Detection and quantification of virulent Aeromonas hydrophila in channel catfish tissues following waterborne challenge

Authors: Zhang, Dunhua; MOREIRA, GABRIEL - Universidad De Sao Paulo; Shoemaker, Craig; NEWTON, JOSEPH - Auburn University; Xu, Dehai

Submitted to: FEMS Microbiology Letters
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/29/2016
Publication Date: 4/3/2016
Citation: Zhang, D., Moreira, G.S., Shoemaker, C.A., Newton, J.C., Xu, D. 2016. Detection and quantification of virulent Aeromonas hydrophila in channel catfish tissues following waterborne challenge. FEMS Microbiology Letters [online]. 363(9). Available: http://femsle.oxfordjournals.org/content/363/9/fnw080. doi:10.1093/femsle/fnw080.

Interpretive Summary: Following a 2009 outbreak of motile Aeromonas septicemia (MAS) in farmed catfish in West Alabama and East Mississippi, a virulent clonal population of A. hydrophila (vAh) was determined to be responsible for the MAS cases. Tens of millions of pounds of food-size catfish have been lost to the disease in production ponds of the southeastern United States since then. It appears that vAh has emerged as a primary pathogen associated with catfish aquaculture in the United States. Although progress has been made in understanding vAh infection in catfish using a waterborne challenge model developed in our previous study, it is still unknown when vAh gets access into fish from water, where the bacterium is distributed in fish tissues and how infection is progressed. This study was to evaluate the distribution of vAh in channel catfish tissues using real-time polymerase chain reaction (qPCR) following waterborne/immersion challenge with vAh. Results of this study showed that the presence of high concentration of vAh at fresh wounding sites could result in fast and systematic infection (between 4 to 8 h). One hour post waterborne challenge, vAh cells were detected in all external tissues and most internal tissues examined (except brain). Within the 24 h time course, gill had the highest vAh cells at 1 h post challenge; spleen and brain had the highest vAh cells at 4 h post challenge; adipose fin, blood, intestine, kidney and skin had the highest vAh cells at 8 h post challenge; and liver had the highest vAh cells at 24 h post challenge. The infected fish started to show mortality at 8 h post challenge with peak mortality at about 24 h post challenge and almost all mortality occurred within 48 h post challenge with mean day to death of 1.5 days. In commercial fish culture, many factors can cause fish skin injuries, such as overcrowding, external parasites and seining practices, which may be attributed to the chance of MAS outbreak.

Technical Abstract: The aim of this study was to understand the pathogenesis of motile aeromonas septicemia caused by virulent A. hydrophila (vAh) in channel catfish, Ictalurus punctatus. Adipose fin clipped catfish were challenged with vAh using waterborne challenge method and the distribution of vAh in catfish tissues over a time course was detected and quantified using real-time polymerase chain reaction (qPCR). The results showed that 77.8% of fish died within 48 h post challenge with mean-day to death of 1.5 days. At 2 h post challenge, vAh 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. 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 to 24 h.