The susceptibility of shi drum to betanodavirus depends on the rearing densities

Introduction: The diversification of species in aquaculture requires the continuous study of animal welfare parameters and the susceptibility to pathogens. An inadequate welfare state results in stress, poor health and increased severity of infectious diseases. In fact, several stress conditions have been related with increased mortalities and betanodavirus (NNV) spread. The shi drum (Umbrina cirrosa) has attracted attention due to its good growth rates and great adaptability to culture conditions together with a great flesh quality. However, this species is susceptible to the four genotypes of NNV and displays stress behavior at high rearing densities. Methodology: In order to determine the response of shi drum upon NNV infection at different stressed conditions, specimens reared at low or high density were experimentally infected. A transcriptomic study was performed comparing the gene expression pattern of several tissues of infected and non-infected specimens from the group showing mortalities. The presence of NNV in the gonad of survivors was analyzed. Results: The mortalities observed were low but only occurred at the group reared at high density. The transcriptomic study showed that the number of tissue-specific genes expressed increased in head-kidney, liver and brain, and decreased in spleen. In the four tissues, cell adhesion, leukocyte migration, cytokine interaction, cell proliferation, cell survival and autophagy related pathways were differentially expressed. NNV was detected in the gonad of surviving fish at all rearing densities analyzed. Conclusions: No differences in the viral capacity of colonizing the gonad were observed between low- and high-density conditions. At this last condition, NNV infection produced a deep systemic alteration in shi drum. The main pathways altered were related with immune response, cell cycle and autophagy, accordingly with studies performed in other fish species.