Introduction
Epitheliotropic viruses can be particularly dangerous in the aquatic environment, which is osmotically and microbiologically hostile to fish. If the pathogen is able to disrupt the mucosal barrier, the breach can lead to severe secondary effects, such as disruption of the osmotic balance or induction of secondary infections. We used a live attenuated virus vaccine against cyprinid herpesvirus 3 (CyHV-3) to investigate which aspects of non-direct protection are important in the aquatic environment and to fill the remaining knowledge gaps on how these vaccines effectively protect fish against the deadly disease caused by CyHV-3.
Materials and methods
Common carp were vaccinated against CyHV-3 using a double-deletion vaccine virus KHV-TΔDUT/TK in the absence or presence of a mixture of common carp beta-defensins 1, 2 and 3. Fish were challenged with a hyper-virulent Polish isolate of CyHV-3 2.5 months after vaccination. Blood, skin, gill and kidney samples were collected at 2, 7, 14, 28 days post vaccination and challenge for monitoring of immune responses by SNT, RT-qPCR using Fluidigm and disease related pathology.
Results
Vaccination induced mild clinical signs, low viral load and slight up-regulation of cd8 and igm gene expression in vaccinated fish, while blood neutralising activity increased from 14 days post-vaccination. A challenge infection with CyHV-3 induced severe disease with 80-100% mortality in unvaccinated fish, whereas no mortality was observed in vaccinated fish and the viral load was >1000-fold lower. Histological analysis showed that vaccination protected against pathological changes in the skin and gills. In the skin of non-vaccinated fish, T and B cell responses were severely downregulated, inflammatory and stress responses were increased upon challenge, whereas vaccinated fish had enhanced neutrophil, T and B cell responses. Disruption of skin and gill barrier elements (tight and adherence junctions, desmosomes, mucins) led to a severe osmotic imbalance and an uncontrolled increase in skin and gill bacterial load, which most likely exacerbated the pathology.
Conclusions
Using a live attenuated virus vaccine, we show that increased neutrophil, T and B cell responses provide protection against CyHV-3 infection and preserve skin integrity and gill function, supporting successful protection against secondary bacterial pathogens and osmotic disruption, enabling vaccinated carp to cope with the hostile aquatic environment.