Journal of Fish Diseases

ABSTRACTDespite recent genomic studies and increased molecular data, epitheliocystis remains an enigmatic fish disease with no experimental in vitro or in vivo models to aid the advancement of research. In this study, we revert to a classical microscopical approach and screen with the electron microscope the epitheliocystis lesions caused by a Ca. Parilichlamydia sp., infecting mucus cells in Greater amberjack. We report distinct morphological features of this bacterial family, characterised by Intermediate Bodies that closely resemble those of previously described Candidatus similchlamydia, and Elementary Bodies that exhibit morphological similarities to Chlamydia trachomatis. We describe the characteristics of a novel Chlamydial Inclusion Membrane (IM) type, with abundant interdigitations, possibly shaped by fusion of the IM with cytoplasmic vesicles, and moreover discuss the presence of multivesicular bodies in the infected cell. Our observation of immune cells in the infected areas indicates an interaction of macrophages with infected cells, a role for granular cells as pathogens reservoirs and an active phagoptosis process in the nearby areas, overall shedding light on cellular immune processes characterising these infections in fish hosts.

AbstractChlamydial infections are recognised as causative agent of epitheliocystis, reported from over 90 fish species. In the present study, the farmed striped catfish Pangasianodon hypophthalmus (14–15 cm, 70–90 g) with a history of cumulative mortality of about 23% during June and July 2015, were brought to the laboratory. The histopathological examination of gills from the affected fish revealed presence of granular basophilic intracellular inclusions, mostly at the base of the interlamellar region and in gill filaments. A concurrent infection with Trichodina spp., Ichthyobodo spp. and Dactylogyrus spp. was observed in the gills. The presence of chlamydial DNA in the gills of affected fish was confirmed by amplification and sequencing of 16S rRNA gene. BLAST‐n analysis of these amplicons revealed maximum similarity (96%) with Candidatus Actinochlamydia clariae. On the basis of phylogenetic analysis, it was inferred that the epitheliocystis agents from striped catfish were novel and belonged to the taxon Ca. Actinochlamydia. It is proposed that epitheliocystis agents from striped catfish will be named as Ca. Actinochlamydia pangasiae. The 16S rRNA gene amplicons from novel chlamydiae were labelled and linked to inclusions by in situ hybridisation. This is the first report of epitheliocystis from India in a new fish host P. hypophthalmus.

AbstractIntracellular inclusions containing chlamydia‐like organisms are frequently observed in the gill epithelial cells of Atlantic salmon, Salmo salar L., cultured in fresh water in Ireland. In this study, the causative agent was identified in four separate freshwater sites, using 16s rRNA sequencing, as ‘Candidatus Clavochlamydia salmonicola’. Histopathology and real‐time (RT) PCR were used to further assess infections. The prevalence of infection ranged from 75–100% between sites and infection intensity was highly variable. No significant lesions were associated with these infections. As a diagnostic tool, RT‐PCR proved marginally more sensitive than histopathology. The fate of ‘Candidatus Clavochlamydia salmonicola’ in Atlantic salmon post‐seawater transfer was investigated in a 12‐week marine longitudinal study. Both RT‐PCR and histopathological examination indicate that the organism disappears from the gills 4–6 weeks post‐transfer.

AbstractAn enteritic syndrome affecting farmed rainbow trout and occurring predominantly in summer has recently been described and related to the massive accumulation of a segmented filamentous bacterium (SFB), ‘Candidatus Arthromitus’, in the digestive tract of fish. Details of the clinical manifestations, the characteristics of the bacterium and the possible aetiology of the disease are presented in this paper. No other bacterial pathogen than ‘Candidatus Arthromitus’ was characterized in trout undergoing the enteritic syndrome. From clinical and histopathological observations, a toxin or toxic product accumulation was suspected in affected trout, and could account for the massive destruction of the intestinal mucosa. However, the origin and aetiological role of the SFB could not be clearly elucidated, although most of the filaments appeared to beviable and to produce and release strings of endospores in the lumen of the gut. The organism could not be cultured in the conditions tested, shared several morphological and structural characteristics with related bacteria described in the intestinal microflora of different vertebrate species, but no attachment structure was definitively recognized. All attempts to detect similar SFB in the fish diet orin the environment of affected fish farms were unsuccessful. It is unclear whether this organism represents a normal and not previously detected inhabitant of the intestine of trout stressed after environmental perturbation, or whether it results directly from external sources of contamination.