Characterization of novel pesti- and astro-like viruses in hatchery-reared European seabass (Dicentrarchus labrax) associated with health disorders and low mortality

Abstract European seabass is a major farmed species in Mediterranean aquaculture. Its early life stages are very sensitive to sanitary and environmental conditions and are therefore closely monitored. In this study, we characterized new viruses recently isolated from fingerlings in a hatchery showing whitish three-dimensional lesions on the fins and sides and distended abdomens and investigated their pathologic potential. A cytopathic effect was observed 17 days post-inoculation (dpi) of homogenized internal organs at 20 °C on a CHSE 214 cell line. Viral presence was confirmed using the indirect fluorescent antibody test (IFAT) targeting double-stranded RNA patterns. Next-generation sequencing (NGS) and phylogenetic analyses were used to characterize two viruses: a pesti-like virus of 13,123 nt detected both in positive supernatant and organ homogenate, named Dicentrarchus labrax Pesti-like virus (DLPLV); and an astro-like virus with a sequence of 5,068 nt detected only in the homogenate and clustering with other described fish astroviruses, and named Dicentrarchus labrax Astro-like virus (DLALV). In vivo, direct or waterborne contact of healthy fish with fish showing clinical signs induced very sporadic and fleeting clinical signs, while bath infection with DLPLV alone induced 8 to 16% mortality for 162-days post-hatching fish. Seroneutralization tests demonstrated the existence of a humoral response specific to DLPLV, regardless of the route of infection. This work highlights the wide diversity of aquatic viruses and illustrates the complexity of reproducing experimentally the clinical signs observed in hatcheries, especially in the case of multifactorial phenomena. The diagnostic tools developed will be usable for surveillance and fast detection of any new clinical cases.