Abstract
Epitheliocystis is a bacterial disease characterized by the intracellular presence of replicating bacteria within inclusions in the gills and epithelial tissues of fish. This disease has been observed in various species of freshwater and marine fish worldwide and has been associated with different causative agents, including both chlamydial and non-chlamydial bacteria. In aquaculture settings, epitheliocystis-related mortalities primarily affect young fish, with mortality rates ranging from gradual and low to sudden and high. Recent findings suggest that this condition plays a significant role in multifactorial mixed gill infections leading to fish mortalities, highlighting the importance of gaining a deeper understanding of the underlying mechanisms governing these intracellular infections. The study of this disease is hindered by limited knowledge regarding the functioning of gill cells in different host species and, more importantly, the lack of viable in vitro cultivation methods f ...
Epitheliocystis is a bacterial disease characterized by the intracellular presence of replicating bacteria within inclusions in the gills and epithelial tissues of fish. This disease has been observed in various species of freshwater and marine fish worldwide and has been associated with different causative agents, including both chlamydial and non-chlamydial bacteria. In aquaculture settings, epitheliocystis-related mortalities primarily affect young fish, with mortality rates ranging from gradual and low to sudden and high. Recent findings suggest that this condition plays a significant role in multifactorial mixed gill infections leading to fish mortalities, highlighting the importance of gaining a deeper understanding of the underlying mechanisms governing these intracellular infections. The study of this disease is hindered by limited knowledge regarding the functioning of gill cells in different host species and, more importantly, the lack of viable in vitro cultivation methods for the bacterial agents involved.In this project, a first objective was to investigate the dynamics of local epizootics and identify the environmental factors contributing to mortalities. To achieve this, a one-year monitoring study was conducted at a single aquaculture farm site, focusing on three distinct fish species: gilthead seabream (Sparus aurata), meagre (Argyrosomus regius), and greater amberjack (Seriola dumerili). The study aimed to assess the presence of various pathogens associated with epitheliocystis, which are known to induce structural changes in the surrounding gill tissue, including both proliferative and non-proliferative reactions. Factors such as fish size and temperature are discussed in respect to parameters such as pathogen prevalence, outbreak onset and consequent mortality rates. In greater amberjack we observed different trends of prevalence for different groups of pathogens, and we related fish mortalities to a novel beta proteobacterial agent responsible for the proliferative reactions. Higher mortality rates were related to lower fish age, suggesting more severe outcomes in younger fish.Following this first screening, greater amberjack was subjected to extensive analysis (immunohistochemistry, histology, micro-CT and ultrastructure) that revealed significant differences between acute proliferative reactions that were attributed to the novel beta proteobacteria, Ca. Ichthyocystis seriolae and non-proliferative chronic lesions caused by Ca. Parilichlamydia sp. The two agents that coinfect the same host, target different cells, with the beta proteobacteria infecting chloride cells and the chlamydial agent infecting mucous cells. Ultrastructural analysis showed different bacterial morphologies in different stages of intracellular infection and, moreover, peculiar distinctive interactions with cell structures and organelles. Host immune cellular response was also observed to be completely different for the two infecting agents, with proliferative lesions displaying granulocytes and phagocytic cells, while infected mucous cells were observed to interact mostly with macrophages. Infected mast cells were also observed, which may have a role as dormant bacterial carriers.Whole Genome Sequencing (WGS) analysis (including long and short read technologies) was finally performed for Ca. Ichthyocystis seriolae, whose genome displays clear signs of genome reduction, typical of an obligate intracellular pathogen. Comparative analysis of gene families and genomic islands revealed differences and similarities with other species of the genus. Different proteins were related to disease characteristics observed previously by histology and transmission electron microscopy, TEM. Special attention was given to virulence factors and toxins that are related to the pathogenic potential of the novel bacterium.
show more
