Abstract
Flavobacterium columnare (F. columnare) is the causative agent of columnaris disease. This bacterium affects both cultured and wild freshwater fish including many susceptible commercially important fish species. F. columnare infections may result in skin lesions, fin erosion and gill necrosis, with a high degree of mortality, leading to severe economic losses. Especially in the last decade, various research groups have performed studies aimed at elucidating the pathogenesis of columnaris disease, leading to significant progress in defining the complex interactions between the organism and its host. Despite these efforts, the pathogenesis of columnaris disease hitherto largely remains unclear, compromising the further development of efficient curative and preventive measures to combat this disease. Besides elaborating on the agent and the disease it causes, this review aims to summarize these pathogenesis data emphasizing the areas meriting further investigation.
Highlights
Flavobacterium columnare (F. columnare) is the causative agent of columnaris disease
Despite the worldwide importance of columnaris disease and the multitude of studies focusing on its causative agent, only snippets of the bacterium-host interactions have hitherto been exposed
Full genome sequencing of the reference strain has been an important step towards gaining insight into the pathogenesis of columnaris disease
Summary
2.1 Clinical signs, histopathology, ultrastructural features and haematology F. columnare causes acute to chronic infections and typically affects the gills, the skin and fins. An aggregative adhesion pattern of a highly virulent F. columnare strain onto gill tissue is a distinct feature in both in vivo and organ culture experiments [35,42,53,61] This results in an irregular gill surface covered by a thick mat consisting of numerous clumps of F. columnare bacterial cells, most likely impeding oxygen uptake and causing death of the fish. Decostere et al observed significantly higher bacterial titers on the gills when organic matter or nitrite were added to an organ bath when performing ex vivo trials with F. columnare [61] They discussed that organic matter could concentrate nutrients to feed the bacteria and that degrading enzymes could be kept in close contact with the host tissue. High mortality rates were observed in elvers kept in standing water, while in aquaria with running water, mortality in elvers was reduced by half [94]
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