Experimental induction of motile Aeromonas septicemia in channel catfish by waterborne challenge with virulent Aeromonas hydrophila

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

Submitted to: Aquaculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/19/2015
Publication Date: 1/9/2016
Citation: Zhang, D., Xu, D., Shoemaker, C.A. 2016. Experimental induction of motile Aeromonas septicemia in channel catfish by waterborne challenge with virulent Aeromonas hydrophila. Aquaculture Reports. 3:18-23.

Interpretive Summary: Since the 2009 outbreak of motile Aeromonas septicemia (MAS) in West Alabama and East Mississippi, the disease has cost catfish aquaculture losses of about three million pounds of food-size fish annually. To date, primary or obvious field conditions leading to the disease outbreaks were largely unknown and none of recommended management practices that have worked in the past seemed to be effective at limiting or preventing the outbreaks. In this study, we investigated factors that may result in an increased susceptibility of fish to vAh infection and establish a reproducible waterborne challenge model that mimics natural occurrence of MAS. Our results revealed that fish body surface wounding was one of the most important factors that predisposed catfish to virulent A. hydrophila infection. Fish wounds may result from fish physical colliding, field seining, parasitism by parasites, etc. To confirm the effect of fish wounds on A. hydrophila infection, relatively uniform wounds were created by clipping part of fish adipose fin (a non-essential function fin). It was found that adipose fin clipped (Af-clipped) fish were highly susceptible to A. hydrophila infection. Using Af-clipped fish, we developed a waterborne challenge model, which can facilitate evaluation of virulence of field isolates of A. hydrophila, assessment of predisposing factors and development of prevention methods against MAS.

Technical Abstract: Motile Aeromonas septicemia (MAS), caused by virulent clonal isolates of Aeromonas hydrophila (vAh), is emerging as a major disease in catfish 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 establish a waterborne challenge model that mimics natural occurrence of MAS. Results of this study indicated that wounding on the fish body surface was one of the key factors for establishment of 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 control treatment (mock challenge). When subjected to vAh challenge, Af-clipped fish were highly susceptible, showing typical symptoms of MAS observed in the field. The rate of mortality 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 concentration of 2 x 107 colony forming units per ml of water (27±1ºC). The waterborne challenge model was further tested using four field isolates of Aeromonas sp. All vAh isolates caused about 90% mortality of Af-clipped fish and one isolate of Aeromonas veronii caused no mortality under the same challenge conditions. The waterborne challenge model described in this study will facilitate urgently-needed studies of MAS prevention (such as wound avoidance and curing) and control (such as prophylactic vaccination; antibiotics treatment and probiotics screening).