Epitheliocystis in Greater Amberjack: Evidence of a Novel Causative Agent, Pathology, Immune Response and Epidemiological Findings

Abstract

Epitheliocystis is a fish gill disease caused by a broad range of intracellular bacteria infecting freshwater and marine fish worldwide. Here we report the occurrence and progression of epitheliocystis in greater amberjack reared in Crete (Greece). The disease appears to be caused mainly by a novel Betaproteobacteria belonging to the Candidatus Ichthyocystis genus with a second agent genetically similar to Ca. Parilichlamydia carangidicola coinfecting the gills in some cases. After a first detection of the disease in 2017, we investigated epitheliocystis in the following year’s cohort of greater amberjack juveniles (cohort 2018) transferred from inland tanks to the same cage farm in the open sea where the first outbreak was detected. This cohort was monitored for over a year together with stocks of gilthead seabream and meagre co-farmed in the same area. Our observations showed that epitheliocystis could be detected in greater amberjack gills as early as a month following the transfer to sea cages, with ionocytes at the base of the gill lamellae being initially infected. Cyst formation appears to trigger a proliferative response, leading to the fusion of lamellae, impairment of gill functions and subsequently to mortality. Lesions are characterized by infiltration of immune cells, indicating activation of the innate immune response. At later stages of the outbreak, cysts were no longer found in ionocytes but were observed in mucocytes at the trailing edge of the filament. Whole cysts appeared finally to be expelled from infected mucocytes directly into the water, which might constitute a novel means of dispersion of the infectious agents. Molecular screening indicates that meagre is not affected by this disease and confirms the presence of previously described epitheliocystis agents, Ca. Ichthyocystis sparus, Ca. Ichthyocystis hellenicum and Ca. Similichlamydia spp., in gilthead seabream. Prevalence data show that the bacteria persist in both gilthead seabream and greater amberjack cohorts after first infection.

Phylogenetic analyses of Candidatus Branchiomonas cysticola refine the taxonomic classification of Betaproteobacteria associated with epitheliocystis in fish

“…4 ). This is in compliance with previous phylogenies using 16S rRNA sequences (Contador et al 2016 ; Seth-Smith et al 2016 ; Cascarano and Katharios 2022 ; Cascarano et al 2022 ). Consequently, it is reasonable to believe that all taxa of this clade share the same common ancestor of Cand .…”

“…Ichtyocystis hellenicum). Recent studies have identified new agents of this genus associated with epitheliocystis in reared greater amberjack ( Seriola dumerili ), wild-caught Pacific herring ( Clupea pallasii ) and Pacific mackerel ( Scomber japonicus ) (Cascarano et al 2022 ), as well as from pompano ( Trachinotus ovatus ) (Cascarano and Katharios 2022 ).…”

“…A single locus 16S rRNA Bayesian phylogenetic analysis including 37 representative type strains of Betaproteobacteria and 17 bacterial strains associated with epitheliocystis in fish (Contador et al 2016 ; Seth-Smith et al 2016 ; Cascarano et al 2022 ; Cascarano and Katharios 2022 ) was conducted in MrBayes. Pseudomonas aeruginosa strain PAO1 T was used as an outgroup.…”

“…Ichthyocystis spp., causing epitheliocystis in fish species, have also been identified, and based on 16S rRNA analyses these bacteria represent close relatives to Cand . B. cysticola (Seth-Smith et al 2016 ; Cascarano et al 2022 ; Cascarano and Katharios 2022 ). Even though 16S rRNA is the only common sequence information available from these Candidatus species, it could be assumed that they all belong to the same family within Betaproteobacteria.…”

“…The continuous expansion of epitheliocystis outbreaks associated with Betaproteobacteria in fish species worldwide (Contador et al 2016 ; Seth-Smith et al 2016 ; Cascarano et al 2022 ; Cascarano and Katharios 2022 ), stresses the need for a clarification/refining of the taxonomic affiliations of these bacteria. We hence propose a revised taxonomic classification of Cand .…”