Pathogenicity of Aeromonas hydrophila and pathogenesis of motile Aeromonas septicemia in channel catfish, Ictalurus punctatus

Authors: Zhang, Dunhua; Xu, Dehai; Shoemaker, Craig; Beck, Benjamin

Submitted to: Book of Abstracts World Aquaculture Society
Publication Type: Abstract Only
Publication Acceptance Date: 8/30/2017
Publication Date: 11/7/2017
Citation: Zhang, D., Xu, D., Shoemaker, C.A., Beck, B.H. 2017. Pathogenicity of Aeromonas hydrophila and pathogenesis of motile Aeromonas septicemia in channel catfish, Ictalurus punctatus [abstract]. Latin American and Caribbean Aquaculture 17. p. 34.

Interpretive Summary:

Technical Abstract: Outbreaks of motile Aeromonas septicemia (MAS) disease in warm-water fishes have caused significant economic losses in aquaculture industries worldwide. Severe outbreaks of the disease have been reported since 2009 on catfish farms of the Southeastern United States with approximate 2,000 tons of dead fish annually. To date, recommended management practices that have worked in the past seemed to be ineffective at limiting or preventing the MAS outbreak. The purpose of this study was to evaluate factors that predispose fish to A. hydrophila infection, virulence of A. hydrophila strains and the possible survival mechanism of A. hydrophila in catfish ponds. Results of this study indicate that naïve and otherwise apparently healthy fish were resistant to A. hydrophila infection while wounding on the surfaces of the fish made fish highly susceptible to infection via the waterborne route, suggesting that the (biological and/or physical) incurring lesions on fish could be a key factor predisposing catfish to MAS disease. Catfish weighing from 5 to 300 g were all susceptible and more than 90% mortality occurred within 48 h post immersion challenge in water containing with 2 x 107 colony forming units mL-1 of the pathogen. Cells of the pathogen were detected by quantitative real-time PCR as early as 1 h post challenge from most internal tissues of infected fish, implying that the pathogen was able to rapidly proliferate and spread through fish blood circulation system following infection. Different strains of A. hydrophila varied in virulence. A lineage of highly virulent strains of A. hydrophila (vAh) was responsible for the recurring outbreak of MAS in most US catfish farms. The genome of vAh encodes a suite of proteins for utilization of chitin, the most abundant organic matter in aquatic ecosystems. Assays in vitro showed that four chitinases, one chitobiase and one chitin binding protein were secreted extracellularly by vAh and participated in chitin degradation. The bacterium was able to effectively use not only N-acetylglucosamine and colloidal chitin but also chitin flakes as sole carbon sources for growth, enabling the bacterium to reach high densities in aquatic niches and constituting a potential threat to susceptible fish when abundant chitin is available. Findings of this study improved our understanding of the virulence of vAh and pathogenesis of MAS, which will facilitate further studies on the disease prevention and control.