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Research Project: Pathogen Characterization, Host Immune Response and Development of Strategies to Reduce Losses to Disease in Aquaculture

Location: Aquatic Animal Health Research

Title: Diseases, Pathology and Recent Developments in US Farm-Raised Catfish (Ictalurus spp.)

Author
item Baumgartner, Wes - Mississippi State University
item Khoo, Lester - Mississippi State University
item Beck, Benjamin
item Rosser, Graham - Mississippi State University
item Reichley, Stephen - Mississippi State University
item Griffin, Matt - Mississippi State University

Submitted to: American College of Veterinary Pathologists Meeting
Publication Type: Proceedings
Publication Acceptance Date: 10/30/2016
Publication Date: 12/3/2016
Citation: Baumgartner, W., Khoo, L., Beck, B.H. 2016. Diseases, Pathology and Recent Developments in US Farm-Raised Catfish (Ictalurus spp.). In: Proceedings of the American College of Veterinary Pathologists Annual Meeting,December 3-7, 2016, New Orleans, Louisiana. p. 55.

Interpretive Summary:

Technical Abstract: Channel catfish (Ictalurus punctatus) and now hybrid catfish (I. punctatus x I. furcatus) have been the most economically important aquaculture species in the United States for many years. As such, they continue to be of considerable research interest. The focus of this presentation concerns some recent advances in farmed catfish disease in the US Southeast. Columnaris disease, a common and important cause of severe disease in freshwater fish, is caused by a Gram-negative slender rod Flavobacterium columnare. Disease is almost entirely external, with severe necrosis of the gills and skin. In catfish, necrosis predominates with relatively little inflammation. Differences in susceptibility have been noted in regard to different catfish species and strains. Adherence to fish surface mucus has been considered a prerequisite to infection, as immersion challenge is far more successful than bacterial injection in causing disease. Mucus is chemotactic for F. columnare and recent investigation has found a carbohydrate receptor, the rhamnose binding lectin (RBL), to be upregulated in mucus during infection (1). RBLs are important effectors of the innate response and function to agglutinate bacteria. However, F. columnare may utilize RBLs to initiate disease, rather than being inhibited by them. Fasting catfish upregulate RBLs, and this has significant bearing on catfish disease management, particularly when considering Edwardsiella ictaluri septicemia (ESC). Mucosal lectins have a critical role in immunological regulation of this disease. Edwardsiella ictaluri, the causative agent of enteric septicemia in catfish (ESC), has been an economically important primary pathogen for many years. The bacterium presents as an acute septicemia where bacteria enter, survive, and replicate in naïve macrophages in the pronephros; it is a facultative pathogen of professional phagocytes. E. ictaluri encodes a Salmonella pathogenicity island 2-like type 3 secretion system (T3SS) which is essential for intracellular replication within macrophage phagosomes. This bacterium also encodes an acid-activated urease enzyme, which uses the urea produced by the macrophage to make ammonia, thereby increasing phagosomal pH and disrupting acidification of the phagosome (2). The transient acidification upregulates the T3SS, where several effectors are translocated through the vacuolar membrane directly into the macrophage cytoplasm (3). Historically Edwardsiella tarda has been considered a relatively uncommon to rare disease in US channel catfish, causing “emphysematous putrefactive disease” in the chronic form, where large necrotizing lesions laden with malodorous gas are characteristic. This has changed recently, as cases of pond outbreaks have increased in the last few years. Recent investigations of genotypic and phenotypic variability in E. tarda isolates have resulted in a division of E. tarda into E. piscicida, E. anguillarum, and E. tarda. E. piscicida isolates are genotypically more closely related to E. ictaluri and E. anguillarum than E. tarda, based on 16S rRNA, gyrB, and sodB sequences. E. piscicida and E. tarda also vary significantly in terms of plasmid content, nutrient metabolism, and insertion elements. Furthermore, E. piscicida was far more virulent in channel catfish, with an LD50 of 5.8 x 105 CFU, while an LD50 for E. tarda could not be determined, as culture doses could not be generated to high enough levels. Currently there are no distinguishing phenotypic traits to discriminate E. tarda from E. piscicida, and E. anguillarum. Interestingly, the comparative LD50 for E. piscicida is a log lower in hybrid catfish than in channel catfish, a finding that is consistent with the increase in pond outbreak cases and the increased stocking of hybrid catfish in the industry (4,5). Aeromonas hydrophila is a ubiquitous Gram negative bacterial inhabitant of fr