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INFECTION AND DISEASE PROGRESS OF MOTILE AEROMONAS SEPTICEMIA CAUSED BY VIRULENT Aeromonas hydrophila IN CHANNEL CATFISH

Dunhua Zhang, Dehai Xu, and Craig Shoemaker
Aquatic Animal Health Research Unit, USDA-ARS
990 Wire Road, Auburn, AL 36832
dunhua.zhang@ars.usda.gov

Motile Aeromonas septicemia (MAS), caused by virulent clonal isolates of Aeromonas hydrophila (vAh), is emerging as a major disease in channel catfish (Ictalurus punctatus) aquaculture in the Southeastern United States. Predisposing conditions leading to vAh infection in catfish were however largely unknown. The objective of this study was to investigate factors that predispose catfish to vAh infection and to understand the pathogenesis of MAS.

Results of this study indicated that wounding on the fish body surface was one of the key factors that predisposed catfish to vAh infection via waterborne route. Relatively uniform wounds were created by clipping part of the fish adipose fin. Adipose fin clipped (Af-clipped) fish behaved normally in terms of swimming and feeding and no mortality occurred in the control treatment (a mock challenge). When subjected to challenge in vAh-infected water, Af-clipped fish were highly susceptible, showing typical symptoms of MAS observed in the field. The mortality rate of Af-clipped fish was significantly associated with vAh concentration, challenge time and water temperature. About 90% mortality occurred within 48 h when Af-clipped fish were challenged for 1 h with vAh at

the concentration of 2 x 107 colony forming units per mL of water (27±1°C). The relative susceptibility of different sizes of fish was statistically not different: 90±10%, 83.3±7.5%, and 73.3±14.9% mortality for fish of 4.9±1.2g, 52.9±11.5g, and 222.6±96.5g, respectively. Cells of vAh were detected in most internal tissues of fish at 1 h post challenge. Results of this study suggest that vAh was able to rapidly proliferate and spread, following wound infection, through fish blood circulation system and cause mortality within 8-24 h.




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