Virus (CNGV) and Koi herpesvirus (KHV), and by the Spring Viremia of Carp Virus (SVCV). CyHV-3 is a large dsDNA virus, which is infectious mostly to koi and common carp, while SVCV is a rhabdovirus with a relatively broad host range. Both viruses induce contagious disease with mortality rate up to 90%.   Strategies for the control of viral infection in fish are of limited use. While efforts to prevent introduction of infectious agents into culture facilities are desirable, such exclusion strategies are far from fail-safe. Extensive vaccination methods that are useful for use in aquaculture facilities produce weak immunity, when used with proteins or inactivated viruses.  Methods to overcome this obstacle are to vaccinate the fish with large amounts of antigen and/or use adjuvant and immune modulators over a long period. These techniques usually require individual handling of the fish.  On the other hand, live attenuated virus is efficient and economical when used as an immersionvaccine.  However, this technique poses certain environmental risks and thus may be difficult to license and scale up.  Another option is a vaccine based on the replication defective virus (RDV) (pseudovirus), which can infect cells, but is unable to produce infectious particles. This vaccine may circumvent many of the problems related to attenuated-live vaccine (e.g., inadvertent infection and reversion to the virulent strain).