Distribution of virulent Aeromonas hydrophila in channel catfish tissues following waterborne exposure

Authors: Xu, Dehai; Zhang, Dunhua; Shoemaker, Craig; NEWTON, JOSEPH - Auburn University

Submitted to: Annual Meeting World Aquaculture Society
Publication Type: Abstract Only
Publication Acceptance Date: 4/14/2017
Publication Date: 6/26/2017
Citation: Xu, D., Zhang, D., Shoemaker, C.A., Newton, J. 2017. Distribution of virulent Aeromonas hydrophila in channel catfish tissues following waterborne exposure [abstract]. World Aquaculture 2017. p.745.

Interpretive Summary:

Technical Abstract: In freshwater aquaculture, Aeromonas hydrophila was historically considered an opportunistic pathogen associated with secondary bacterial infections. After a 2009 outbreak of motile Aeromonas septicemia (MAS) in farmed catfish in West Alabama and East Mississippi, virulent A. hydrophila (vAh) was identified as the etiologic agent. Tens of millions of pounds of market-sized catfish have been lost due to vAh. The vAh has emerged as a primary pathogen associated with catfish and carp aquaculture in the United States and China, respectively. In order to evaluate MAS disease progress and find effective methods to prevent/control vAh, we developed a reproducible waterborne challenge model. Although progress has been made in understanding vAh infection in catfish, it is still unknown how vAh infects fish from water, where the bacterium is distributed in fish tissues and how the infection progresses. This study evaluated the distribution of vAh in channel catfish tissues using real-time polymerase chain reaction (qPCR) following waterborne exposure with vAh. Channel catfish were distributed in nine 57-L aquaria with 15 fish per tank to receive three treatments: 1) challenge with vAh at 2 × 107 CFU/mL of water for determination of fish mortality, 2) challenge with vAh at 2 × 107 CFU/mL of water for collection of fish tissues, and 3) mock challenge using sterile tryptic soy broth (TSB) as negative controls. After fish were anesthetized with MS-222 and adipose fin was clipped, fish were transferred into 57-L tanks filled with 15 L of water. For vAh challenge, 100 mL of vAh cell suspension was added to each tank resulting in 2 × 107 CFU/mL of water. Following 1 h exposure, water flow was resumed. Fish mortality was monitored and recorded daily for 1 week post challenge. A total of 6 fish were randomly sampled at 1, 2, 4, 8, 24 and 48 h post vAh challenge. MS-222 euthanized fish tissues (blood, adipose fin near clipped site, gill, skin on the left side of lateral line just above pelvic fin, brain, intestine, liver, spleen and trunk kidney) were sampled for vAh quantification. Genomic DNA of fish tissues was extracted using the DNeasy blood and tissue kit (Qiagen). A qPCR method was used to quantify cells of vAh in fish tissues. Challenge with vAh resulted in 77.8% mortality and most mortality (about 91%) occurred within 48 h post challenge with mean-day to death of 1.5 days. Dead fish showed typical clinical signs of reddened fins, external/internal septicemia and hemorrhage. All dead fish sampled for confirmation were positive for the presence of vAh in liver tissue. At 2 h post challenge, vAh (genomic DNA copies or genome equivalents) was detected in all external and internal tissues sampled. Gill had the highest vAh cells at 1 h post challenge. Spleen harbored the most vAh cells among internal organs at 4 h post challenge. The tissues/organs with most vAh cells detected at 8 h post challenge were adipose fin, blood, intestine, kidney and skin while liver showed the highest vAh cells at 24 h post challenge. These results suggest that vAh was able to rapidly proliferate and spread, following wounding, through the fish circulatory system and cause mortality within 8 to 24 h.