Abstract
The European sea bass (Dicentrarchus labrax) is an important farmed fish species in the Mediterranean area, very sensitive to the infection by encephalopathy and retinopathy virus (VERv), or Betanodavirus, which causes massive mortalities. Effective vaccines to fight the pathology are not yet available and in this work we describe a promising intraperitoneal immunization route against VERv of sea bass juveniles. We performed intraperitoneal and immersion immunization trials with a VERv (isolate 283.2009 RGNNV) inactivated by formalin, β-propiolactone and heat treatment. Interestingly, the intraperitoneal immunization with formalin-inactivated VERv induced a significant antigen-specific IgM production, differently from other inactivation protocols. However, the same formalin-inactivated antigen resulted in very low IgM antibodies when administered by immersion. Following the intraperitoneal injection with formalin-inactivated virus, the quantitative expression of the antiviral MxA gene showed a modulation of transcripts in the gut after 48 h and on head kidney after 24 h, whereas ISG12 gene was significantly up-regulated after 48 h on both tissues. In immersion immunization with formalin-inactivated VERv, a modulation of MxA and ISG12 genes after 24 h post-treatment was detected in the gills. An effective uptake of VERv particles in the gills was confirmed by immunohistochemistry using anti-VERv antibodies. Lastly, in challenge experiments using live VERv after intraperitoneal immunization with formalin-inactivated VERv, we observed a significant increase (81.9%) in relative survival percentage with respect to non-immunized fish, whereas immersion immunization resulted in no protection. Our results suggest that intraperitoneal immunization with formalin-inactivated VERv could be a safe and effective strategy to fight Betanodavirus infection in European sea bass.Electronic supplementary materialThe online version of this article (doi:10.1186/s13567-016-0376-3) contains supplementary material, which is available to authorized users.
Highlights
Viral encephalopathy and retinopathy (VER), known as viral nervous necrosis (VNN), is a severe infectious disease characterized by neuropathological changes and associated with high mortality in several fish species.Viruses belonging to the Nodaviridae family, genus Betanodavirus, are non-enveloped single strand positive RNA virus characterized by an extremely high resistance to chemical and physical agents [1]
Immunological analyses Anti-encephalopathy and retinopathy virus (VERv) specific immunoglobulin M (IgM) was detected by Indirect enzyme-linked immunosorbent assay (ELISA) in serum samples after immersion and intraperitoneal immunization with the virus inactivated by different ways
Betanodavirus infection is characterized by a high pathogenicity and is associated with extensive mortality in fish farms [27]
Summary
Viral encephalopathy and retinopathy (VER), known as viral nervous necrosis (VNN), is a severe infectious disease characterized by neuropathological changes and associated with high mortality in several fish species.Viruses belonging to the Nodaviridae family, genus Betanodavirus, are non-enveloped single strand positive RNA virus characterized by an extremely high resistance to chemical and physical agents [1]. Viral encephalopathy and retinopathy (VER), known as viral nervous necrosis (VNN), is a severe infectious disease characterized by neuropathological changes and associated with high mortality in several fish species. Most of them have been tested on grouper (Epinephelus spp.), very likely the most important and valuable fish amongst the VER-susceptible species [8]. Sea bass (Dicentrarchus labrax) is the second most farmed species in the Mediterranean Sea (FEAP 2005–2014) and highly susceptible to VER. All stages of sea bass are highly sensitive to Betanodavirus but mortality can vary depending on the fish age and water temperature [10]. Infected larvae and juvenile stages often show flexing of the body, muscle tremors and abnormal swimming behaviour, which includes vertical positioning and spinning resulting from traumatic lesions [11]. In adults, where the mortaly rate can reach 50–60% [10, 12], the most common clinical sign is abnormal swimming
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