| dc.description.abstract | Gill disease is one of the main challenges in Norwegian aquaculture, with the bacterial disease epitheliocystis playing a crucial role in both marine and freshwater reservoirs. The pathology of epitheliocystis is characterized by intracytoplasmic and intravacuolar inclusions containing replicating bacteria in gill epithelial cells, eventually causing respiratory distress due to hyperplasia, hypertrophy, and necrosis of gill tissue. Following its first description in 2012, the betaproteobacteria Candidatus Branchiomonas cysticola has been shown to be one of the dominating causative agents of epitheliocystis. The obligate, intracellular nature of the bacterium complicates genetic characterization beyond the 16S rRNA gene due to the lack of cultivation methods. A recent study (Mjølnerød et al., 2022) identified several housekeeping (HK) genes through next-generation sequencing and fluorescence in situ hybridization, creating the basis for a genotyping scheme at the intraspecific level. Bayesian inferred phylogenetic analyses on the newly identified HK genes of Ca. Branchiomonas cysticola revealed a high degree of genetic homogeneity between isolates, suggesting the population structure of the bacterium constitutes one dominating clonal complex associated with epitheliocystis in Atlantic salmon. Additionally, with significant variations in amplified HK gene sequences and supporting evidence from nucleotide polymorphism in the rRNA operon, this study has also identified a potential novel epitheliocystis agent infecting rainbow trout. A histological analysis of Ca. Branchiomonas cysticola qPCR positive gills revealed a heterogenous distribution of cysts, frequently accompanied by surrounding pathological alterations. In addition, a clustering of epitheliocysts in neighboring lamellae suggests that the bacterium is transmitted horizontally from cell to cell. In addition to being the most prevalent cause of epitheliocystis, Candidatus Branchiomonas cysticola is also one of the most frequently detected pathogens in Norwegian farmed salmonids. Investigation into the phylogeny and pathological alterations of the bacterium have generated valuable knowledge of epidemiological relevance for future diagnostics while also enabling accurate and reliable detection of the bacterium. | |
